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Revert 122959, it needs more thought. Add it back to README.txt with additional notes.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123030 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2011-01-07 20:42:20 +00:00
parent 53bf3ee82b
commit eaff66a895
3 changed files with 22 additions and 31 deletions
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22
lib/Target/README.txt
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@@ -2009,6 +2009,28 @@ entry:
//===---------------------------------------------------------------------===//
This code can be seen in viterbi:
%64 = call noalias i8* @malloc(i64 %62) nounwind
...
%67 = call i64 @llvm.objectsize.i64(i8* %64, i1 false) nounwind
%68 = call i8* @__memset_chk(i8* %64, i32 0, i64 %62, i64 %67) nounwind
llvm.objectsize.i64 should be taught about malloc/calloc, allowing it to
fold to %62. This is a security win (overflows of malloc will get caught)
and also a performance win by exposing more memsets to the optimizer.
This occurs several times in viterbi.
Note that this would change the semantics of @llvm.objectsize which by its
current definition always folds to a constant. We also should make sure that
we remove checking in code like
char *p = malloc(strlen(s)+1);
__strcpy_chk(p, s, __builtin_objectsize(p, 0));
//===---------------------------------------------------------------------===//
This code (from Benchmarks/Dhrystone/dry.c):
define i32 @Func1(i32, i32) nounwind readnone optsize ssp {