<ol>
<li><a href="#intro">Introduction</a></li>
- <li><a href="#questions">The Questions</a>
+ <li><a href="#addresses">Address Computation</a>
<ol>
<li><a href="#extra_index">Why is the extra 0 index required?</a></li>
<li><a href="#deref">What is dereferenced by GEP?</a></li>
subsequent ones?</a></li>
<li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
<li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
+ <li><a href="#vectors">Can GEP index into vector elements?</a>
+ <li><a href="#unions">Can GEP index into unions?</a>
+ <li><a href="#addrspace">What effect do address spaces have on GEPs?</a>
+ <li><a href="#int">How is GEP different from ptrtoint, arithmetic, and inttoptr?</a></li>
+ <li><a href="#be">I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?</a>
+ <li><a href="#vla">How does VLA addressing work with GEPs?</a>
+ </ol></li>
+ <li><a href="#rules">Rules</a>
+ <ol>
+ <li><a href="#bounds">What happens if an array index is out of bounds?</a>
+ <li><a href="#negative">Can array indices be negative?</a>
+ <li><a href="#compare">Can I compare two values computed with GEPs?</a>
+ <li><a href="#types">Can I do GEP with a different pointer type than the type of the underlying object?</a>
+ <li><a href="#null">Can I cast an object's address to integer and add it to null?</a>
+ <li><a href="#ptrdiff">Can I compute the distance between two objects, and add that value to one address to compute the other address?</a>
+ <li><a href="#tbaa">Can I do type-based alias analysis on LLVM IR?</a>
+ <li><a href="#overflow">What happens if a GEP computation overflows?</a>
+ <li><a href="#check">How can I tell if my front-end is following the rules?</a>
+ </ol></li>
+ <li><a href="#rationale">Rationale</a>
+ <ol>
+ <li><a href="#goals">Why is GEP designed this way?</a></li>
+ <li><a href="#i32">Why do struct member indices always use i32?</a></li>
+ <li><a href="#uglygep">What's an uglygep?</a>
</ol></li>
<li><a href="#summary">Summary</a></li>
</ol>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
<!-- *********************************************************************** -->
+
<div class="doc_text">
<p>This document seeks to dispel the mystery and confusion surrounding LLVM's
- GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
- probably the most frequently occuring questions once a developer gets down to
+ GetElementPtr (GEP) instruction. Questions about the wily GEP instruction are
+ probably the most frequently occurring questions once a developer gets down to
coding with LLVM. Here we lay out the sources of confusion and show that the
GEP instruction is really quite simple.
</p>
</div>
<!-- *********************************************************************** -->
-<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
+<div class="doc_section"><a name="addresses"><b>Address Computation</b></a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>When people are first confronted with the GEP instruction, they tend to
relate it to known concepts from other programming paradigms, most notably C
- array indexing and field selection. However, GEP is a little different and
- this leads to the following questions, all of which are answered in the
- following sections.</p>
- <ol>
- <li><a href="extra_index">Why is the extra 0 index required?</a></li>
- <li><a href="deref">What is dereferenced by GEP?</a></li>
- <li><a href="firstptr">Why can you index through the first pointer but not
- subsequent ones?</a></li>
- <li><a href="lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
- <li><a href="trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
- </ol>
+ array indexing and field selection. GEP closely resembles C array indexing
+ and field selection, however it's is a little different and this leads to
+ the following questions.</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
+</div>
+<div class="doc_text">
+ <p>Quick answer: The index stepping through the first operand.</p>
+ <p>The confusion with the first index usually arises from thinking about
+ the GetElementPtr instruction as if it was a C index operator. They aren't the
+ same. For example, when we write, in "C":</p>
+
+<div class="doc_code">
+<pre>
+AType *Foo;
+...
+X = &Foo->F;
+</pre>
+</div>
+
+ <p>it is natural to think that there is only one index, the selection of the
+ field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
+ pointer must be indexed explicitly in LLVM. C, on the other hand, indices
+ through it transparently. To arrive at the same address location as the C
+ code, you would provide the GEP instruction with two index operands. The
+ first operand indexes through the pointer; the second operand indexes the
+ field <tt>F</tt> of the structure, just as if you wrote:</p>
+
+<div class="doc_code">
+<pre>
+X = &Foo[0].F;
+</pre>
+</div>
+
+ <p>Sometimes this question gets rephrased as:</p>
+ <blockquote><p><i>Why is it okay to index through the first pointer, but
+ subsequent pointers won't be dereferenced?</i></p></blockquote>
+ <p>The answer is simply because memory does not have to be accessed to
+ perform the computation. The first operand to the GEP instruction must be a
+ value of a pointer type. The value of the pointer is provided directly to
+ the GEP instruction as an operand without any need for accessing memory. It
+ must, therefore be indexed and requires an index operand. Consider this
+ example:</p>
+
+<div class="doc_code">
+<pre>
+struct munger_struct {
+ int f1;
+ int f2;
+};
+void munge(struct munger_struct *P) {
+ P[0].f1 = P[1].f1 + P[2].f2;
+}
+...
+munger_struct Array[3];
+...
+munge(Array);
+</pre>
+</div>
+
+ <p>In this "C" example, the front end compiler (llvm-gcc) will generate three
+ GEP instructions for the three indices through "P" in the assignment
+ statement. The function argument <tt>P</tt> will be the first operand of each
+ of these GEP instructions. The second operand indexes through that pointer.
+ The third operand will be the field offset into the
+ <tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
+ <tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
+ like:</p>
+
+<div class="doc_code">
+<pre>
+void %munge(%struct.munger_struct* %P) {
+entry:
+ %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
+ %tmp = load i32* %tmp
+ %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
+ %tmp7 = load i32* %tmp6
+ %tmp8 = add i32 %tmp7, %tmp
+ %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
+ store i32 %tmp8, i32* %tmp9
+ ret void
+}
+</pre>
+</div>
+
+ <p>In each case the first operand is the pointer through which the GEP
+ instruction starts. The same is true whether the first operand is an
+ argument, allocated memory, or a global variable. </p>
+ <p>To make this clear, let's consider a more obtuse example:</p>
+
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global i32
+...
+%idx1 = getelementptr i32* %MyVar, i64 0
+%idx2 = getelementptr i32* %MyVar, i64 1
+%idx3 = getelementptr i32* %MyVar, i64 2
+</pre>
+</div>
+
+ <p>These GEP instructions are simply making address computations from the
+ base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
+ </p>
+
+<div class="doc_code">
+<pre>
+idx1 = (char*) &MyVar + 0
+idx2 = (char*) &MyVar + 4
+idx3 = (char*) &MyVar + 8
+</pre>
+</div>
+
+ <p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
+ 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
+ memory is accessed to make these computations because the address of
+ <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
+ <p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
+ <tt>%idx3</tt>. They result in the computation of addresses that point to
+ memory past the end of the <tt>%MyVar</tt> global, which is only one
+ <tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
+ it is inadvisable because any load or store with the pointer that results
+ from these GEP instructions would produce undefined results.</p>
</div>
<!-- *********************************************************************** -->
<p>Quick answer: there are no superfluous indices.</p>
<p>This question arises most often when the GEP instruction is applied to a
global variable which is always a pointer type. For example, consider
- this:</p><pre>
- %MyStruct = uninitialized global { float*, int }
- ...
