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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>The Often Misunderstood GEP Instruction</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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TABLE { text-align: left; border: 1px solid black; border-collapse: collapse; margin: 0 0 0 0; }
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<body>
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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><i>Why is it okay to index through the first pointer, but
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subsequent pointers won't be dereferenced?</i></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, int 1, uint 0
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%tmp = load int* %tmp
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%tmp6 = getelementptr %struct.munger_struct* %P, int 2, uint 1
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%tmp7 = load int* %tmp6
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%tmp8 = add int %tmp7, %tmp
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%tmp9 = getelementptr %struct.munger_struct* %P, int 0, uint 0
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store int %tmp8, int* %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 int
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...
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%idx1 = getelementptr int* %MyVar, long 0
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%idx2 = getelementptr int* %MyVar, long 1
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%idx3 = getelementptr int* %MyVar, long 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>int</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>int</tt> long, not three <tt>int</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*, int }
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...
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%idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1</pre>
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<p>The GEP above yields an <tt>int*</tt> by indexing the <tt>int</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>long 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*, int }</tt>
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but rather <tt>{ float*, int }*</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>int</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*, int }*</tt>.</li>
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<li>The first index, <tt>long 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>ubyte 1</tt> selects the second field of the
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structure (the <tt>int</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 int ]* }
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...
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%idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 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 int]* }* %, long 0, ubyte 0
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%arr = load [40 x int]** %idx
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%idx = getelementptr [40 x int]* %arr, long 0, long 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 int ] }
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...
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%idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 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>int</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 int ] }
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%idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, ubyte 0, long 1
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%idx2 = getlementptr { [10 x int ] }* %MyVar, long 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>int*</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 int] }*</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 int ] }
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%idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, ubyte 0, long 0
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%idx2 = getlementptr { [10 x int ] }* %MyVar, long 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>int*</tt>. The value of <tt>%idx2</tt> is also
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<tt>MyVar+40</tt> but its type is <tt>{ [10 x int] }*</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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<div class="doc_section"><a name="discussion"><b>Appendix: Discussion</b></a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The following is a real discussion from the
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<a href="irc://irc.oftc.net/#llvm">#llvm IRC channel</a> about the GEP
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instruction. You may find this instructive as it was the basis for this
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document.</p>
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<table>
