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Another missed optimization of trivial vector code.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123112 91177308-0d34-0410-b5e6-96231b3b80d8
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Chandler Carruth 2011-01-09 09:58:36 +00:00
parent d8723a9b55
commit e5ca494ae6
1 changed files with 33 additions and 0 deletions
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33
lib/Target/README.txt
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@ -2193,3 +2193,36 @@ is the fact that all three null stores in %entry are dead, but not eliminated.
Also surprising is that %conv isn't simplified to 0 in %....exit.thread.i.i.
//===---------------------------------------------------------------------===//
clang -O3 -fno-exceptions currently compiles this code:
void f(int N) {
std::vector<int> v(N);
g(v);
}
into almost the same as the previous note, but replace its final BB with:
for.body.lr.ph: ; preds = %cond.true.i.i.i.i
%mul.i.i.i.i.i = shl i64 %conv, 2
%call3.i.i.i.i.i = call noalias i8* @_Znwm(i64 %mul.i.i.i.i.i) nounwind
%0 = bitcast i8* %call3.i.i.i.i.i to i32*
store i32* %0, i32** %v8.sub, align 8, !tbaa !0
%add.ptr.i.i.i = getelementptr inbounds i32* %0, i64 %conv
store i32* %add.ptr.i.i.i, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
call void @llvm.memset.p0i8.i64(i8* %call3.i.i.i.i.i, i8 0, i64 %mul.i.i.i.i.i, i32 4, i1 false)
store i32* %add.ptr.i.i.i, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
%tmp18 = add i32 %N, -1
%tmp19 = zext i32 %tmp18 to i64
%tmp20 = shl i64 %tmp19, 2
%tmp21 = add i64 %tmp20, 4
call void @llvm.memset.p0i8.i64(i8* %call3.i.i.i.i.i, i8 0, i64 %tmp21, i32 4, i1 false)
br label %for.end
First off, why (((zext %N - 1) << 2) + 4) instead of the ((sext %N) << 2) done
previously? (or better yet, re-use that one?)
Then, the really painful one is the second memset, of the same memory, to the
same value.
//===---------------------------------------------------------------------===//