- %idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1</pre>
- <p>The GEP above yields an <tt>int*</tt> by indexing the <tt>int</tt> typed
+ this:</p>
+
+<div class="doc_code">
+<pre>
+%MyStruct = uninitialized global { float*, i32 }
+...
+%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
+</pre>
+</div>
+
+ <p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
field of the structure <tt>%MyStruct</tt>. When people first look at it, they
- wonder why the <tt>long 0</tt> index is needed. However, a closer inspection
- of how globals and GEPs work reveals the need. Becoming aware of the following
- facts will dispell the confusion:</p>
+ wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
+ of how globals and GEPs work reveals the need. Becoming aware of the following
+ facts will dispel the confusion:</p>
<ol>
- <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, int }</tt>
- but rather <tt>{ float*, int }*</tt>. That is, <tt>%MyStruct</tt> is a
+ <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
+ but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
pointer to a structure containing a pointer to a <tt>float</tt> and an
- <tt>int</tt>.</li>
+ <tt>i32</tt>.</li>
<li>Point #1 is evidenced by noticing the type of the first operand of
the GEP instruction (<tt>%MyStruct</tt>) which is
- <tt>{ float*, int }*</tt>.</li>
- <li>The first index, <tt>long 0</tt> is required to dereference the
- pointer associated with <tt>%MyStruct</tt>.</li>
- <li>The second index, <tt>ubyte 1</tt> selects the second field of the
- structure (the <tt>int</tt>). </li>
+ <tt>{ float*, i32 }*</tt>.</li>
+ <li>The first index, <tt>i64 0</tt> is required to step over the global
+ variable <tt>%MyStruct</tt>. Since the first argument to the GEP
+ instruction must always be a value of pointer type, the first index
+ steps through that pointer. A value of 0 means 0 elements offset from that
+ pointer.</li>
+ <li>The second index, <tt>i32 1</tt> selects the second field of the
+ structure (the <tt>i32</tt>). </li>
</ol>
</div>
<div class="doc_text">
<p>Quick answer: nothing.</p>
<p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
- access memory in any way. That's what the Load instruction is for. GEP is
- only involved in the computation of addresses. For example, consider this:</p>
- <pre>
- %MyVar = uninitialized global { [40 x int ]* }
- ...
- %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17</pre>
+ access memory in any way. That's what the Load and Store instructions are for.
+ GEP is only involved in the computation of addresses. For example, consider
+ this:</p>
+
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global { [40 x i32 ]* }
+...
+%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
+</pre>
+</div>
+
<p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
pointer to a structure containing a pointer to an array of 40 ints. The
- GEP instruction seems to be accessing the 18th integer of of the structure's
+ GEP instruction seems to be accessing the 18th integer of the structure's
array of ints. However, this is actually an illegal GEP instruction. It
won't compile. The reason is that the pointer in the structure <i>must</i>
be dereferenced in order to index into the array of 40 ints. Since the
GEP instruction never accesses memory, it is illegal.</p>
<p>In order to access the 18th integer in the array, you would need to do the
following:</p>
- <pre>
- %idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
- %arr = load [40 x int]** %idx
- %idx = getelementptr [40 x int]* %arr, long 0, long 17</pre>
+
+<div class="doc_code">
+<pre>
+%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
+%arr = load [40 x i32]** %idx
+%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
+</pre>
+</div>
+
<p>In this case, we have to load the pointer in the structure with a load
instruction before we can index into the array. If the example was changed
to:</p>
- <pre>
- %MyVar = uninitialized global { [40 x int ] }
- ...
- %idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17</pre>
- <p>then everything works fine. In this case, the structure does not contain a
- pointer and the GEP instruction can index through the global variable pointer,
- into the first field of the structure and access the 18th <tt>int</tt> in the
- array there.</p>
-</div>
-<!-- *********************************************************************** -->
-<div class="doc_subsection">
- <a name="firstptr"><b>Why can you index through the first pointer?</b></a>
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global { [40 x i32 ] }
+...
+%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
+</pre>
</div>
-<div class="doc_text">
- <p>Quick answer: Because its already present.</p>
- <p>Having understood the <a href="#deref">previous question</a>, a new
- question then arises:</p>
- <blockquote><i>Why is it okay to index through the first pointer, but
- subsequent pointers won't be dereferenced?</i></blockquote>
- <p>The answer is simply because
- memory does not have to be accessed to perform the computation. The first
- operand to the GEP instruction must be a value of a pointer type. The value
- of the pointer is provided directly to the GEP instruction without any need
- for accessing memory. It must, therefore be indexed like any other operand.
- Consider this example:</p>
- <pre>
- %MyVar = unintialized global int
- ...
- %idx1 = getelementptr int* %MyVar, long 0
- %idx2 = getelementptr int* %MyVar, long 1
- %idx3 = getelementptr int* %MyVar, long 2</pre>
- <p>These GEP instructions are simply making address computations from the
- base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):</p>
- <ul>
- <li> idx1 = &MyVar + 0</li>
- <li> idx2 = &MyVar + 4</li>
- <li> idx3 = &MyVar = 8</li>
- </ul>
- <p>Since the type <tt>int</tt> is known to be four bytes long, the indices
- 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
- memory is accessed to make these computations because the address of
- <tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
- <p>Note that the cases of <tt>%idx2</tt> and <tt>%idx3</tt> are a bit silly.
- They are computing addresses of something of unknown type (and thus
- potentially breaking type safety) because <tt>%MyVar</tt> is only one
- integer long.</p>
+
+ <p>then everything works fine. In this case, the structure does not contain a
+ pointer and the GEP instruction can index through the global variable,
+ into the first field of the structure and access the 18th <tt>i32</tt> in the
+ array there.</p>
</div>
<!-- *********************************************************************** -->
<p>If you look at the first indices in these GEP
instructions you find that they are different (0 and 1), therefore the address
computation diverges with that index. Consider this example:</p>
- <pre>
- %MyVar = global { [10 x int ] }
- %idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, byte 0, long 1
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
+%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
+</pre>
+</div>
+
<p>In this example, <tt>idx1</tt> computes the address of the second integer
- in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>. The
- type of <tt>idx1</tt> is <tt>int*</tt>. However, <tt>idx2</tt> computes the
- address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
- <tt>idx2</tt> is <tt>{ [10 x int] }*</tt> and its value is equivalent
- to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
- in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
- alias.</p>
+ in the array that is in the structure in <tt>%MyVar</tt>, that is
+ <tt>MyVar+4</tt>. The type of <tt>idx1</tt> is <tt>i32*</tt>. However,
+ <tt>idx2</tt> computes the address of <i>the next</i> structure after
+ <tt>%MyVar</tt>. The type of <tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its
+ value is equivalent to <tt>MyVar + 40</tt> because it indexes past the ten
+ 4-byte integers in <tt>MyVar</tt>. Obviously, in such a situation, the
+ pointers don't alias.</p>
+
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
- <a name="lead0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
+ <a name="trail0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
</div>
<div class="doc_text">
<p>Quick Answer: They compute the same address location.</p>
<p>These two GEP instructions will compute the same address because indexing
through the 0th element does not change the address. However, it does change
the type. Consider this example:</p>
- <pre>
- %MyVar = global { [10 x int ] }
- %idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, byte 0, long 0
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
+</pre>
+</div>
+
<p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
- its type is <tt>int*</tt>. The value of <tt>%idx2</tt> is also
- <tt>MyVar+40</tt> but its type is <tt>{ [10 x int] }*</tt>.</p>
+ its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
+ <tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="vectors"><b>Can GEP index into vector elements?</b></a>
+</div>
+<div class="doc_text">
+ <p>This hasn't always been forcefully disallowed, though it's not recommended.
