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Add an "advanced" GetElementPtr FAQ document, with answers to

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questions left unanswered by the first GetElementPtr FAQ.

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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 Revenge Of 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>
<div class="doc_title">
The Revenge Of The Often Misunderstood GEP Instruction
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro"><b>Introduction</b></a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>GEP was mysterious and wily at first, but it turned out that the basic
workings were fairly comprehensible. However the dragon was merely subdued;
now it's back, and it has more fundamental complexity to confront. This
document seeks to uncover misunderstandings of the GEP operator that tend
to persist past initial confusion about the funky "extra 0" thing. Here we
show that the GEP instruction is really not quite as simple as it seems,
even after the initial confusion is overcome.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>How is GEP different from ptrtoint, arithmetic,
and inttoptr?</b></a>
</div>
<div class="doc_text">
<p>It's very similar; there are only subtle differences.</p>
<p>With ptrtoint, you have to pick an integer type. One approach is to pick i64;
this is safe on everything LLVM supports (LLVM internally assumes pointers
are never wider than 64 bits in many places), and the optimizer will actually
narrow the i64 arithmetic down to the actual pointer size on targets which
don't support 64-bit arithmetic in most cases. However, there are some cases
where it doesn't do this. With GEP you can avoid this problem.
<p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a
GEP from one object and address into a different separately allocated
object. IR producers (front-ends) must follow this rule, and consumers
(optimizers, specifically alias analysis) benefit from being able to rely
on it.</p>
<p>And, GEP is more concise in common cases.</p>
<p>However, for of the underlying integer computation implied, there
is no difference.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>I'm writing a backend for a target which needs custom
lowering for GEP. How do I do this?</b></a>
</div>
<div class="doc_text">
<p>You don't. The integer computation implied by a GEP is target-independent.
Typically what you'll need to do is make your backend pattern-match
expressions trees involving ADD, MUL, etc., which are what GEP is lowered
into. This has the advantage of letting your code work correctly in more
cases.</p>
<p>GEP does use target-dependent parameters for the size and layout of data
types, which targets can customize.</p>
<p>If you require support for addressing units which are not 8 bits, you'll
need to fix a lot of code in the backend, with GEP lowering being only a
small piece of the overall picture.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Why do struct member indices always use i32?</b></a>
</div>
<div class="doc_text">
<p>The specific type i32 is probably just a historical artifact, however it's
wide enough for all practical purposes, so there's been no need to change it.
It doesn't necessarily imply i32 address arithmetic; it's just an identifier
which identifies a field in a struct. Requiring that all struct indices be
the same reduces the range of possibilities for cases where two GEPs are
effectively the same but have distinct operand types.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>How does VLA addressing work with GEPs?</b></a>
</div>
<div class="doc_text">
<p>GEPs don't natively support VLAs. LLVM's type system is entirely static,
and GEP address computations are guided by an LLVM type.</p>
<p>VLA indices can be implemented as linearized indices. For example, an
expression like X[a][b][c], must be effectively lowered into a form
like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional
array reference.</p>
<p>This means if you want to write an analysis which understands array
indices and you want to support VLAs, your code will have to be
prepared to reverse-engineer the linearization. One way to solve this
problem is to use the ScalarEvolution library, which always presents
VLA and non-VLA indexing in the same manner.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>What happens if an array index is out of bounds?</b></a>
</div>
<div class="doc_text">
<p>There are two senses in which an array index can be out of bounds.</p>
<p>First, there's the array type which comes from the (static) type of
the first operand to the GEP. Indices greater than the number of elements
in the corresponding static array type are valid. There is no problem with
out of bounds indices in this sense. Indexing into an array only depends
on the size of the array element, not the number of elements.</p>
<p>A common example of how this is used is arrays where the size is not known.
It's common to use array types with zero length to represent these. The
fact that the static type says there are zero elements is irrelevant; it's
perfectly valid to compute arbitrary element indices, as the computation
only depends on the size of the array element, not the number of
elements. Note that zero-sized arrays are not a special case here.</p>
<p>This sense is unconnected with <tt>inbounds</tt> keyword. The
<tt>inbounds</tt> keyword is designed to describe low-level pointer
arithmetic overflow conditions, rather than high-level array
indexing rules.