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<tr><th>User</th><th>Comment</th></tr>
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<tr><td>Yorion</td><td>If x & y must alias, are [ getelementptr x,0,0,1,2 ] and [ getelementptr x,1,2 ] aliased? (they obviously have different types, but they should alias...)</td></tr>
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<tr><td>Yorion</td><td>oops, for the second one I meant [ getelementptr y,1,2 ]</td></tr>
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<tr><td>Reid</td><td>I don't see how that could be, Yorion but I'm not the authority on this</td></tr>
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<tr><td>Yorion</td><td>hmm.. </td></tr>
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<tr><td>Reid</td><td>the two geps, by definition, are going to produce different pointers which are not aliased</td></tr>
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<tr><td>Yorion</td><td>would [ GEP x,1,0 ] and [ GEP y,1 ] be aliased?</td></tr>
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<tr><td>Reid</td><td>if the second gep was [gep y,0,0,1,2] then they should be aliased as well</td></tr>
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<tr><td>Reid</td><td>no, I wouldn't expect that to work either :)</td></tr>
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<tr><td>Reid</td><td>you can't just arbitrarily drop leading or trailing indices :)</td></tr>
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<tr><td>Reid</td><td>(.. leading or trailing 0 indices, I mean)</td></tr>
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<tr><td>Reid</td><td>this instruction walks through a data structure and generates a pointer to the resulting thing</td></tr>
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<tr><td>Reid</td><td>if the number of indices are different, you're ending up at a different place and by definition they'll have different addresses</td></tr>
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<tr><td>Yorion</td><td>oh, I see, because of different types, [ GEP x,0,1 ]
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& [ GEP x,1 ] actually might refer to different fields, but might also refer to the same ones... </td></tr>
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<tr><td>Reid</td><td>or, at least, that's my crude understanding of it :)</td></tr>
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<tr><td>Reid</td><td>no, they'll definitely refer to different fields</td></tr>
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<tr><td>nicholas</td><td>GEP x,0,1 ==> &((*(x+0))+1)? vs. GEP x,1 ==> &(*(x+1))?</td></tr>
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<tr><td>Reid</td><td>lemme grok that for a sec</td></tr>
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<tr><td>Reid</td><td>that might be true in some limited definition of x, but it wouldn't be generally</td></tr>
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<tr><td>nicholas</td><td>oh. fields of different sizes in a structure.</td></tr>
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<tr><td>Reid</td><td>yup</td></tr>
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<tr><td>Yorion</td><td>is perhaps the type unification the reason why [ GEP x,0,1 ] and [ GEP x,1 ] cannot alias?</td></tr>
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<tr><td>Reid</td><td>no</td></tr>
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<tr><td>Reid</td><td>they may or may not have the same type, but they are definitely different pointers</td></tr>
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<tr><td>Reid</td><td>lets use a concrete example for "x"</td></tr>
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<tr><td>Reid</td><td>suppose x is "struct {int a, float b} *"</td></tr>
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<tr><td>Reid</td><td>GEP X,0,1 is going to return the address of b</td></tr>
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<tr><td>Reid</td><td>GEP X,1 is going to return the address of the *second* "a" (after the first b)</td></tr>
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<tr><td>Yorion</td><td>ah, I see... </td></tr>
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<tr><td>Yorion</td><td>trailing zeros are still a bit confusing... </td></tr>
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<tr><td>Reid</td><td>same thing .. you're just selecting the 0th member of an array or structure</td></tr>
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<tr><td>Yorion</td><td>you don't move away from the pointer, only the type is changed</td></tr>
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<tr><td>Reid</td><td>no, you still move away from the pointer .. the type might change, or not</td></tr>
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<tr><td>Reid</td><td>the pointer definitely changes</td></tr>
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<tr><td>Reid</td><td>lets look at an example for trailing zero</td></tr>
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<tr><td>Reid</td><td>suppose x is "int x[10][10][10][10]" (in C)</td></tr>