+ It leads to awkward special cases in the optimizers, and fundamental
+ inconsistency in the IR. In the future, it will probably be outright
+ disallowed.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="unions"><b>Can GEP index into unions?</b></a>
+</div>
+<div class="doc_text">
+ <p>Unknown.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="addrspace"><b>What effect do address spaces have on GEPs?</b></a>
+</div>
+<div class="doc_text">
+ <p>None, except that the address space qualifier on the first operand pointer
+ type always matches the address space qualifier on the result type.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="int"><b>How is GEP different from ptrtoint, arithmetic,
+ and inttoptr?</b></a>
+</div>
+<div class="doc_text">
+ <p>It's very similar; there are only subtle differences.</p>
+
+ <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
+ this is safe on everything LLVM supports (LLVM internally assumes pointers
+ are never wider than 64 bits in many places), and the optimizer will actually
+ narrow the i64 arithmetic down to the actual pointer size on targets which
+ don't support 64-bit arithmetic in most cases. However, there are some cases
+ where it doesn't do this. With GEP you can avoid this problem.
+
+ <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
+ GEP from one object, address into a different separately allocated
+ object, and dereference it. IR producers (front-ends) must follow this rule,
+ and consumers (optimizers, specifically alias analysis) benefit from being
+ able to rely on it. See the <a href="#rules">Rules</a> section for more
+ information.</p>
+
+ <p>And, GEP is more concise in common cases.</p>
+
+ <p>However, for the underlying integer computation implied, there
+ is no difference.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="be"><b>I'm writing a backend for a target which needs custom
+ lowering for GEP. How do I do this?</b></a>
+</div>
+<div class="doc_text">
+ <p>You don't. The integer computation implied by a GEP is target-independent.
+ Typically what you'll need to do is make your backend pattern-match
+ expressions trees involving ADD, MUL, etc., which are what GEP is lowered
+ into. This has the advantage of letting your code work correctly in more
+ cases.</p>
+
+ <p>GEP does use target-dependent parameters for the size and layout of data
+ types, which targets can customize.</p>
+
+ <p>If you require support for addressing units which are not 8 bits, you'll
+ need to fix a lot of code in the backend, with GEP lowering being only a
+ small piece of the overall picture.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="vla"><b>How does VLA addressing work with GEPs?</b></a>
+</div>
+<div class="doc_text">
+ <p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
+ and GEP address computations are guided by an LLVM type.</p>
+
+ <p>VLA indices can be implemented as linearized indices. For example, an
+ expression like X[a][b][c], must be effectively lowered into a form
+ like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
+ array reference.</p>
+
+ <p>This means if you want to write an analysis which understands array
+ indices and you want to support VLAs, your code will have to be
+ prepared to reverse-engineer the linearization. One way to solve this
+ problem is to use the ScalarEvolution library, which always presents
+ VLA and non-VLA indexing in the same manner.</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="rules"><b>Rules</b></a></div>
+<!-- *********************************************************************** -->
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="bounds"><b>What happens if an array index is out of bounds?</b></a>
+</div>
+<div class="doc_text">
+ <p>There are two senses in which an array index can be out of bounds.</p>
+
+ <p>First, there's the array type which comes from the (static) type of
+ the first operand to the GEP. Indices greater than the number of elements
+ in the corresponding static array type are valid. There is no problem with
+ out of bounds indices in this sense. Indexing into an array only depends
+ on the size of the array element, not the number of elements.</p>
+
+ <p>A common example of how this is used is arrays where the size is not known.
+ It's common to use array types with zero length to represent these. The
+ fact that the static type says there are zero elements is irrelevant; it's
+ perfectly valid to compute arbitrary element indices, as the computation
+ only depends on the size of the array element, not the number of
+ elements. Note that zero-sized arrays are not a special case here.</p>
+
+ <p>This sense is unconnected with <tt>inbounds</tt> keyword. The
+ <tt>inbounds</tt> keyword is designed to describe low-level pointer
+ arithmetic overflow conditions, rather than high-level array
+ indexing rules.
