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Add a note about a missed memset optimization from std::fill.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123103 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2011-01-09 01:32:55 +00:00
parent b258135301
commit 75fbd3793f
1 changed files with 30 additions and 0 deletions
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30
lib/Target/README.txt
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@ -2108,3 +2108,33 @@ into a call to calloc. We should make sure that we analyze calloc as
aggressively as malloc though.
//===---------------------------------------------------------------------===//
clang -03 currently compiles this code
void f1(int* begin, int* end) {
std::fill(begin, end, 0);
}
into
define void @_Z2f1PiS_(i32* %begin, i32* %end) nounwind {
entry:
%cmp7.i.i = icmp eq i32* %begin, %end
br i1 %cmp7.i.i, label %_ZSt4fillIPiiEvT_S1_RKT0_.exit, label %for.body.i.i
for.body.i.i: ; preds = %entry, %for.body.i.i
%indvar.i.i = phi i64 [ %tmp, %for.body.i.i ], [ 0, %entry ]
%tmp = add i64 %indvar.i.i, 1
%ptrincdec.i.i = getelementptr i32* %begin, i64 %tmp
%__first.addr.08.i.i = getelementptr i32* %begin, i64 %indvar.i.i
store i32 0, i32* %__first.addr.08.i.i, align 4, !tbaa !0
%cmp.i.i = icmp eq i32* %ptrincdec.i.i, %end
br i1 %cmp.i.i, label %_ZSt4fillIPiiEvT_S1_RKT0_.exit, label %for.body.i.i
_ZSt4fillIPiiEvT_S1_RKT0_.exit: ; preds = %for.body.i.i, %entry
ret void
}
It should compile it to a memset.
//===---------------------------------------------------------------------===//