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