+
+ <p>Analysis passes which wish to understand array indexing should not
+ assume that the static array type bounds are respected.</p>
+
+ <p>The second sense of being out of bounds is computing an address that's
+ beyond the actual underlying allocated object.</p>
+
+ <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
+ undefined if the address is outside the actual underlying allocated
+ object and not the address one-past-the-end.</p>
+
+ <p>Without the <tt>inbounds</tt> keyword, there are no restrictions
+ on computing out-of-bounds addresses. Obviously, performing a load or
+ a store requires an address of allocated and sufficiently aligned
+ memory. But the GEP itself is only concerned with computing addresses.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="negative"><b>Can array indices be negative?</b></a>
+</div>
+<div class="doc_text">
+ <p>Yes. This is basically a special case of array indices being out
+ of bounds.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="compare"><b>Can I compare two values computed with GEPs?</b></a>
+</div>
+<div class="doc_text">
+ <p>Yes. If both addresses are within the same allocated object, or
+ one-past-the-end, you'll get the comparison result you expect. If either
+ is outside of it, integer arithmetic wrapping may occur, so the
+ comparison may not be meaningful.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="types"><b>Can I do GEP with a different pointer type than the type of
+ the underlying object?</b></a>
+</div>
+<div class="doc_text">
+ <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
+ pointer type. The types in a GEP serve only to define the parameters for the
+ underlying integer computation. They need not correspond with the actual
+ type of the underlying object.</p>
+
+ <p>Furthermore, loads and stores don't have to use the same types as the type
+ of the underlying object. Types in this context serve only to specify
+ memory size and alignment. Beyond that there are merely a hint to the
+ optimizer indicating how the value will likely be used.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="null"><b>Can I cast an object's address to integer and add it
+ to null?</b></a>
+</div>
+<div class="doc_text">
+ <p>You can compute an address that way, but if you use GEP to do the add,
+ you can't use that pointer to actually access the object, unless the
+ object is managed outside of LLVM.</p>
+
+ <p>The underlying integer computation is sufficiently defined; null has a
+ defined value -- zero -- and you can add whatever value you want to it.</p>
+
+ <p>However, it's invalid to access (load from or store to) an LLVM-aware
+ object with such a pointer. This includes GlobalVariables, Allocas, and
+ objects pointed to by noalias pointers.</p>
+
+ <p>If you really need this functionality, you can do the arithmetic with
+ explicit integer instructions, and use inttoptr to convert the result to
+ an address. Most of GEP's special aliasing rules do not apply to pointers
+ computed from ptrtoint, arithmetic, and inttoptr sequences.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="ptrdiff"><b>Can I compute the distance between two objects, and add
+ that value to one address to compute the other address?</b></a>
+</div>
+<div class="doc_text">
+ <p>As with arithmetic on null, You can use GEP to compute an address that
+ way, but you can't use that pointer to actually access the object if you
+ do, unless the object is managed outside of LLVM.</p>
+
+ <p>Also as above, ptrtoint and inttoptr provide an alternative way to do this
+ which do not have this restriction.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="tbaa"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
+</div>
+<div class="doc_text">
+ <p>You can't do type-based alias analysis using LLVM's built-in type system,
+ because LLVM has no restrictions on mixing types in addressing, loads or
+ stores.</p>
+
+ <p>It would be possible to add special annotations to the IR, probably using
+ metadata, to describe a different type system (such as the C type system),
+ and do type-based aliasing on top of that. This is a much bigger
+ undertaking though.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="overflow"><b>What happens if a GEP computation overflows?</b></a>
+</div>
+<div class="doc_text">
+ <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
+ undefined.</p>
+
+ <p>Otherwise, the result value is the result from evaluating the implied
+ two's complement integer computation. However, since there's no
+ guarantee of where an object will be allocated in the address space,
+ such values have limited meaning.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="check"><b>How can I tell if my front-end is following the
+ rules?</b></a>
+</div>
+<div class="doc_text">
+ <p>There is currently no checker for the getelementptr rules. Currently,
+ the only way to do this is to manually check each place in your front-end
+ where GetElementPtr operators are created.</p>
+
+ <p>It's not possible to write a checker which could find all rule
+ violations statically. It would be possible to write a checker which
+ works by instrumenting the code with dynamic checks though. Alternatively,
+ it would be possible to write a static checker which catches a subset of
+ possible problems. However, no such checker exists today.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="rationale"><b>Rationale</b></a></div>
+<!-- *********************************************************************** -->
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="goals"><b>Why is GEP designed this way?</b></a>
+</div>
+<div class="doc_text">
+ <p>The design of GEP has the following goals, in rough unofficial
+ order of priority:</p>
+ <ul>
+ <li>Support C, C-like languages, and languages which can be
+ conceptually lowered into C (this covers a lot).</li>
+ <li>Support optimizations such as those that are common in
+ C compilers.</li>
+ <li>Provide a consistent method for computing addresses so that
+ address computations don't need to be a part of load and
+ store instructions in the IR.</li>
+ <li>Support non-C-like languages, to the extent that it doesn't
+ interfere with other goals.</li>
+ <li>Minimize target-specific information in the IR.</li>
+ </ul>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="i32"><b>Why do struct member indices always use i32?</b></a>
+</div>
+<div class="doc_text">
+ <p>The specific type i32 is probably just a historical artifact, however it's
