<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
+ this leads to the following questions; all of which are answered in the
following sections.</p>
<ol>
<li><a href="#firstptr">What is the first index of the GEP instruction?</a>
<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>
- <pre>
- AType* Foo;
- ...
- X = &Foo->F;</pre>
+
+<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, indexs
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>
- <pre>
- X = &Foo[0].F;</pre>
+
+<div class="doc_code">
+<pre>
+X = &Foo[0].F;
+</pre>
+</div>
+
<p>Sometimes this question gets rephrased as:</p>
- <blockquote><i>Why is it okay to index through the first pointer, but
- subsequent pointers won't be dereferenced?</i></blockquote>
+ <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>
- <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 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
<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>
- <pre>
- void %munge(%struct.munger_struct* %P) {
- entry:
- %tmp = getelementptr %struct.munger_struct* %P, int 1, uint 0
- %tmp = load int* %tmp
- %tmp6 = getelementptr %struct.munger_struct* %P, int 2, uint 1
- %tmp7 = load int* %tmp6
- %tmp8 = add int %tmp7, %tmp
- %tmp9 = getelementptr %struct.munger_struct* %P, int 0, uint 0
- store int %tmp8, int* %tmp9
- ret void
- }</pre>
+
+<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>
- <pre>
- %MyVar = unintialized global int
- ...
- %idx1 = getelementptr int* %MyVar, long 0
- %idx2 = getelementptr int* %MyVar, long 1
- %idx3 = getelementptr int* %MyVar, long 2</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = unintialized 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>
- <ul>
- <li> idx1 = (char*) &MyVar + 0</li>
- <li> idx2 = (char*) &MyVar + 4</li>
- <li> idx3 = (char*) &MyVar + 8</li>
- </ul>
- <p>Since the type <tt>int</tt> is known to be four bytes long, the indices
+
+<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>int</tt> long, not three <tt>int</tt>s long. While this is legal in LLVM,
+ <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
+ 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 dispell 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 step over the global
+ <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>ubyte 1</tt> selects the second field of the
- structure (the <tt>int</tt>). </li>
+ <li>The second index, <tt>i32 1</tt> selects the second field of the
+ structure (the <tt>i32</tt>). </li>
</ol>
</div>
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>
- <pre>
- %MyVar = uninitialized global { [40 x int ]* }
- ...
- %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17</pre>
+
+<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 the structure's
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>
+
+<div class="doc_code">
+<pre>
+%MyVar = uninitialized global { [40 x i32 ] }
+...
+%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
+</pre>
+</div>
+
<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>int</tt> in the
+ 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, ubyte 0, long 1
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
+%idx2 = getlementptr { [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
+ 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 int] }*</tt> and its value is equivalent
+ <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>
<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, ubyte 0, long 0
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1</pre>
+
+<div class="doc_code">
+<pre>
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+%idx2 = getlementptr { [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>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
- src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
<a href="http://validator.w3.org/check/referer"><img
- src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
+ src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br/>
Last modified: $Date$
</address>
projects / oota-llvm.git / blobdiff