<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><p><i>Why is it okay to index through the first pointer, but
subsequent pointers won't be dereferenced?</i></p></blockquote>
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, 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 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 i32
- ...
- %idx1 = getelementptr i32* %MyVar, i64 0
- %idx2 = getelementptr i32* %MyVar, i64 1
- %idx3 = getelementptr i32* %MyVar, i64 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>
+
+<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
<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*, i32 }
- ...
- %idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1</pre>
+ 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>i64 0</tt> index is needed. However, a closer inspection
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 i32 ]* }
- ...
- %idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 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 i32]* }* %, i64 0, i32 0
- %arr = load [40 x i32]** %idx
- %idx = getelementptr [40 x i32]* %arr, i64 0, i64 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 i32 ] }
- ...
- %idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 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>i32</tt> in the
<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 i32 ] }
- %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
- %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 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>i32*</tt>. However, <tt>idx2</tt> computes the
<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 i32 ] }
- %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
- %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 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>i32*</tt>. The value of <tt>%idx2</tt> is also
<tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
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