<p>Analysis passes which wish to understand array indexing should not
assume that the static array type bounds are respected.</p>
<p>The second sense of being out of bounds is computing an address that's
beyond of the actual underlying allocated object.</p>
<p>With the <tt>inbounds</tt> keyword, the result value of the GEP is
undefined if the address is outside the actual underlying allocated
object and not the address one-past-the-end.</p>
<p>Without the <tt>inbounds</tt> keyword, there are no restrictions
on computing out-of-bounds addresses. Obviously, performing a load or
a store requires an address of allocated and sufficiently aligned
memory. But the GEP itself is only concerned with computing addresses.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can array indices be negative?</b></a>
</div>
<div class="doc_text">
<p>Yes. This is basically a special case of array indices being out
of bounds.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can I compare two values computed with GEPs?</b></a>
</div>
<div class="doc_text">
<p>Yes. If both addresses are within the same allocated object, or
one-past-the-end, you'll get the comparison result you expect. If either
is outside of it, integer arithmetic wrapping may occur, so the
comparison may not be meaningful.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can I do GEP with a different pointer type than the type of
the underlying object?</b></a>
</div>
<div class="doc_text">
<p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary
pointer type. The types in a GEP serve only to define the parameters for the
underlying integer computation. They need not correspond with the actual
type of the underlying object.</p>
<p>Furthermore, loads and stores don't have to use the same types as the type
of the underlying object. Types in this context serve only to specify
memory size and alignment. Beyond that there are merely a hint to the
optimizer indicating how the value will likely be used.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can I cast an object's address to integer and add it
to null?</b></a>
</div>
<div class="doc_text">
<p>You can compute an address that way, but you can't use that pointer to
actually access the object if you do, unless the object is managed
outside of LLVM.</p>
<p>The underlying integer computation is sufficiently defined; null has a
defined value -- zero -- and you can add whatever value you want to it.</p>
<p>However, it's invalid to access (load from or store to) an LLVM-aware
object with such a pointer. This includes GlobalVariables, Allocas, and
objects pointed to by noalias pointers.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can I compute the distance between two objects, and add
that value to one address to compute the other address?</b></a>
</div>
<div class="doc_text">
<p>As with arithmetic on null, You can compute an address that way, but
you can't use that pointer to actually access the object if you do,
unless the object is managed outside of LLVM.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can I do type-based alias analysis on LLVM IR?</b></a>
</div>
<div class="doc_text">
<p>You can't do type-based alias analysis using LLVM's built-in type system,
because LLVM has no restrictions on mixing types in addressing, loads or
stores.</p>
<p>It would be possible to add special annotations to the IR, probably using
metadata, to describe a different type system (such as the C type system),
and do type-based aliasing on top of that. This is a much bigger
undertaking though.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>What's an uglygep?</b></a>
</div>
<div class="doc_text">
<p>Some LLVM optimizers operate on GEPs by internally lowering them into
more primitive integer expressions, which allows them to be combined
with other integer expressions and/or split into multiple separate
integer expressions. If they've made non-trivial changes, translating
back into LLVM IR can involve reverse-engineering the structure of
the addressing in order to fit it into the static type of the original
first operand. It isn't always possibly to fully reconstruct this
structure; sometimes the underlying addressing doesn't correspond with
the static type at all. In such cases the optimizer instead will emit
a GEP with the base pointer casted to a simple address-unit pointer,
using the name "uglygep". This isn't pretty, but it's just as
valid, and it's sufficient to preserve the pointer aliasing guarantees
that GEP provides.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can GEP index into vector elements?</b></a>
</div>
<div class="doc_text">
<p>Sort of. This hasn't always been forcefully disallowed, though it's
not recommended. It leads to awkward special cases in the optimizers.
In the future, it may be outright disallowed.</p>
<p>Instead, you should cast your pointer types and use arrays instead of
vectors for addressing. Arrays have the same in-memory representation
as vectors, so the addressing is interchangeable.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Can GEP index into unions?</b></a>
</div>
<div class="doc_text">
<p>Unknown.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>What happens if a GEP computation overflows?</b></a>
</div>
<div class="doc_text">
<p>If the GEP has the <tt>inbounds</tt> keyword, the result value is
undefined.</p>
<p>Otherwise, the result value is the result from evaluating the implied
two's complement integer computation. However, since there's no
guarantee of where an object will be allocated in the address space,
such values have limited meaning.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>What effect do address spaces have on GEPs?</b></a>
</div>
<div class="doc_text">
<p>None, except that the address space qualifier on the first operand pointer
type always matches the address space qualifier on the result type.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="lead0"><b>Why is GEP designed this way?</b></a>
</div>
<div class="doc_text">
<p>The design of GEP has the following goals, in rough unofficial
order of priority:</p>
<p>
<ol>
<li>Support C, C-like languages, and languages which can be
conceptually lowered into C (this covers a lot).</li>
<li>Support optimizations such as those that are common in
C compilers.</li>
<li>Provide a consistent method for computing addresses so that
address computations don't need to be a part of load and
store instructions in the IR.</li>
<li>Support non-C-like languages, to the extent that it doesn't
interfere with other goals.</li>
<li>Minimize target-specific information in the IR.</li>
</ol>
</p>
</div>
<!-- *********************************************************************** -->
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@ -54,6 +54,9 @@ Lifelong Program Analysis &amp; Transformation</a> - Design overview.</li>
Multi-Stage Optimization</a> - More details (quite old now).</li>
<li><a href="GetElementPtr.html">GetElementPtr FAQ</a> - Answers to some very
frequent questions about LLVM's most frequently misunderstood instruction.</li>
<li><a href="AdvancedGetElementPtr.html">Advanced GetElementPtr FAQ</a> - Answers
to questions about the GetElementPtr operator for those who have made it through
the first FAQ.</li>
</ul>
<!--=======================================================================-->