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<tr><td>Reid</td><td>GEP X,0,0 will yield you a 3 dimensional array</td></tr>
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<tr><td>Reid</td><td>GEP X,0,0,0,0,0 will yield you an "int"</td></tr>
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<tr><td>Reid</td><td>make sense?</td></tr>
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<tr><td>Yorion</td><td>yes</td></tr>
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<tr><td>Reid</td><td>so, I think there's a law here: if the number of indices in two GEP instructions are not equivalent, there is no way the resulting pointers can alias</td></tr>
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<tr><td>Reid</td><td>(assuming the x and y alias)</td></tr>
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<tr><td>Yorion</td><td>I was confused with some code in BasicAliasAnalysis that says that two pointers are equal if they differ only in trailing zeros</td></tr>
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<tr><td>Yorion</td><td>BasicAliasAnalysis.cpp:504-518</td></tr>
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<tr><td>Reid</td><td>lemme look</td></tr>
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<tr><td>nicholas</td><td>if y1 = GEP X, 0, 0 and y2 = GEP X, 0, 0, 0, 0, 0 (from Reid's example)</td></tr>
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<tr><td>nicholas</td><td>then doesn't *y1 and *y2 both refer to the same "int"?</td></tr>
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<tr><td>Reid</td><td>they shouldn't</td></tr>
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<tr><td>Reid</td><td>hmm .. actually, maybe you're right :)</td></tr>
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<tr><td>Reid</td><td>they definitely have different *types*</td></tr>
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<tr><td>Yorion</td><td>true</td></tr>
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<tr><td>nicholas</td><td>different types just doesn't cut it. :)</td></tr>
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<tr><td>Reid</td><td>.. thinking on this :)</td></tr>
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<tr><td>nicholas</td><td>similarly, i could create a yucky with a struct that has a char *, then have you GEP right through the pointer into the pointed-to data. That could mean that the resulting point might alias anything.</td></tr>
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<tr><td>Yorion</td><td>my theory (after reading BAA) is that all zeros can be omitted, and that the pointers alias if they have the same sequence of indices</td></tr>
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<tr><td>Yorion</td><td>however, this screws the typing, so that's why zeros are for</td></tr>
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<tr><td>Yorion</td><td>nicholas, does that match your hunch?</td></tr>
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<tr><td>nicholas</td><td>I have to admit, I've had much grief with GEPIs already. I wish the semantics were plainly documented as part of their own language, instead of just relying on C aliasing rules and C semantics...</td></tr>
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<tr><td>nicholas</td><td>Yorion: leading zeroes can't be omitted.</td></tr>
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<tr><td>Reid</td><td>okay, if you have two GEPs and their leading indices are an exact match, if the one with more indices only has trailing 0s then they should alias</td></tr>
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<tr><td>nicholas</td><td>must alias, i think.</td></tr>
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<tr><td>Reid</td><td>yes, must alias, sorry</td></tr>
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<tr><td>Yorion</td><td>okay</td></tr>
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<tr><td>Yorion</td><td>I'm glad we cleared this up</td></tr>
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<tr><td>Reid</td><td>so, if y1 = GEP X, 1,2,0 and if y2 = GEP X, 1,2,0,0,0 then y1 "must alias" y2 :)</td></tr>
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<tr><td>Reid</td><td>but that doesn't work for leading 0s :)</td></tr>
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<tr><td>Yorion</td><td>yes, true... I was wrong </td></tr>
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<tr><td>Reid</td><td>I too have been having fun with GEP recently :)</td></tr>
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<tr><td>Yorion</td><td>but, there're cases like [a = GEP x,1,0; b = GEP a,1,0; c = GEP b,1,0], and that should be equivalent to GEP x,1,0,1,0,1</td></tr>
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<tr><td>Reid</td><td>not quite</td></tr>
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<tr><td>nicholas</td><td>I'm sure another rule can be written for GEPIs, but they would only apply to type-safe code.</td></tr>
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<tr><td>nicholas</td><td>another *tautology</td></tr>
