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| author | Nick Lewycky <nicholas@mxc.ca> | 2008-12-15 01:34:58 +0000 |
|---|---|---|
| committer | Nick Lewycky <nicholas@mxc.ca> | 2008-12-15 01:34:58 +0000 |
| commit | e642658480ed0c9cde023a165edca237ba90bd31 (patch) | |
| tree | edaaf7d65af6cc9b3c06311be66b9cf764f65902 /docs/AliasAnalysis.html | |
| parent | 30baa95afdf1199c2c5514310907b01ce5584cf5 (diff) | |
| download | external_llvm-e642658480ed0c9cde023a165edca237ba90bd31.zip external_llvm-e642658480ed0c9cde023a165edca237ba90bd31.tar.gz external_llvm-e642658480ed0c9cde023a165edca237ba90bd31.tar.bz2 | |
Introducing nocapture, a parameter attribute for pointers to indicate that the
callee will not introduce any new aliases of that pointer.
The attributes had all bits allocated already, so I decided to collapse
alignment. Alignment was previously stored as a 16-bit integer from bits 16 to
32 of the attribute, but it was required to be a power of 2. Now it's stored in
log2 encoded form in five bits from 16 to 21. That gives us 11 more bits of
space.
You may have already noticed that you only need four bits to encode a 16-bit
power of two, so why five bits? Because the AsmParser accepted 32-bit
alignments, even though we couldn't store them (they were silently discarded).
Now we can store them in memory, but not in the bitcode.
The bitcode format was already storing these as 64-bit VBR integers. So, the
bitcode format stays the same, keeping the alignment values stored as 16 bit
raw values. There's some hideous code in the reader and writer that deals with
this, waiting to be ripped out the moment we run out of bits again and have to
replace the parameter attributes table encoding.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61019 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/AliasAnalysis.html')
| -rw-r--r-- | docs/AliasAnalysis.html | 10 |
1 files changed, 5 insertions, 5 deletions
diff --git a/docs/AliasAnalysis.html b/docs/AliasAnalysis.html index 1569fb8..97a8133 100644 --- a/docs/AliasAnalysis.html +++ b/docs/AliasAnalysis.html @@ -192,11 +192,11 @@ and returns MustAlias, MayAlias, or NoAlias as appropriate. <div class="doc_text"> <p>The NoAlias response is used when the two pointers refer to distinct objects, -even regardless of whether the pointers compare equal. For example, freed -pointers don't alias any pointers that were allocated afterwards. As a -degenerate case, pointers returned by malloc(0) have no bytes for an object, -and are considered NoAlias even when malloc returns the same pointer. The same -rule applies to NULL pointers.</p> +regardless of whether the pointers compare equal. For example, freed pointers +don't alias any pointers that were allocated afterwards. As a degenerate case, +pointers returned by malloc(0) have no bytes for an object, and are considered +NoAlias even when malloc returns the same pointer. The same rule applies to +NULL pointers.</p> <p>The MayAlias response is used whenever the two pointers might refer to the same object. If the two memory objects overlap, but do not start at the same |