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

questions left unanswered by the first GetElementPtr FAQ. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96526 91177308-0d34-0410-b5e6-96231b3b80d8
author: Dan Gohman <gohman@apple.com> 2010-02-17 22:47:06 +0000
committer: Dan Gohman <gohman@apple.com> 2010-02-17 22:47:06 +0000
commit: dfecbe9b0c9f14f60bf285f9e62bbb1c728f09e2 (patch)
tree: 47842023e7d3979e688c7b00d2bb5efd414827b0
parent: 69bfbd61c6a9f9f9a6fcddacc7a9be7e02ea9a2d (diff)
download: external_llvm-dfecbe9b0c9f14f60bf285f9e62bbb1c728f09e2.zip
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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>
+
+<!-- *********************************************************************** -->
+
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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+
diff --git a/docs/index.html b/docs/index.html
index 28f4cde..936eae1 100644
--- a/docs/index.html
+++ b/docs/index.html
@@ -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>
 
 <!--=======================================================================-->
author	Dan Gohman <gohman@apple.com>	2010-02-17 22:47:06 +0000
committer	Dan Gohman <gohman@apple.com>	2010-02-17 22:47:06 +0000
commit	dfecbe9b0c9f14f60bf285f9e62bbb1c728f09e2 (patch)
tree	47842023e7d3979e688c7b00d2bb5efd414827b0
parent	69bfbd61c6a9f9f9a6fcddacc7a9be7e02ea9a2d (diff)
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