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<tr><td>Yorion</td><td>Reid: why not, only the type should be different...</td></tr>
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<tr><td>Reid</td><td>its should be equivalent to GEP x,1,0,1,0,1,0</td></tr>
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<tr><td>Yorion</td><td>and that must alias GEP x,1,0,1,0,1</td></tr>
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<tr><td>Reid</td><td>hmm, by the previous rule, I guess you're right :)</td></tr>
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<tr><td>Yorion</td><td>I'm a bit scared that even you're a bit confused about GEP</td></tr>
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<tr><td>Reid</td><td>I'm glad I'm not the only one that gets a little confused wrapping my head around this stuff :)</td></tr>
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<tr><td>Reid</td><td>GEP has always confused me .. partly because I think its wrong :)</td></tr>
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<tr><td>Reid</td><td>well, actually, not so much that GEP is wrong, but that gvars being pointers without storage</td></tr>
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<tr><td>Reid</td><td>i.e. when you say "%x = global int" in LLVM, the type of X is int*</td></tr>
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<tr><td>Reid</td><td>yet, there is no storage for that pointer</td></tr>
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<tr><td>Reid</td><td>its magically deduced</td></tr>
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<tr><td>nicholas</td><td>well, it makes no sense to have globals be SSA...</td></tr>
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<tr><td>Reid</td><td>heh</td></tr>
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<tr><td>Reid</td><td>yeah, well .. practicalities :)</td></tr>
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<tr><td>Yorion</td><td>true</td></tr>
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<tr><td>Yorion</td><td>sabre gave me a reference on how globals are handled in SSA</td></tr>
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<tr><td>Reid</td><td>anyway, gotta run</td></tr>
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<tr><td>Yorion</td><td>the paper was crappy, but I do understand now why is it implemented that way in LLVM</td></tr>
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<tr><td>Yorion</td><td>thx for the interesting discussion Reid</td></tr>
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<tr><td>Reid</td><td>heh .. its one that Chris and I keep having .. he just tells me that C has rotted my brain :)</td></tr>
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<tr><td>nicholas</td><td>lol</td></tr>
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<tr><td>Yorion</td><td>hehehe</td></tr>
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<tr><td>Reid</td><td>he might be right :)</td></tr>
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<tr><td>Yorion</td><td>sabre: have you seen the discussion on GEP?</td></tr>
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<tr><td>sabre</td><td>no</td></tr>
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<tr><td>sabre</td><td>I'll read the backlog, j/s</td></tr>
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<tr><td>sabre</td><td>ok, there's a lot</td></tr>
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<tr><td>sabre</td><td>what's the executive summary?</td></tr>
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<tr><td>sabre</td><td>do you have a q?</td></tr>
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<tr><td>Yorion</td><td>is it possible that GEP x,0,0,1 and GEP x,1 alias?</td></tr>
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<tr><td>sabre</td><td>no</td></tr>
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<tr><td>Yorion</td><td>and b) GEP x,1,0,0 and GEP x,1 should alias, right?</td></tr>
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<tr><td>sabre</td><td>I assume you mean for size = 1 ?</td></tr>
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<tr><td>sabre</td><td>b) yes</td></tr>
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<tr><td>Yorion</td><td>although they have different types</td></tr>
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<tr><td>sabre</td><td>right</td></tr>
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<tr><td>Yorion</td><td>okay</td></tr>
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<tr><td>Yorion</td><td>I'm still not 100% convinced that: a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0 cannot alias Z=GEP x,1,1,1</td></tr>
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<tr><td>Yorion</td><td>(that c and z cannot alias)</td></tr>
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<tr><td>sabre</td><td>that's becuse they do alias</td></tr>
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<tr><td>sabre</td><td>mustalias in fact</td></tr>
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<tr><td>Yorion</td><td>but then: GEP x,1,0,1,0,1,0 = GEP x,1,1,1</td></tr>