+ wide enough for all practical purposes, so there's been no need to change it.
+ It doesn't necessarily imply i32 address arithmetic; it's just an identifier
+ which identifies a field in a struct. Requiring that all struct indices be
+ the same reduces the range of possibilities for cases where two GEPs are
+ effectively the same but have distinct operand types.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+
+<div class="doc_subsection">
+ <a name="uglygep"><b>What's an uglygep?</b></a>
+</div>
+<div class="doc_text">
+ <p>Some LLVM optimizers operate on GEPs by internally lowering them into
+ more primitive integer expressions, which allows them to be combined
+ with other integer expressions and/or split into multiple separate
+ integer expressions. If they've made non-trivial changes, translating
+ back into LLVM IR can involve reverse-engineering the structure of
+ the addressing in order to fit it into the static type of the original
+ first operand. It isn't always possibly to fully reconstruct this
+ structure; sometimes the underlying addressing doesn't correspond with
+ the static type at all. In such cases the optimizer instead will emit
+ a GEP with the base pointer casted to a simple address-unit pointer,
+ using the name "uglygep". This isn't pretty, but it's just as
+ valid, and it's sufficient to preserve the pointer aliasing guarantees
+ that GEP provides.</p>
+
</div>
<!-- *********************************************************************** -->
</ol>
</div>
-<!-- *********************************************************************** -->
-<div class="doc_section"><a name="discussion"><b>Appendix: Discussion</b></a></div>
-<!-- *********************************************************************** -->
-<div class="doc_text">
- <p>The following is a real discussion from the
- <a href="irc://irc.oftc.net/#llvm">#llvm IRC channel</a> about the GEP
- instruction. You may find this instructive as it was the basis for this
- document.</p>
- <table>
- <tr><th>User</th><th>Comment</th></tr>
- <tr><td>Yorion</td><td>If x & y must alias, are [ getelementptr x,0,0,1,2 ] and [ getelementptr x,1,2 ] aliased? (they obviously have different types, but they should alias...)</td></tr>
- <tr><td>Yorion</td><td>oops, for the second one I meant [ getelementptr y,1,2 ]</td></tr>
- <tr><td>Reid</td><td>I don't see how that could be, Yorion but I'm not the authority on this</td></tr>
- <tr><td>Yorion</td><td>hmm.. </td></tr>
- <tr><td>Reid</td><td>the two geps, by definition, are going to produce different pointers which are not aliased</td></tr>
- <tr><td>Yorion</td><td>would [ GEP x,1,0 ] and [ GEP y,1 ] be aliased?</td></tr>
- <tr><td>Reid</td><td>if the second gep was [gep y,0,0,1,2] then they should be aliased as well</td></tr>
- <tr><td>Reid</td><td>no, I wouldn't expect that to work either :)</td></tr>
- <tr><td>Reid</td><td>you can't just arbitrarily drop leading or trailing indices :)</td></tr>
- <tr><td>Reid</td><td>(.. leading or trailing 0 indices, I mean)</td></tr>
- <tr><td>Reid</td><td>this instruction walks through a data structure and generates a pointer to the resulting thing</td></tr>
- <tr><td>Reid</td><td>if the number of indices are different, you're ending up at a different place and by definition they'll have different addresses</td></tr>
- <tr><td>Yorion</td><td>oh, I see, because of different types, [ GEP x,0,1 ]
- & [ GEP x,1 ] actually might refer to different fields, but might also refer to the same ones... </td></tr>
- <tr><td>Reid</td><td>or, at least, that's my crude understanding of it :)</td></tr>
- <tr><td>Reid</td><td>no, they'll definitely refer to different fields</td></tr>
- <tr><td>nicholas</td><td>GEP x,0,1 ==> &((*(x+0))+1)? vs. GEP x,1 ==> &(*(x+1))?</td></tr>
- <tr><td>Reid</td><td>lemme grok that for a sec</td></tr>
- <tr><td>Reid</td><td>that might be true in some limited definition of x, but it wouldn't be generally</td></tr>
- <tr><td>nicholas</td><td>oh. fields of different sizes in a structure.</td></tr>
- <tr><td>Reid</td><td>yup</td></tr>
- <tr><td>Yorion</td><td>is perhaps the type unification the reason why [ GEP x,0,1 ] and [ GEP x,1 ] cannot alias?</td></tr>
- <tr><td>Reid</td><td>no</td></tr>
- <tr><td>Reid</td><td>they may or may not have the same type, but they are definitely different pointers</td></tr>
- <tr><td>Reid</td><td>lets use a concrete example for "x"</td></tr>
- <tr><td>Reid</td><td>suppose x is "struct {int a, float b} *"</td></tr>
- <tr><td>Reid</td><td>GEP X,0,1 is going to return the address of b</td></tr>
- <tr><td>Reid</td><td>GEP X,1 is going to return the address of the *second* "a" (after the first b)</td></tr>
- <tr><td>Yorion</td><td>ah, I see... </td></tr>
- <tr><td>Yorion</td><td>trailing zeros are still a bit confusing... </td></tr>
- <tr><td>Reid</td><td>same thing .. you're just selecting the 0th member of an array or structure</td></tr>
- <tr><td>Yorion</td><td>you don't move away from the pointer, only the type is changed</td></tr>
- <tr><td>Reid</td><td>no, you still move away from the pointer .. the type might change, or not</td></tr>
- <tr><td>Reid</td><td>the pointer definitely changes</td></tr>
- <tr><td>Reid</td><td>lets look at an example for trailing zero</td></tr>
- <tr><td>Reid</td><td>suppose x is "int x[10][10][10][10]" (in C)</td></tr>
- <tr><td>Reid</td><td>GEP X,0,0 will yield you a 3 dimensional array</td></tr>
- <tr><td>Reid</td><td>GEP X,0,0,0,0,0 will yield you an "int"</td></tr>
- <tr><td>Reid</td><td>make sense?</td></tr>
- <tr><td>Yorion</td><td>yes</td></tr>
- <tr><td>Reid</td><td>so, I think there's a law here: if the number of indices in two GEP instructions are not equivalent, there is no way the resulting pointers can alias</td></tr>
- <tr><td>Reid</td><td>(assuming the x and y alias)</td></tr>
- <tr><td>Yorion</td><td>I was confused with some code in BasicAliasAnalysis that says that two pointers are equal if they differ only in trailing zeros</td></tr>
- <tr><td>Yorion</td><td>BasicAliasAnalysis.cpp:504-518</td></tr>
- <tr><td>Reid</td><td>lemme look</td></tr>
- <tr><td>nicholas</td><td>if y1 = GEP X, 0, 0 and y2 = GEP X, 0, 0, 0, 0, 0 (from Reid's example)</td></tr>
- <tr><td>nicholas</td><td>then doesn't *y1 and *y2 both refer to the same "int"?</td></tr>
- <tr><td>Reid</td><td>they shouldn't</td></tr>
- <tr><td>Reid</td><td>hmm .. actually, maybe you're right :)</td></tr>
- <tr><td>Reid</td><td>they definitely have different *types*</td></tr>
- <tr><td>Yorion</td><td>true</td></tr>
- <tr><td>nicholas</td><td>different types just doesn't cut it. :)</td></tr>
