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<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="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>
+ <li><a href="#firstptr">What is the first index of the GEP instruction?</a>
+ </li>
+ <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="#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>
</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, indexs
+ 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 = 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>
+
+<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>
+
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="extra_index"><b>Why is the extra 0 index required?</b></a>
<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
+ 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 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 = 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>
<!-- *********************************************************************** -->
<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 = 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>
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