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<tr><td>sabre</td><td>Yorion: no</td></tr>
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<tr><td>sabre</td><td>c != GEP x,1,0,1,0,1,0</td></tr>
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<tr><td>sabre</td><td>the first index doesn't work like that</td></tr>
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<tr><td>Yorion</td><td>how does then the first index work? c and z must alias, but GEP x,1,0,1,0 != GEP x,1,1 ??</td></tr>
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<tr><td>sabre</td><td>*sigh*</td></tr>
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<tr><td>Reid</td><td>:)</td></tr>
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<tr><td>Reid</td><td>we need an FAQ on this</td></tr>
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<tr><td>sabre</td><td>Yorion: how did you get </td></tr>
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<tr><td>sabre</td><td>"GEP x,1,0,1,0"? </td></tr>
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<tr><td>Yorion</td><td>look</td></tr>
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<tr><td>sabre</td><td>you can't just concatenate subscripts</td></tr>
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<tr><td>Yorion</td><td>why?</td></tr>
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<tr><td>sabre</td><td>because... it doesn't work that way?</td></tr>
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<tr><td>sabre</td><td>consider C</td></tr>
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<tr><td>Yorion</td><td>how does it work?</td></tr>
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<tr><td>sabre</td><td>if I have blah* P</td></tr>
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<tr><td>sabre</td><td>P[0][1][2][3][4]</td></tr>
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<tr><td>sabre</td><td>this is *not* the same as:</td></tr>
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<tr><td>sabre</td><td>t = &P[0][1][2] ... t[3][4]</td></tr>
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<tr><td>sabre</td><td>Yorion: Consider: struct *P </td></tr>
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<tr><td>sabre</td><td>P->X == P[0].X</td></tr>
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<tr><td>sabre</td><td>You're losing the 0.</td></tr>
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<tr><td>sabre</td><td>P->X->Y == P[0].X[0].Y</td></tr>
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<tr><td>sabre</td><td>Not P.X.Y</td></tr>
|
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<tr><td>sabre</td><td>actually that's a bad analogy</td></tr>
|
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<tr><td>sabre</td><td>because C dereferences in this case</td></tr>
|
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<tr><td>sabre</td><td>Try: (&(P->X))->Y</td></tr>
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<tr><td>Yorion</td><td>so, a=GEP x,1,0; b=GEP a,1,0; c=GEP b,1,0, can you construct the definition of c in terms of x?</td></tr>
|
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<tr><td>sabre</td><td>yes, but you're going out of bounds :)</td></tr>
|
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<tr><td>sabre</td><td>consider this:</td></tr>
|
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<tr><td>sabre</td><td>{ float, { double , { int } } } *P</td></tr>
|
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<tr><td>sabre</td><td>int *X = gep P, 0, 1, 1, 0</td></tr>
|
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<tr><td>sabre</td><td>do you understand the leading zero?</td></tr>
|
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<tr><td>sabre</td><td>alternatively:</td></tr>
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<tr><td>sabre</td><td>t = gep P, 0, 1</td></tr>
|
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<tr><td>sabre</td><td>t2 = gep t, 0, 1</td></tr>
|
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<tr><td>sabre</td><td>X = gep t, 0, 0</td></tr>
|
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<tr><td>Yorion</td><td>what's t2 for?</td></tr>
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<tr><td>sabre</td><td>oh</td></tr>
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<tr><td>sabre</td><td>sorry :)</td></tr>
|
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<tr><td>sabre</td><td>X = gep t2, 0, 0</td></tr>
|
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<tr><td>Yorion</td><td>give me a minute please</td></tr>
|
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<tr><td>sabre</td><td>ok</td></tr>
|
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<tr><td>Yorion</td><td>sabre: shouldn't the type be: { float, { double, { int }* } }* P ?</td></tr>
|
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<tr><td>sabre</td><td>nope</td></tr>
|
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<tr><td>sabre</td><td>why the extra * ?</td></tr>
|
