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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>The Often Misunderstood GEP Instruction</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
<style type="text/css">
TABLE { text-align: left; border: 1px solid black; border-collapse: collapse; margin: 0 0 0 0; }
</style>
</head>
<body>
<h1>
The Often Misunderstood GEP Instruction
</h1>
<ol>
<li><a href="#intro">Introduction</a></li>
<li><a href="#addresses">Address Computation</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>
<li><a href="#vectors">Can GEP index into vector elements?</a>
<li><a href="#addrspace">What effect do address spaces have on GEPs?</a>
<li><a href="#int">How is GEP different from ptrtoint, arithmetic, and inttoptr?</a></li>
<li><a href="#be">I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?</a>
<li><a href="#vla">How does VLA addressing work with GEPs?</a>
</ol></li>
<li><a href="#rules">Rules</a>
<ol>
<li><a href="#bounds">What happens if an array index is out of bounds?</a>
<li><a href="#negative">Can array indices be negative?</a>
<li><a href="#compare">Can I compare two values computed with GEPs?</a>
<li><a href="#types">Can I do GEP with a different pointer type than the type of the underlying object?</a>
<li><a href="#null">Can I cast an object's address to integer and add it to null?</a>
<li><a href="#ptrdiff">Can I compute the distance between two objects, and add that value to one address to compute the other address?</a>
<li><a href="#tbaa">Can I do type-based alias analysis on LLVM IR?</a>
<li><a href="#overflow">What happens if a GEP computation overflows?</a>
<li><a href="#check">How can I tell if my front-end is following the rules?</a>
</ol></li>
<li><a href="#rationale">Rationale</a>
<ol>
<li><a href="#goals">Why is GEP designed this way?</a></li>
<li><a href="#i32">Why do struct member indices always use i32?</a></li>
<li><a href="#uglygep">What's an uglygep?</a>
</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>
<!-- *********************************************************************** -->
<h2><a name="intro">Introduction</a></h2>
<!-- *********************************************************************** -->
<div>
<p>This document seeks to dispel the mystery and confusion surrounding LLVM's
<a href="LangRef.html#i_getelementptr">GetElementPtr</a> (GEP) instruction.
Questions about the wily GEP instruction are
probably the most frequently occurring 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>
<!-- *********************************************************************** -->
<h2><a name="addresses">Address Computation</a></h2>
<!-- *********************************************************************** -->
<div>
<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. GEP closely resembles C array indexing
and field selection, however it's is a little different and this leads to
the following questions.</p>
<!-- *********************************************************************** -->
<h3>
<a name="firstptr">What is the first index of the GEP instruction?</a>
</h3>
<div>
<p>Quick answer: The index stepping through the first operand.</p>
<p>The confusion with the first index usually arises from thinking about
the GetElementPtr instruction as if it was a C index operator. They aren't the
same. For example, when we write, in "C":</p>
<div class="doc_code">
<pre>
AType *Foo;
...
X = &Foo->F;
</pre>
</div>
<p>it is natural to think that there is only one index, the selection of the
field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That
pointer must be indexed explicitly in LLVM. C, on the other hand, indices
through it transparently. To arrive at the same address location as the C
code, you would provide the GEP instruction with two index operands. The
first operand indexes through the pointer; the second operand indexes the
field <tt>F</tt> of the structure, just as if you wrote:</p>
<div class="doc_code">
<pre>
X = &Foo[0].F;
</pre>
</div>
<p>Sometimes this question gets rephrased as:</p>
<blockquote><p><i>Why is it okay to index through the first pointer, but
subsequent pointers won't be dereferenced?</i></p></blockquote>
<p>The answer is simply because memory does not have to be accessed to
perform the computation. The first operand to the GEP instruction must be a
value of a pointer type. The value of the pointer is provided directly to
the GEP instruction as an operand without any need for accessing memory. It
must, therefore be indexed and requires an index operand. Consider this
example:</p>
<div class="doc_code">
<pre>
struct munger_struct {
int f1;
int f2;
};
void munge(struct munger_struct *P) {
P[0].f1 = P[1].f1 + P[2].f2;
}
...
munger_struct Array[3];
...
munge(Array);
</pre>
</div>
<p>In this "C" example, the front end compiler (llvm-gcc) will generate three
GEP instructions for the three indices through "P" in the assignment
statement. The function argument <tt>P</tt> will be the first operand of each
of these GEP instructions. The second operand indexes through that pointer.
The third operand will be the field offset into the
<tt>struct munger_struct</tt> type, for either the <tt>f1</tt> or
<tt>f2</tt> field. So, in LLVM assembly the <tt>munge</tt> function looks
like:</p>
<div class="doc_code">
<pre>
void %munge(%struct.munger_struct* %P) {
entry:
%tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
%tmp = load i32* %tmp
%tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
%tmp7 = load i32* %tmp6
%tmp8 = add i32 %tmp7, %tmp
%tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
store i32 %tmp8, i32* %tmp9
ret void
}
</pre>
</div>
<p>In each case the first operand is the pointer through which the GEP
instruction starts. The same is true whether the first operand is an
argument, allocated memory, or a global variable. </p>
<p>To make this clear, let's consider a more obtuse example:</p>
<div class="doc_code">
<pre>
%MyVar = uninitialized global i32
...
%idx1 = getelementptr i32* %MyVar, i64 0
%idx2 = getelementptr i32* %MyVar, i64 1
%idx3 = getelementptr i32* %MyVar, i64 2
</pre>
</div>
<p>These GEP instructions are simply making address computations from the
base address of <tt>MyVar</tt>. They compute, as follows (using C syntax):
</p>
<div class="doc_code">
<pre>
idx1 = (char*) &MyVar + 0
idx2 = (char*) &MyVar + 4
idx3 = (char*) &MyVar + 8
</pre>
</div>
<p>Since the type <tt>i32</tt> is known to be four bytes long, the indices
0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No
memory is accessed to make these computations because the address of
<tt>%MyVar</tt> is passed directly to the GEP instructions.</p>
<p>The obtuse part of this example is in the cases of <tt>%idx2</tt> and
<tt>%idx3</tt>. They result in the computation of addresses that point to
memory past the end of the <tt>%MyVar</tt> global, which is only one
<tt>i32</tt> long, not three <tt>i32</tt>s long. While this is legal in LLVM,
it is inadvisable because any load or store with the pointer that results
from these GEP instructions would produce undefined results.</p>
</div>
<!-- *********************************************************************** -->
<h3>
<a name="extra_index">Why is the extra 0 index required?</a>
</h3>
<!-- ***************************
|