- <tr><td>Reid</td><td>.. thinking on this :)</td></tr>
- <tr><td>nicholas</td><td>similarly, i could create a yucky with a struct that has a char *, then have you GEP right through the pointer into the pointed-to data. That could mean that the resulting point might alias anything.</td></tr>
- <tr><td>Yorion</td><td>my theory (after reading BAA) is that all zeros can be omitted, and that the pointers alias if they have the same sequence of indices</td></tr>
- <tr><td>Yorion</td><td>however, this screws the typing, so that's why zeros are for</td></tr>
- <tr><td>Yorion</td><td>nicholas, does that match your hunch?</td></tr>
- <tr><td>nicholas</td><td>I have to admit, I've had much grief with GEPIs already. I wish the semantics were plainly documented as part of their own language, instead of just relying on C aliasing rules and C semantics...</td></tr>
- <tr><td>nicholas</td><td>Yorion: leading zeroes can't be omitted.</td></tr>
- <tr><td>Reid</td><td>okay, if you have two GEPs and their leading indices are an exact match, if the one with more indices only has trailing 0s then they should alias</td></tr>
- <tr><td>nicholas</td><td>must alias, i think.</td></tr>
- <tr><td>Reid</td><td>yes, must alias, sorry</td></tr>
- <tr><td>Yorion</td><td>okay</td></tr>
- <tr><td>Yorion</td><td>I'm glad we cleared this up</td></tr>
- <tr><td>Reid</td><td>so, if y1 = GEP X, 1,2,0 and if y2 = GEP X, 1,2,0,0,0 then y1 "must alias" y2 :)</td></tr>
- <tr><td>Reid</td><td>but that doesn't work for leading 0s :)</td></tr>
- <tr><td>Yorion</td><td>yes, true... I was wrong </td></tr>
- <tr><td>Reid</td><td>I too have been having fun with GEP recently :)</td></tr>
- <tr><td>Yorion</td><td>but, there're cases like [a = GEP x,1,0; b = GEP a,1,0; c = GEP b,1,0], and that should be equivalent to GEP x,1,0,1,0,1</td></tr>
- <tr><td>Reid</td><td>not quite</td></tr>
- <tr><td>nicholas</td><td>I'm sure another rule can be written for GEPIs, but they would only apply to type-safe code.</td></tr>
- <tr><td>nicholas</td><td>another *tautology</td></tr>
- <tr><td>Yorion</td><td>Reid: why not, only the type should be different...</td></tr>
- <tr><td>Reid</td><td>its should be equivalent to GEP x,1,0,1,0,1,0</td></tr>
- <tr><td>Yorion</td><td>and that must alias GEP x,1,0,1,0,1</td></tr>
- <tr><td>Reid</td><td>hmm, by the previous rule, I guess you're right :)</td></tr>
- <tr><td>Yorion</td><td>I'm a bit scared that even you're a bit confused about GEP</td></tr>
- <tr><td>Reid</td><td>I'm glad I'm not the only one that gets a little confused wrapping my head around this stuff :)</td></tr>
- <tr><td>Reid</td><td>GEP has always confused me .. partly because I think its wrong :)</td></tr>
- <tr><td>Reid</td><td>well, actually, not so much that GEP is wrong, but that gvars being pointers without storage</td></tr>
- <tr><td>Reid</td><td>i.e. when you say "%x = global int" in LLVM, the type of X is int*</td></tr>
- <tr><td>Reid</td><td>yet, there is no storage for that pointer</td></tr>
- <tr><td>Reid</td><td>its magically deduced</td></tr>
- <tr><td>nicholas</td><td>well, it makes no sense to have globals be SSA...</td></tr>
- <tr><td>Reid</td><td>heh</td></tr>
- <tr><td>Reid</td><td>yeah, well .. practicalities :)</td></tr>
- <tr><td>Yorion</td><td>true</td></tr>
- <tr><td>Yorion</td><td>sabre gave me a reference on how globals are handled in SSA</td></tr>
- <tr><td>Reid</td><td>anyway, gotta run</td></tr>
- <tr><td>Yorion</td><td>the paper was crappy, but I do understand now why is it implemented that way in LLVM</td></tr>
- <tr><td>Yorion</td><td>thx for the interesting discussion Reid</td></tr>
- <tr><td>Reid</td><td>heh .. its one that Chris and I keep having .. he just tells me that C has rotted my brain :)</td></tr>
- <tr><td>nicholas</td><td>lol</td></tr>
- <tr><td>Yorion</td><td>hehehe</td></tr>
- <tr><td>Reid</td><td>he might be right :)</td></tr>
- <tr><td>Yorion</td><td>sabre: have you seen the discussion on GEP?</td></tr>
- <tr><td>sabre</td><td>no</td></tr>
- <tr><td>sabre</td><td>I'll read the backlog, j/s</td></tr>
- <tr><td>sabre</td><td>ok, there's a lot</td></tr>
- <tr><td>sabre</td><td>what's the executive summary?</td></tr>
- <tr><td>sabre</td><td>do you have a q?</td></tr>
- <tr><td>Yorion</td><td>is it possible that GEP x,0,0,1 and GEP x,1 alias?</td></tr>
- <tr><td>sabre</td><td>no</td></tr>
- <tr><td>Yorion</td><td>and b) GEP x,1,0,0 and GEP x,1 should alias, right?</td></tr>
- <tr><td>sabre</td><td>I assume you mean for size = 1 ?</td></tr>
- <tr><td>sabre</td><td>b) yes</td></tr>
- <tr><td>Yorion</td><td>although they have different types</td></tr>
- <tr><td>sabre</td><td>right</td></tr>
- <tr><td>Yorion</td><td>okay</td></tr>
- <tr><td>Yorion</td><td>I'm still not 100% convinced that: a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0 cannot alias Z=GEP x,1,1,1</td></tr>
- <tr><td>Yorion</td><td>(that c and z cannot alias)</td></tr>
- <tr><td>sabre</td><td>that's becuse they do alias</td></tr>
- <tr><td>sabre</td><td>mustalias in fact</td></tr>
- <tr><td>Yorion</td><td>but then: GEP x,1,0,1,0,1,0 = GEP x,1,1,1</td></tr>
- <tr><td>sabre</td><td>Yorion: no</td></tr>
- <tr><td>sabre</td><td>c != GEP x,1,0,1,0,1,0</td></tr>
- <tr><td>sabre</td><td>the first index doesn't work like that</td></tr>
- <tr><td>Yorion</td><td>how does then the first index work? c and z must alias, but GEP x,1,0,1,0 != GEP x,1,1 ??</td></tr>
- <tr><td>sabre</td><td>*sigh*</td></tr>
- <tr><td>Reid</td><td>:)</td></tr>
- <tr><td>Reid</td><td>we need an FAQ on this</td></tr>
- <tr><td>sabre</td><td>Yorion: how did you get </td></tr>
- <tr><td>sabre</td><td>"GEP x,1,0,1,0"? </td></tr>
- <tr><td>Yorion</td><td>look</td></tr>
- <tr><td>sabre</td><td>you can't just concatenate subscripts</td></tr>
- <tr><td>Yorion</td><td>why?</td></tr>
- <tr><td>sabre</td><td>because... it doesn't work that way?</td></tr>
- <tr><td>sabre</td><td>consider C</td></tr>
- <tr><td>Yorion</td><td>how does it work?</td></tr>
- <tr><td>sabre</td><td>if I have blah* P</td></tr>
- <tr><td>sabre</td><td>P[0][1][2][3][4]</td></tr>
- <tr><td>sabre</td><td>this is *not* the same as:</td></tr>
- <tr><td>sabre</td><td>t = &P[0][1][2] ... t[3][4]</td></tr>
- <tr><td>sabre</td><td>Yorion: Consider: struct *P </td></tr>
- <tr><td>sabre</td><td>P->X == P[0].X</td></tr>
- <tr><td>sabre</td><td>You're losing the 0.</td></tr>
- <tr><td>sabre</td><td>P->X->Y == P[0].X[0].Y</td></tr>
- <tr><td>sabre</td><td>Not P.X.Y</td></tr>
- <tr><td>sabre</td><td>actually that's a bad analogy</td></tr>