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<tr><td>sabre</td><td>if it helps, the type of t is { double, {int}}* and t2 is {int}* and X is int*</td></tr>
|
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<tr><td>Yorion</td><td>wait... 0 represents dereference, natural number i
|
|
represents &A[i] ?</td></tr>
|
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<tr><td>sabre</td><td>gep does no dereferences, ever</td></tr>
|
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<tr><td>sabre</td><td>gep P, 0, 1 is equivalent to &P[0].X</td></tr>
|
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<tr><td>sabre</td><td>aka &P->X</td></tr>
|
|
<tr><td>sabre</td><td>gep P, 1 == &P[1] aka P+1</td></tr>
|
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<tr><td>sabre</td><td>so gep P, 0, 1 can't alias gep P, 1 just like
|
|
&P->Y can't alias P+1</td></tr>
|
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<tr><td>sabre</td><td>assuming P is a pointer to struct {X, Y }</td></tr>
|
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<tr><td>Yorion</td><td>sabre: is it possible to come up with a general rule for "arithmetic of GEP indices"? </td></tr>
|
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<tr><td>sabre</td><td>Yorion: of course, it's very simple</td></tr>
|
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<tr><td>sabre</td><td>just not what you're expecting :)</td></tr>
|
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<tr><td>sabre</td><td>See langref.html</td></tr>
|
|
<tr><td>Yorion</td><td>for example, a=GEP x,0,0,1 b=GEP a,0,0,1, c=GEP b,0,0,1, that should be equal to GEP x,0,1,1,0, right?</td></tr>
|
|
<tr><td>Yorion</td><td>I did</td></tr>
|
|
<tr><td>Yorion</td><td>oops, equal to GEP x,0,1,1,1,0</td></tr>
|
|
<tr><td>sabre</td><td>that would be:</td></tr>
|
|
<tr><td>sabre</td><td>GEP X, 0, 0, 1, 0, 1, 0, 1</td></tr>
|
|
<tr><td>Yorion</td><td>you're killing me</td></tr>
|
|
<tr><td>sabre</td><td>The basic rule when turning: A = GEP B, C D = GEP A, 0, E</td></tr>
|
|
<tr><td>sabre</td><td>is that you drop the 0, turning it into</td></tr>
|
|
<tr><td>sabre</td><td>GEP B, C, E</td></tr>
|
|
<tr><td>Yorion</td><td>okay, that's good. any other rules?</td></tr>
|
|
<tr><td>nicholas</td><td>what if it isn't a 0?</td></tr>
|
|
<tr><td>sabre</td><td>more generally: A = GEP Ptr, B, C, ... D = GEP A, 0, E, F, ... </td></tr>
|
|
<tr><td>sabre</td><td>D = GEP Ptr, B, C, ... E, F, ...</td></tr>
|
|
<tr><td>sabre</td><td>if it's not zero, you generally cannot concatenate them</td></tr>
|
|
<tr><td>sabre</td><td>unless the first gep has one subscript</td></tr>
|
|
<tr><td>sabre</td><td>in which case you drop the zero</td></tr>
|
|
<tr><td>sabre</td><td>if you look in InstCombiner::visitGetElementPtrInst, it should have this logic</td></tr>
|
|
<tr><td>Yorion</td><td>what if there is no zero? how can I compute the offset from the base pointer in that case?</td></tr>
|
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<tr><td>Yorion</td><td>like A=GEP B,C and D=GEP A,E,F</td></tr>
|
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<tr><td>sabre</td><td>you don't have to combine them to compute an offset</td></tr>
|
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<tr><td>sabre</td><td>are you *just* trying to get a byte offset from the pointer?</td></tr>
|
|
<tr><td>Yorion</td><td>I'm trying to get offset of D from B</td></tr>
|
|
<tr><td>sabre</td><td>why didn't you say so? :)</td></tr>
|
|
<tr><td>sabre</td><td>with all constant subscripts, it's trivial</td></tr>
|
|
<tr><td>sabre</td><td>look at SelectionDAGLowering::visitGetElementPtr</td></tr>
|
|
<tr><td>sabre</td><td>in CodeGen/SelectionDAG/SelectionDAGISel.cpp</td></tr>
|
|
<tr><td>sabre</td><td>basically the rule is that you multiply the index by the size of the thing indexed</td></tr>
|
|
<tr><td>sabre</td><td>there is also a Support/GetElementPtrIterator or something</td></tr>
|
|
<tr><td>sabre</td><td>that makes it trivial to see what type is indexed by which subscript</td></tr>
|
|
<tr><td>sabre</td><td>for each subscript it gives you a type</td></tr>
|
|
<tr><td>sabre</td><td>For an array subscript you multiply the index by the indexed type</td></tr>
|
|
<tr><td>sabre</td><td>for a struct subscript, you add the field offset</td></tr>
|
|
<tr><td>sabre</td><td>s/array/sequentialtype/ if you're in a pedantic mood</td></tr>
|
|
<tr><td>Yorion</td><td>let's focus on structs: in that case, the above given example would be: D = GEP B,C,E,F?</td></tr>
|
|
<tr><td>sabre</td><td>no</td></tr>
|
|
<tr><td>sabre</td><td>you drop the E if it's zero</td></tr>
|
|
<tr><td>sabre</td><td>if it's not you can't concat</td></tr>
|
|
<tr><td>sabre</td><td>are you trying to trick me into saying "yes, just append the indices"? :)</td></tr>
|
|
<tr><td>Yorion</td><td>okay, let's assume E is not zero, how do I compute offset from B for D for a struct?</td></tr>
|
|
<tr><td>sabre</td><td>Why are you framing this in terms of concatenation?</td></tr>
|
|
<tr><td>Yorion</td><td>no, I'm trying to understand it</td></tr>
|
|
<tr><td>sabre</td><td>computing an offset and concatenating are entirely different</td></tr>
|
|
