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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
View File

@ -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 {

4
lib/Transforms/InstCombine/InstCombineCalls.cpp
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@ -304,10 +304,6 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
if (Value *NElems = getMallocArraySize(MI, TD, true))
if (ConstantInt *NElements = dyn_cast<ConstantInt>(NElems))
Size = NElements->getZExtValue() * TD->getTypeAllocSize(MallocType);
// If there is no offset we can just return the size passed to malloc.
if (Offset == 0)
return ReplaceInstUsesWith(CI, MI->getArgOperand(0));
}
// Do not return "I don't know" here. Later optimization passes could

27
test/Transforms/InstCombine/objsize.ll
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@ -160,30 +160,3 @@ define i32 @test7() {
ret i32 %objsize
}
define i32 @test8(i32 %x) {
; CHECK: @test8
%alloc = call noalias i8* @malloc(i32 %x) nounwind
%objsize = call i32 @llvm.objectsize.i32(i8* %alloc, i1 false) nounwind readonly
; CHECK-NEXT: ret i32 %x
ret i32 %objsize
}
define i32 @test9(i32 %x) {
; CHECK: @test9
%alloc = call noalias i8* @malloc(i32 %x) nounwind
%gep = getelementptr inbounds i8* %alloc, i32 16
%objsize = call i32 @llvm.objectsize.i32(i8* %gep, i1 false) nounwind readonly
; CHECK-NOT: ret i32 %x
ret i32 %objsize
}
define i8* @test10(i32 %x) {
; CHECK: @test10
%alloc = call noalias i8* @malloc(i32 %x) nounwind
%objsize = call i32 @llvm.objectsize.i32(i8* %alloc, i1 false) nounwind readonly
tail call i8* @__memset_chk(i8* %alloc, i32 0, i32 %x, i32 %objsize) nounwind
; CHECK-NOT: @llvm.objectsize
; CHECK: @llvm.memset
ret i8* %alloc
; CHECK: ret i8*
}