- <tr><td>sabre</td><td>because C dereferences in this case</td></tr>
- <tr><td>sabre</td><td>Try: (&(P->X))->Y</td></tr>
- <tr><td>Yorion</td><td>so, a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0, can you construct the definition of c in terms of x?</td></tr>
- <tr><td>sabre</td><td>yes, but you're going out of bounds :)</td></tr>
- <tr><td>sabre</td><td>consider this:</td></tr>
- <tr><td>sabre</td><td>{ float, { double , { int } } } *P</td></tr>
- <tr><td>sabre</td><td>int *X = gep P, 0, 1, 1, 0</td></tr>
- <tr><td>sabre</td><td>do you understand the leading zero?</td></tr>
- <tr><td>sabre</td><td>alternatively:</td></tr>
- <tr><td>sabre</td><td>t = gep P, 0, 1</td></tr>
- <tr><td>sabre</td><td>t2 = gep t, 0, 1</td></tr>
- <tr><td>sabre</td><td>X = gep t, 0, 0</td></tr>
- <tr><td>Yorion</td><td>what's t2 for?</td></tr>
- <tr><td>sabre</td><td>oh</td></tr>
- <tr><td>sabre</td><td>sorry :)</td></tr>
- <tr><td>sabre</td><td>X = gep t2, 0, 0</td></tr>
- <tr><td>Yorion</td><td>give me a minute please</td></tr>
- <tr><td>sabre</td><td>ok</td></tr>
- <tr><td>Yorion</td><td>sabre: shouldn't the type be: { float, { double, { int }* } }* P ?</td></tr>
- <tr><td>sabre</td><td>nope</td></tr>
- <tr><td>sabre</td><td>why the extra * ?</td></tr>
- <tr><td>sabre</td><td>if it helps, the type of t is { double, {int}}* and t2 is {int}* and X is int*</td></tr>
- <tr><td>Yorion</td><td>wait... 0 represents dereference, natural number i
- represents &A[i] ?</td></tr>
- <tr><td>sabre</td><td>gep does no dereferences, ever</td></tr>
- <tr><td>sabre</td><td>gep P, 0, 1 is equivalent to &P[0].X</td></tr>
- <tr><td>sabre</td><td>aka &P->X</td></tr>
- <tr><td>sabre</td><td>gep P, 1 == &P[1] aka P+1</td></tr>
- <tr><td>sabre</td><td>so gep P, 0, 1 can't alias gep P, 1 just like
- &P->Y can't alias P+1</td></tr>
- <tr><td>sabre</td><td>assuming P is a pointer to struct {X, Y }</td></tr>
- <tr><td>Yorion</td><td>sabre: is it possible to come up with a general rule for "arithmetic of GEP indices"? </td></tr>
- <tr><td>sabre</td><td>Yorion: of course, it's very simple</td></tr>
- <tr><td>sabre</td><td>just not what you're expecting :)</td></tr>
- <tr><td>sabre</td><td>See langref.html</td></tr>
- <tr><td>Yorion</td><td>for example, a=GEP x,0,0,1 b=GEP a,0,0,1, c=GEP b,0,0,1, that should be equal to GEP x,0,1,1,0, right?</td></tr>
- <tr><td>Yorion</td><td>I did</td></tr>
- <tr><td>Yorion</td><td>oops, equal to GEP x,0,1,1,1,0</td></tr>
- <tr><td>sabre</td><td>that would be:</td></tr>
- <tr><td>sabre</td><td>GEP X, 0, 0, 1, 0, 1, 0, 1</td></tr>
- <tr><td>Yorion</td><td>you're killing me</td></tr>
- <tr><td>sabre</td><td>The basic rule when turning: A = GEP B, C D = GEP A, 0, E</td></tr>
- <tr><td>sabre</td><td>is that you drop the 0, turning it into</td></tr>
- <tr><td>sabre</td><td>GEP B, C, E</td></tr>
- <tr><td>Yorion</td><td>okay, that's good. any other rules?</td></tr>
- <tr><td>nicholas</td><td>what if it isn't a 0?</td></tr>
- <tr><td>sabre</td><td>more generally: A = GEP Ptr, B, C, ... D = GEP A, 0, E, F, ... </td></tr>
- <tr><td>sabre</td><td>D = GEP Ptr, B, C, ... E, F, ...</td></tr>
- <tr><td>sabre</td><td>if it's not zero, you generally cannot concatenate them</td></tr>
- <tr><td>sabre</td><td>unless the first gep has one subscript</td></tr>
- <tr><td>sabre</td><td>in which case you drop the zero</td></tr>
- <tr><td>sabre</td><td>if you look in InstCombiner::visitGetElementPtrInst, it should have this logic</td></tr>
- <tr><td>Yorion</td><td>what if there is no zero? how can I compute the offset from the base pointer in that case?</td></tr>
- <tr><td>Yorion</td><td>like A=GEP B,C and D=GEP A,E,F</td></tr>
- <tr><td>sabre</td><td>you don't have to combine them to compute an offset</td></tr>
- <tr><td>sabre</td><td>are you *just* trying to get a byte offset from the pointer?</td></tr>
- <tr><td>Yorion</td><td>I'm trying to get offset of D from B</td></tr>
- <tr><td>sabre</td><td>why didn't you say so? :)</td></tr>
- <tr><td>sabre</td><td>with all constant subscripts, it's trivial</td></tr>
- <tr><td>sabre</td><td>look at SelectionDAGLowering::visitGetElementPtr</td></tr>
- <tr><td>sabre</td><td>in CodeGen/SelectionDAG/SelectionDAGISel.cpp</td></tr>
- <tr><td>sabre</td><td>basically the rule is that you multiply the index by the size of the thing indexed</td></tr>
- <tr><td>sabre</td><td>there is also a Support/GetElementPtrIterator or something</td></tr>
- <tr><td>sabre</td><td>that makes it trivial to see what type is indexed by which subscript</td></tr>
- <tr><td>sabre</td><td>for each subscript it gives you a type</td></tr>
- <tr><td>sabre</td><td>For an array subscript you multiply the index by the indexed type</td></tr>
- <tr><td>sabre</td><td>for a struct subscript, you add the field offset</td></tr>
- <tr><td>sabre</td><td>s/array/sequentialtype/ if you're in a pedantic mood</td></tr>
- <tr><td>Yorion</td><td>let's focus on structs: in that case, the above given example would be: D = GEP B,C,E,F?</td></tr>
- <tr><td>sabre</td><td>no</td></tr>
- <tr><td>sabre</td><td>you drop the E if it's zero</td></tr>
- <tr><td>sabre</td><td>if it's not you can't concat</td></tr>
- <tr><td>sabre</td><td>are you trying to trick me into saying "yes, just append the indices"? :)</td></tr>
- <tr><td>Yorion</td><td>okay, let's assume E is not zero, how do I compute offset from B for D for a struct?</td></tr>
- <tr><td>sabre</td><td>Why are you framing this in terms of concatenation?</td></tr>
- <tr><td>Yorion</td><td>no, I'm trying to understand it</td></tr>
- <tr><td>sabre</td><td>computing an offset and concatenating are entirely different</td></tr>
- <tr><td>sabre</td><td>Lets consider a specific example</td></tr>
- <tr><td>Yorion</td><td>because I want to express certain properties in the terms of base pointers either globals or parameters</td></tr>
- <tr><td>Yorion</td><td>I want to eliminate locals from my analysis</td></tr>
- <tr><td>sabre</td><td>you realize that parmeters can point into the middle of structs?</td></tr>
- <tr><td>Yorion</td><td>yes</td></tr>
- <tr><td>sabre</td><td>you realize invalid access paths can be constructed with geps/</td></tr>
- <tr><td>sabre</td><td>?</td></tr>
- <tr><td>Yorion</td><td>what do you mean by invalid access paths? </td></tr>
- <tr><td>Yorion</td><td>like offseting out of the struct which is passed to the function?</td></tr>
- <tr><td>sabre</td><td>The case where the subscript isn't zero is invalid code</td></tr>
- <tr><td>sabre</td><td>from a type-safety perspective</td></tr>
- <tr><td>DannyB</td><td>he means untypesafe things that seem valid :)</td></tr>