<tr><td>sabre</td><td>Lets consider a specific example</td></tr>
|
|
<tr><td>Yorion</td><td>because I want to express certain properties in the terms of base pointers either globals or parameters</td></tr>
|
|
<tr><td>Yorion</td><td>I want to eliminate locals from my analysis</td></tr>
|
|
<tr><td>sabre</td><td>you realize that parmeters can point into the middle of structs?</td></tr>
|
|
<tr><td>Yorion</td><td>yes</td></tr>
|
|
<tr><td>sabre</td><td>you realize invalid access paths can be constructed with geps/</td></tr>
|
|
<tr><td>sabre</td><td>?</td></tr>
|
|
<tr><td>Yorion</td><td>what do you mean by invalid access paths? </td></tr>
|
|
<tr><td>Yorion</td><td>like offseting out of the struct which is passed to the function?</td></tr>
|
|
<tr><td>sabre</td><td>The case where the subscript isn't zero is invalid code</td></tr>
|
|
<tr><td>sabre</td><td>from a type-safety perspective</td></tr>
|
|
<tr><td>DannyB</td><td>he means untypesafe things that seem valid :)</td></tr>
|
|
<tr><td>DannyB</td><td>IE they point somewhere in the struct, but not to any particular field</td></tr>
|
|
<tr><td>DannyB</td><td>(or whatever)</td></tr>
|
|
<tr><td>sabre</td><td>right</td></tr>
|
|
<tr><td>Yorion</td><td>okay</td></tr>
|
|
<tr><td>sabre</td><td>or they might point in some other struct :)</td></tr>
|
|
<tr><td>sabre</td><td>It's the equivalent to saying:</td></tr>
|
|
<tr><td>sabre</td><td>struct Foo { int A, int B; }</td></tr>
|
|
<tr><td>sabre</td><td>Foo* P = </td></tr>
|
|
<tr><td>sabre</td><td>T = &P->B;</td></tr>
|
|
<tr><td>sabre</td><td>S = T+1</td></tr>
|
|
<tr><td>sabre</td><td>that is:</td></tr>
|
|
<tr><td>sabre</td><td>T = gep 0, 1</td></tr>
|
|
<tr><td>sabre</td><td>S = gep T, 1</td></tr>
|
|
<tr><td>sabre</td><td>you can't concat those and get a type-safe access path</td></tr>
|
|
<tr><td>sabre</td><td>remember T = &P->B === T = &P[0].B</td></tr>
|
|
<tr><td>sabre</td><td>understand?</td></tr>
|
|
<tr><td>Yorion</td><td>let me think for a minute</td></tr>
|
|
<tr><td>sabre</td><td>Consider what the C case does, it will be most clear if you're used to C</td></tr>
|
|
<tr><td>sabre</td><td>:)</td></tr>
|
|
<tr><td>sabre</td><td>Consider the byte offset and why it doesn't point into P-> anything</td></tr>
|
|
<tr><td>sabre</td><td>it points into P[1] not P[0]</td></tr>
|
|
<tr><td>Yorion</td><td>it's perfectly fine if GEP offsets out of the type. I'd still need to express GEP in the terms of base pointers. Take the example above: T=GEP P,0,1; S=GEP T,1</td></tr>
|
|
<tr><td>Yorion</td><td>type safety is not crucial in my case</td></tr>
|
|
<tr><td>sabre</td><td>That specific example is GEP P, 1, 0</td></tr>
|
|
<tr><td>sabre</td><td>however, you can form geps that are NOT equivalent to anything else</td></tr>
|
|
<tr><td>sabre</td><td>for example, consider:</td></tr>
|
|
<tr><td>sabre</td><td>struct X { int, char}</td></tr>
|
|
<tr><td>Yorion</td><td>that is fine. they're equivalent to something in the calling context</td></tr>
|
|
<tr><td>sabre</td><td>the same sequence points into padding</td></tr>
|
|
<tr><td>sabre</td><td>and there is no gep that can do that</td></tr>
|
|
<tr><td>Yorion</td><td>the bottom line is: if the program is valid, interprocedural analysis will match that offset with something in one of the functions on the call stack</td></tr>
|
|
<tr><td>Yorion</td><td>and that's all I care about</td></tr>
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<tr><td>Yorion</td><td>can you give me a formula for structs for computing
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offsets that takes into account the case GEP T,&lt:non_zeros> and the size of the structs/fields?</td></tr>
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<tr><td>sabre</td><td>yes, I did above</td></tr>
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<tr><td>sabre</td><td>Two things:</td></tr>
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<tr><td>sabre</td><td>GEP Ptr, A, X, Y, Z</td></tr>
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<tr><td>sabre</td><td>The result is Ptr + A * sizeof(struct) + fieldoffs(X) + fieldoffs(Y) + fieldoffs(Z)</td></tr>
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<tr><td>sabre</td><td>simple enough?</td></tr>
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<tr><td>sabre</td><td>you see why "A" is special?</td></tr>
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<tr><td>Yorion</td><td>give me a min, I'm constructing an example</td></tr>
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<tr><td>Reid</td><td>sabre: I think I finally understand</td></tr>
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<tr><td>Reid</td><td>your comment that GEP *never* dereferences makes a lot of sense</td></tr>
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<tr><td>Reid</td><td>it is only doing address calculation, so the first one is taking the address of the var</td></tr>
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<tr><td>sabre</td><td>If C didn't conflate lvalues and rvalues, GEP would be much easier to understand for people</td></tr>
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<tr><td>Reid</td><td>yeah</td></tr>
