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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, address into a different separately allocated
object, and dereference it. 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 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 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 if you use GEP to do the add,
you can't use that pointer to actually access the object, 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>
<p>If you really need this functionality, you can do the arithmetic with
explicit integer instructions, and use inttoptr to convert the result to
an address. Most of GEP's special aliasing rules do not apply to pointers
computed from ptrtoint, arithmetic, and inttoptr sequences.</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 use GEP to 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>Also as above, ptrtoint and inttoptr provide an alternative way to do this
which do not have this restriction.</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>
</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>
<ul>
<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>
</ul>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
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</body>
</html>

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@ -17,7 +17,7 @@
<ol>
<li><a href="#intro">Introduction</a></li>
<li><a href="#questions">The Questions</a>
<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>
@ -25,6 +25,30 @@
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="#unions">Can GEP index into unions?</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>
@ -37,9 +61,10 @@
<!-- *********************************************************************** -->
<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
GetElementPtr (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.
@ -47,22 +72,14 @@
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="questions"><b>The Questions</b></a></div>
<div class="doc_section"><a name="addresses"><b>Address Computation</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>
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>
</div>
<!-- *********************************************************************** -->
@ -85,7 +102,7 @@ X = &amp;Foo-&gt;F;
<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
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
@ -155,7 +172,7 @@ entry:
<div class="doc_code">
<pre>
%MyVar = unintialized global i32
%MyVar = uninitialized global i32
...
%idx1 = getelementptr i32* %MyVar, i64 0
%idx2 = getelementptr i32* %MyVar, i64 1
@ -210,7 +227,7 @@ idx3 = (char*) &amp;MyVar + 8
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>
facts will dispel 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
@ -297,8 +314,8 @@ idx3 = (char*) &amp;MyVar + 8
<div class="doc_code">
<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
%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
</pre>
</div>
@ -326,8 +343,8 @@ idx3 = (char*) &amp;MyVar + 8
<div class="doc_code">
<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
%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1
</pre>
</div>
@ -336,6 +353,352 @@ idx3 = (char*) &amp;MyVar + 8
<tt>MyVar+40</tt> but its type is <tt>{ [10 x i32] }*</tt>.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="vectors"><b>Can GEP index into vector elements?</b></a>
</div>
<div class="doc_text">
<p>This hasn't always been forcefully disallowed, though it's not recommended.
It leads to awkward special cases in the optimizers, and fundamental
inconsistency in the IR. In the future, it will probably be outright
disallowed.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="unions"><b>Can GEP index into unions?</b></a>
</div>
<div class="doc_text">
<p>Unknown.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="addrspace"><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="int"><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, address into a different separately allocated
object, and dereference it. IR producers (front-ends) must follow this rule,
and consumers (optimizers, specifically alias analysis) benefit from being
able to rely on it. See the <a href="#rules">Rules</a> section for more
information.</p>
<p>And, GEP is more concise in common cases.</p>
<p>However, for the underlying integer computation implied, there
is no difference.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="be"><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="vla"><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_section"><a name="rules"><b>Rules</b></a></div>
<!-- *********************************************************************** -->
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="bounds"><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 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="negative"><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="compare"><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="types"><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="null"><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 if you use GEP to do the add,
you can't use that pointer to actually access the object, 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>
<p>If you really need this functionality, you can do the arithmetic with
explicit integer instructions, and use inttoptr to convert the result to
an address. Most of GEP's special aliasing rules do not apply to pointers
computed from ptrtoint, arithmetic, and inttoptr sequences.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="ptrdiff"><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 use GEP to 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>Also as above, ptrtoint and inttoptr provide an alternative way to do this
which do not have this restriction.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="tbaa"><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="overflow"><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="check"><b>How can I tell if my front-end is following the
rules?</b></a>
</div>
<div class="doc_text">
<p>There is currently no checker for the getelementptr rules. Currently,
the only way to do this is to manually check each place in your front-end
where GetElementPtr operators are created.</p>
<p>It's not possible to write a checker which could find all rule
violations statically. It would be possible to write a checker which
works by instrumenting the code with dynamic checks though. Alternatively,
it would be possible to write a static checker which catches a subset of
possible problems. However, no such checker exists today.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"><a name="rationale"><b>Rationale</b></a></div>
<!-- *********************************************************************** -->
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="goals"><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>
<ul>
<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>
</ul>
</div>
<!-- *********************************************************************** -->
<div class="doc_subsection">
<a name="i32"><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="uglygep"><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_section"><a name="summary"><b>Summary</b></a></div>
<!-- *********************************************************************** -->

3
docs/index.html
View File

@ -54,9 +54,6 @@ 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>
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