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<title>The Often Misunderstood GEP Instruction</title>
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TABLE { text-align: left; border: 1px solid black; border-collapse: collapse; margin: 0 0 0 0; }
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<div class="doc_title">
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The Often Misunderstood GEP Instruction
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</div>
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<ol>
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<li><a href="#intro">Introduction</a></li>
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<li><a href="#questions">The Questions</a>
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<ol>
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<li><a href="#extra_index">Why is the extra 0 index required?</a></li>
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<li><a href="#deref">What is dereferenced by GEP?</a></li>
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<li><a href="#firstptr">Why can you index through the first pointer but not
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subsequent ones?</a></li>
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<li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
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<li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
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</ol></li>
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<li><a href="#summary">Summary</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
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<div class="doc_text">
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<p>This document seeks to dispel the mystery and confusion surrounding LLVM's
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GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are
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probably the most frequently occuring questions once a developer gets down to
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coding with LLVM. Here we lay out the sources of confusion and show that the
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GEP instruction is really quite simple.
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
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<div class="doc_text">
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<p>When people are first confronted with the GEP instruction, they tend to
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relate it to known concepts from other programming paradigms, most notably C
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array indexing and field selection. However, GEP is a little different and
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this leads to the following questions, all of which are answered in the
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following sections.</p>
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<ol>
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<li><a href="#firstptr">What is the first index of the GEP instruction?</a>
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</li>
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<li><a href="#extra_index">Why is the extra 0 index required?</a></li>
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<li><a href="#deref">What is dereferenced by GEP?</a></li>
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<li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li>
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<li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_subsection">
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<a name="firstptr"><b>What is the first index of the GEP instruction?</b></a>
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</div>
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<div class="doc_text">
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<p>Quick answer: The index stepping through the first operand.</p>
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<p>The confusion with the first index usually arises from thinking about
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the GetElementPtr instruction as if it was a C index operator. They aren't the
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same. For example, when we write, in "C":</p>
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<pre>
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AType* Foo;
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...
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X = &Foo->F;</pre>
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<p>it is natural to think that there is only one index, the selection of the
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field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
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pointer must be indexed explicitly in LLVM. C, on the other hand, indexs
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through it transparently. To arrive at the same address location as the C
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code, you would provide the GEP instruction with two index operands. The
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first operand indexes through the pointer; the second operand indexes the
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field <tt>F</tt> of the structure, just as if you wrote:</p>
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<pre>
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X = &Foo[0].F;</pre>
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<p>Sometimes this question gets rephrased as:</p>
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<blockquote><p><i>Why is it okay to index through the first pointer, but
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subsequent pointers won't be dereferenced?</i></p></blockquote>
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<p>The answer is simply because memory does not have to be accessed to
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perform the computation. The first operand to the GEP instruction must be a
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value of a pointer type. The value of the pointer is provided directly to
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the GEP instruction as an operand without any need for accessing memory. It
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must, therefore be indexed and requires an index operand. Consider this
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example:</p>
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<pre>
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struct munger_struct {
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int f1;
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int f2;
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};
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void munge(struct munger_struct *P)
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{
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P[0].f1 = P[1].f1 + P[2].f2;
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}
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...
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munger_struct Array[3];
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...
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munge(Array);</pre>
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<p>In this "C" example, the front end compiler (llvm-gcc) will generate three
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GEP instructions for the three indices through "P" in the assignment
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statement. The function argument <tt>P</tt> will be the first operand of each
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of these GEP instructions. The second operand indexes through that pointer.
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The third operand will be the field offset into the
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<tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
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<tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
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like:</p>
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<pre>
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void %munge(%struct.munger_struct* %P) {
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entry:
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%tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
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%tmp = load i32* %tmp
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%tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
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%tmp7 = load i32* %tmp6
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%tmp8 = add i32 %tmp7, %tmp
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%tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
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store i32 %tmp8, i32* %tmp9
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ret void
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}</pre>
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<p>In each case the first operand is the pointer through which the GEP
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instruction starts. The same is true whether the first operand is an
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argument, allocated memory, or a global variable. </p>
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<p>To make this clear, let's consider a more obtuse example:</p>
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<pre>
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%MyVar = unintialized global i32
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...
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%idx1 = getelementptr i32* %MyVar, i64 0
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%idx2 = getelementptr i32* %MyVar, i64 1
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%idx3 = getelementptr i32* %MyVar, i64 2</pre>
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<p>These GEP instructions are simply making address computations from the
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base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
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</p>
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<ul>
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<li> idx1 = (char*) &MyVar + 0</li>
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<li> idx2 = (char*) &MyVar + 4</li>
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<li> idx3 = (char*) &MyVar + 8</li>
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</ul>
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<p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
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0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
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memory is accessed to make these computations because the address of
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<tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
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<p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
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<tt>%idx3</tt>. They result in the computation of addresses that point to
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memory past the end of the <tt>%MyVar</tt> global, which is only one
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<tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
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it is inadvisable because any load or store with the pointer that results
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from these GEP instructions would produce undefined results.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_subsection">
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<a name="extra_index"><b>Why is the extra 0 index required?</b></a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Quick answer: there are no superfluous indices.</p>
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<p>This question arises most often when the GEP instruction is applied to a
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global variable which is always a pointer type. For example, consider
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this:</p><pre>
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%MyStruct = uninitialized global { float*, i32 }
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...