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<tr><td>Yorion</td><td>so, for example: M=GEP A,B,C; N=GEP M,D,E; N = [ A + B*sizeof(struct) + fieldoffs(C) ]:(of type T) + D*sizeof(T) + fieldoffs(E)</td></tr>
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<tr><td>Reid</td><td>I just remember learning a hard lesson about the difference between char* A and char A[] .. long time ago when I was learning C</td></tr>
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<tr><td>sabre</td><td>of type T*</td></tr>
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<tr><td>sabre</td><td>otherwise fine</td></tr>
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<tr><td>Yorion</td><td>okay, I think I finally understand it</td></tr>
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<tr><td>sabre</td><td>without the T* your D sizeof will be wrong</td></tr>
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<tr><td>Yorion</td><td>a suggestion: the formula you gave above explains it all</td></tr>
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<tr><td>Yorion</td><td>I'd suggest explaining it that way in documentation</td></tr>
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<tr><td>sabre</td><td>That's not right though</td></tr>
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<tr><td>sabre</td><td>it doesn't include arrays or packed types</td></tr>
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<tr><td>sabre</td><td>so it is, at best, a half truth</td></tr>
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<tr><td>Yorion</td><td>tell me, how to compute the fieldoffs for an index?</td></tr>
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<tr><td>sabre</td><td>arrays can be in structs :)</td></tr>
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<tr><td>Yorion</td><td>in bytes</td></tr>
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<tr><td>sabre</td><td>idx * sizeof(arrayelt)</td></tr>
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<tr><td>sabre</td><td>just like for pointers (the first index)</td></tr>
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<tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
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<tr><td>sabre</td><td>for sequentials you use idx*sizeof(sequenced type)</td></tr>
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<tr><td>sabre</td><td>for structs you add their offset</td></tr>
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<tr><td>sabre</td><td>it's really very simple :)</td></tr>
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<tr><td>sabre</td><td>the first index of a gep is always over the pointer</td></tr>
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<tr><td>Yorion</td><td>no I meant in LLVM, how do I convert the field offset into bytes?</td></tr>
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<tr><td>sabre</td><td>which is why it's strange</td></tr>
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<tr><td>sabre</td><td>if you only think about structs</td></tr>
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<tr><td>sabre</td><td>TargetData::getFieldOffset </td></tr>
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<tr><td>sabre</td><td>or something</td></tr>
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<tr><td>sabre</td><td>look in SelectionDAGISel.cpp (visitGEP) as I suggested.</td></tr>
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<tr><td>Yorion</td><td>do you still have the energy to go over sequential types? :-)</td></tr>
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<tr><td>Yorion</td><td>what is the offset formula for sequential types?</td></tr>
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<tr><td>Reid</td><td>we just went over that: idx * sizeof(elementType)</td></tr>
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<tr><td>Yorion</td><td>so, if there's an array hidden somewhere in the struct, essentially I need to compute idx*sizeof() instead of fieldoffs() and that's it?</td></tr>
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<tr><td>sabre</td><td>yes</td></tr>
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<tr><td>Reid</td><td>yes</td></tr>
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<tr><td>Yorion</td><td>excellent.</td></tr>
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<tr><td>sabre</td><td>There are two cases: structs and sequentials</td></tr>
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<tr><td>sabre</td><td>[9:17pm] sabre: for sequentials you use idx*sizeof(sequenced type)</td></tr>
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<tr><td>sabre</td><td>[9:17pm] sabre: for structs you add their offset</td></tr>
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<tr><td>sabre</td><td>[9:17pm] sabre: it's really very simple :)</td></tr>
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<tr><td>Yorion</td><td>now when I understand it, it is simple... </td></tr>
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<tr><td>Yorion</td><td>thx</td></tr>
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