- <tr><td>DannyB</td><td>IE they point somewhere in the struct, but not to any particular field</td></tr>
- <tr><td>DannyB</td><td>(or whatever)</td></tr>
- <tr><td>sabre</td><td>right</td></tr>
- <tr><td>Yorion</td><td>okay</td></tr>
- <tr><td>sabre</td><td>or they might point in some other struct :)</td></tr>
- <tr><td>sabre</td><td>It's the equivalent to saying:</td></tr>
- <tr><td>sabre</td><td>struct Foo { int A, int B; }</td></tr>
- <tr><td>sabre</td><td>Foo* P = </td></tr>
- <tr><td>sabre</td><td>T = &P->B;</td></tr>
- <tr><td>sabre</td><td>S = T+1</td></tr>
- <tr><td>sabre</td><td>that is:</td></tr>
- <tr><td>sabre</td><td>T = gep 0, 1</td></tr>
- <tr><td>sabre</td><td>S = gep T, 1</td></tr>
- <tr><td>sabre</td><td>you can't concat those and get a type-safe access path</td></tr>
- <tr><td>sabre</td><td>remember T = &P->B === T = &P[0].B</td></tr>
- <tr><td>sabre</td><td>understand?</td></tr>
- <tr><td>Yorion</td><td>let me think for a minute</td></tr>
- <tr><td>sabre</td><td>Consider what the C case does, it will be most clear if you're used to C</td></tr>
- <tr><td>sabre</td><td>:)</td></tr>
- <tr><td>sabre</td><td>Consider the byte offset and why it doesn't point into P-> anything</td></tr>
- <tr><td>sabre</td><td>it points into P[1] not P[0]</td></tr>
- <tr><td>Yorion</td><td>it's perfectly fine if GEP offsets out of the type. I'd still need to express GEP in the terms of base pointers. Take the example above: T=GEP P,0,1; S=GEP T,1</td></tr>
- <tr><td>Yorion</td><td>type safety is not crucial in my case</td></tr>
- <tr><td>sabre</td><td>That specific example is GEP P, 1, 0</td></tr>
- <tr><td>sabre</td><td>however, you can form geps that are NOT equivalent to anything else</td></tr>
- <tr><td>sabre</td><td>for example, consider:</td></tr>
- <tr><td>sabre</td><td>struct X { int, char}</td></tr>
- <tr><td>Yorion</td><td>that is fine. they're equivalent to something in the calling context</td></tr>
- <tr><td>sabre</td><td>the same sequence points into padding</td></tr>
- <tr><td>sabre</td><td>and there is no gep that can do that</td></tr>
- <tr><td>Yorion</td><td>the bottom line is: if the program is valid, interprocedural analysis will match that offset with something in one of the functions on the call stack</td></tr>
- <tr><td>Yorion</td><td>and that's all I care about</td></tr>
- <tr><td>Yorion</td><td>can you give me a formula for structs for computing
- offsets that takes into account the case GEP T,&lt:non_zeros> and the size of the structs/fields?</td></tr>
- <tr><td>sabre</td><td>yes, I did above</td></tr>
- <tr><td>sabre</td><td>Two things:</td></tr>
- <tr><td>sabre</td><td>GEP Ptr, A, X, Y, Z</td></tr>
- <tr><td>sabre</td><td>The result is Ptr + A * sizeof(struct) + fieldoffs(X) + fieldoffs(Y) + fieldoffs(Z)</td></tr>
- <tr><td>sabre</td><td>simple enough?</td></tr>
- <tr><td>sabre</td><td>you see why "A" is special?</td></tr>
- <tr><td>Yorion</td><td>give me a min, I'm constructing an example</td></tr>
- <tr><td>Reid</td><td>sabre: I think I finally understand</td></tr>
- <tr><td>Reid</td><td>your comment that GEP *never* dereferences makes a lot of sense</td></tr>
- <tr><td>Reid</td><td>it is only doing address calculation, so the first one is taking the address of the var</td></tr>
- <tr><td>sabre</td><td>If C didn't conflate lvalues and rvalues, GEP would be much easier to understand for people</td></tr>
- <tr><td>Reid</td><td>yeah</td></tr>
- <tr><td>Yorion</td><td>so, for example: M=GEP A,B,C; N=GEP M,D,E; N = [ A + B*sizeof(struct) + fieldoffs(C) ]:(of type T) + D*sizeof(T) + fieldoffs(E)</td></tr>
- <tr><td>Reid</td><td>I just remember learning a hard lesson about the difference between char* A and char A[] .. long time ago when I was learning C</td></tr>
- <tr><td>sabre</td><td>of type T*</td></tr>
- <tr><td>sabre</td><td>otherwise fine</td></tr>
- <tr><td>Yorion</td><td>okay, I think I finally understand it</td></tr>
- <tr><td>sabre</td><td>without the T* your D sizeof will be wrong</td></tr>
- <tr><td>Yorion</td><td>a suggestion: the formula you gave above explains it all</td></tr>
- <tr><td>Yorion</td><td>I'd suggest explaining it that way in documentation</td></tr>
- <tr><td>sabre</td><td>That's not right though</td></tr>
- <tr><td>sabre</td><td>it doesn't include arrays or packed types</td></tr>
- <tr><td>sabre</td><td>so it is, at best, a half truth</td></tr>
- <tr><td>Yorion</td><td>tell me, how to compute the fieldoffs for an index?</td></tr>
- <tr><td>sabre</td><td>arrays can be in structs :)</td></tr>
- <tr><td>Yorion</td><td>in bytes</td></tr>
- <tr><td>sabre</td><td>idx * sizeof(arrayelt)</td></tr>
- <tr><td>sabre</td><td>just like for pointers (the first index)</td></tr>
- <tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
- <tr><td>sabre</td><td>for sequentials you use idx*sizeof(sequenced type)</td></tr>
- <tr><td>sabre</td><td>for structs you add their offset</td></tr>
- <tr><td>sabre</td><td>it's really very simple :)</td></tr>
- <tr><td>sabre</td><td>the first index of a gep is always over the pointer</td></tr>
- <tr><td>Yorion</td><td>no I meant in LLVM, how do I convert the field offset into bytes?</td></tr>
- <tr><td>sabre</td><td>which is why it's strange</td></tr>
- <tr><td>sabre</td><td>if you only think about structs</td></tr>
- <tr><td>sabre</td><td>TargetData::getFieldOffset </td></tr>
- <tr><td>sabre</td><td>or something</td></tr>
- <tr><td>sabre</td><td>look in SelectionDAGISel.cpp (visitGEP) as I suggested.</td></tr>
- <tr><td>Yorion</td><td>do you still have the energy to go over sequential types? :-)</td></tr>
- <tr><td>Yorion</td><td>what is the offset formula for sequential types?</td></tr>
- <tr><td>Reid</td><td>we just went over that: idx * sizeof(elementType)</td></tr>
- <tr><td>Yorion</td><td>so, if there's an array hidden somewhere in the struct, essentially I need to compute idx*sizeof() instead of fieldoffs() and that's it?</td></tr>
- <tr><td>sabre</td><td>yes</td></tr>
- <tr><td>Reid</td><td>yes</td></tr>
- <tr><td>Yorion</td><td>excellent.</td></tr>
- <tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
- <tr><td>sabre</td><td>[9:17pm] sabre: for sequentials you use idx*sizeof(sequenced type)</td></tr>
- <tr><td>sabre</td><td>[9:17pm] sabre: for structs you add their offset</td></tr>
- <tr><td>sabre</td><td>[9:17pm] sabre: it's really very simple :)</td></tr>
- <tr><td>Yorion</td><td>now when I understand it, it is simple... </td></tr>
- <tr><td>Yorion</td><td>thx</td></tr>
- </table>
-
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