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%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1</pre>
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<p>The GEP above yields an <tt>i32*</tt> by indexing the <tt>i32</tt> typed
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field of the structure <tt>%MyStruct</tt>. When people first look at it, they
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wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection
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of how globals and GEPs work reveals the need. Becoming aware of the following
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facts will dispell the confusion:</p>
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<ol>
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<li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt>
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but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a
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pointer to a structure containing a pointer to a <tt>float</tt> and an
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<tt>i32</tt>.</li>
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<li>Point #1 is evidenced by noticing the type of the first operand of
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the GEP instruction (<tt>%MyStruct</tt>) which is
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<tt>{ float*, i32 }*</tt>.</li>
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<li>The first index, <tt>i64 0</tt> is required to step over the global
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variable <tt>%MyStruct</tt>. Since the first argument to the GEP
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instruction must always be a value of pointer type, the first index
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steps through that pointer. A value of 0 means 0 elements offset from that
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pointer.</li>
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<li>The second index, <tt>i32 1</tt> selects the second field of the
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structure (the <tt>i32</tt>). </li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_subsection">
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<a name="deref"><b>What is dereferenced by GEP?</b></a>
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</div>
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<div class="doc_text">
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<p>Quick answer: nothing.</p>
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<p>The GetElementPtr instruction dereferences nothing. That is, it doesn't
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access memory in any way. That's what the Load and Store instructions are for.
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GEP is only involved in the computation of addresses. For example, consider
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this:</p>
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<pre>
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%MyVar = uninitialized global { [40 x i32 ]* }
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...
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%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17</pre>
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<p>In this example, we have a global variable, <tt>%MyVar</tt> that is a
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pointer to a structure containing a pointer to an array of 40 ints. The
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GEP instruction seems to be accessing the 18th integer of the structure's
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array of ints. However, this is actually an illegal GEP instruction. It
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won't compile. The reason is that the pointer in the structure <i>must</i>
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be dereferenced in order to index into the array of 40 ints. Since the
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GEP instruction never accesses memory, it is illegal.</p>
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<p>In order to access the 18th integer in the array, you would need to do the
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following:</p>
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<pre>
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%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
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%arr = load [40 x i32]** %idx
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%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17</pre>
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<p>In this case, we have to load the pointer in the structure with a load
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instruction before we can index into the array. If the example was changed
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to:</p>
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<pre>
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%MyVar = uninitialized global { [40 x i32 ] }
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...
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%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17</pre>
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<p>then everything works fine. In this case, the structure does not contain a
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pointer and the GEP instruction can index through the global variable,
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into the first field of the structure and access the 18th <tt>i32</tt> in the
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array there.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_subsection">
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<a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
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</div>
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<div class="doc_text">
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<p>Quick Answer: They compute different address locations.</p>
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<p>If you look at the first indices in these GEP
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instructions you find that they are different (0 and 1), therefore the address
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computation diverges with that index. Consider this example:</p>
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<pre>
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%MyVar = global { [10 x i32 ] }
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%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
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%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1</pre>
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<p>In this example, <tt>idx1</tt> computes the address of the second integer
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in the array that is in the structure in %MyVar, that is <tt>MyVar+4</tt>. The
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type of <tt>idx1</tt> is <tt>i32*</tt>. However, <tt>idx2</tt> computes the
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address of <i>the next</i> structure after <tt>%MyVar</tt>. The type of
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<tt>idx2</tt> is <tt>{ [10 x i32] }*</tt> and its value is equivalent
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to <tt>MyVar + 40</tt> because it indexes past the ten 4-byte integers
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in <tt>MyVar</tt>. Obviously, in such a situation, the pointers don't
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alias.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_subsection">
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<a name="trail0"><b>Why do GEP x,1,0,0 and GEP x,1 alias?</b></a>
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</div>
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<div class="doc_text">
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<p>Quick Answer: They compute the same address location.</p>
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<p>These two GEP instructions will compute the same address because indexing
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through the 0th element does not change the address. However, it does change
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the type. Consider this example:</p>
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<pre>
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%MyVar = global { [10 x i32 ] }
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%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
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%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1</pre>
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<p>In this example, the value of <tt>%idx1</tt> is <tt>%MyVar+40</tt> and
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its type is <tt>i32*</tt>. The value of <tt>%idx2</tt> is also
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<tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>In summary, here's some things to always remember about the GetElementPtr
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instruction:</p>
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<ol>
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<li>The GEP instruction never accesses memory, it only provides pointer
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computations.</li>
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<li>The first operand to the GEP instruction is always a pointer and it must
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be indexed.</li>
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<li>There are no superfluous indices for the GEP instruction.</li>
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<li>Trailing zero indices are superfluous for pointer aliasing, but not for
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the types of the pointers.</li>
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<li>Leading zero indices are not superfluous for pointer aliasing nor the
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types of the pointers.</li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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