mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-12-14 11:32:34 +00:00
e921792509
not restricted by the GEP rules. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96598 91177308-0d34-0410-b5e6-96231b3b80d8
363 lines
15 KiB
HTML
363 lines
15 KiB
HTML
<!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>
|
|
|
|
<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>
|
|
<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>
|
|
<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>
|
|
|