[llvm-commits] [llvm] r96526 - in /llvm/trunk/docs: AdvancedGetElementPtr.html index.html
Dan Gohman
gohman at apple.com
Wed Feb 17 14:47:06 PST 2010
Author: djg
Date: Wed Feb 17 16:47:06 2010
New Revision: 96526
URL: http://llvm.org/viewvc/llvm-project?rev=96526&view=rev
Log:
Add an "advanced" GetElementPtr FAQ document, with answers to
questions left unanswered by the first GetElementPtr FAQ.
Added:
llvm/trunk/docs/AdvancedGetElementPtr.html (with props)
Modified:
llvm/trunk/docs/index.html
Added: llvm/trunk/docs/AdvancedGetElementPtr.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/AdvancedGetElementPtr.html?rev=96526&view=auto
==============================================================================
--- llvm/trunk/docs/AdvancedGetElementPtr.html (added)
+++ llvm/trunk/docs/AdvancedGetElementPtr.html Wed Feb 17 16:47:06 2010
@@ -0,0 +1,356 @@
+<!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
+ src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
+ <a href="http://validator.w3.org/check/referer"><img
+ src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
+ <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br/>
+ Last modified: $Date$
+</address>
+</body>
+</html>
+
Propchange: llvm/trunk/docs/AdvancedGetElementPtr.html
------------------------------------------------------------------------------
svn:keywords = Author Date Id Revision
Modified: llvm/trunk/docs/index.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/index.html?rev=96526&r1=96525&r2=96526&view=diff
==============================================================================
--- llvm/trunk/docs/index.html (original)
+++ llvm/trunk/docs/index.html Wed Feb 17 16:47:06 2010
@@ -54,6 +54,9 @@
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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