It's not necessary to do rounding for alloca operations when the requested
alignment is equal to the stack alignment.
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+ <title>The Often Misunderstood GEP Instruction</title>
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+
+<div class="doc_title">
+ The Often Misunderstood GEP Instruction
+</div>
+
+<ol>
+ <li><a href="#intro">Introduction</a></li>
+ <li><a href="#questions">The Questions</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>
+ <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></li>
+ <li><a href="#summary">Summary</a></li>
+</ol>
+
+<div class="doc_author">
+ <p>Written by: <a href="mailto:rspencer@reidspencer.com">Reid Spencer</a>.</p>
+</div>
+
+
+<!-- *********************************************************************** -->
+<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
+ 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_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="#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>
+ <pre>
+ AType* Foo;
+ ...
+ X = &Foo->F;</pre>
+ <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>
+ <pre>
+ X = &Foo[0].F;</pre>
+ <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>
+ <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>
+ <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>
+ <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>
+ <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>
+ <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>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>
+</div>
+<!-- *********************************************************************** -->
+<div class="doc_text">
+ <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>
+ <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
+ 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*, 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>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*, 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_subsection">
+ <a name="deref"><b>What is dereferenced by GEP?</b></a>
+</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 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>
+ <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
+ 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 i32]* }* %, i64 0, i32 0
+ %arr = load [40 x i32]** %idx
+ %idx = getelementptr [40 x i32]* %arr, i64 0, i64 17</pre>
+ <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>
+ <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>
+
+<!-- *********************************************************************** -->
+<div class="doc_subsection">
+ <a name="lead0"><b>Why don't GEP x,0,0,1 and GEP x,1 alias?</b></a>
+</div>
+<div class="doc_text">
+ <p>Quick Answer: They compute different address locations.</p>
+ <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>
+ <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
+ 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="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 i32 ] }
+ %idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+ %idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1</pre>
+ <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>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="summary"><b>Summary</b></a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+ <p>In summary, here's some things to always remember about the GetElementPtr
+ instruction:</p>
+ <ol>
+ <li>The GEP instruction never accesses memory, it only provides pointer
+ computations.</li>
+ <li>The first operand to the GEP instruction is always a pointer and it must
+ be indexed.</li>
+ <li>There are no superfluous indices for the GEP instruction.</li>
+ <li>Trailing zero indices are superfluous for pointer aliasing, but not for
+ the types of the pointers.</li>
+ <li>Leading zero indices are not superfluous for pointer aliasing nor the
+ types of the pointers.</li>
+ </ol>
+</div>
+
+<!-- *********************************************************************** -->
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