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Add a note about vector's size-constructor producing dead stores.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123111 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2011-01-09 09:58:33 +00:00
parent 0804ead404
commit d8723a9b55
1 changed files with 55 additions and 0 deletions
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55
lib/Target/README.txt
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@ -2138,3 +2138,58 @@ _ZSt4fillIPiiEvT_S1_RKT0_.exit: ; preds = %for.body.i.i, %entr
It should compile it to a memset.
//===---------------------------------------------------------------------===//
clang -O3 -fno-exceptions currently compiles this code:
void f(int N) {
std::vector<int> v(N);
g(v);
}
into
define void @_Z1fi(i32 %N) nounwind {
entry:
%v2 = alloca [3 x i32*], align 8
%v2.sub = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 0
%tmpcast = bitcast [3 x i32*]* %v2 to %"class.std::vector"*
%conv = sext i32 %N to i64
store i32* null, i32** %v2.sub, align 8, !tbaa !0
%tmp3.i.i.i.i.i = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 1
store i32* null, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
%tmp4.i.i.i.i.i = getelementptr inbounds [3 x i32*]* %v2, i64 0, i64 2
store i32* null, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
%cmp.i.i.i.i = icmp eq i32 %N, 0
br i1 %cmp.i.i.i.i, label %_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.thread.i.i, label %cond.true.i.i.i.i
_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.thread.i.i: ; preds = %entry
store i32* null, i32** %v2.sub, align 8, !tbaa !0
store i32* null, i32** %tmp3.i.i.i.i.i, align 8, !tbaa !0
%add.ptr.i5.i.i = getelementptr inbounds i32* null, i64 %conv
store i32* %add.ptr.i5.i.i, i32** %tmp4.i.i.i.i.i, align 8, !tbaa !0
br label %_ZNSt6vectorIiSaIiEEC1EmRKiRKS0_.exit
cond.true.i.i.i.i: ; preds = %entry
%cmp.i.i.i.i.i = icmp slt i32 %N, 0
br i1 %cmp.i.i.i.i.i, label %if.then.i.i.i.i.i, label %_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.i.i
if.then.i.i.i.i.i: ; preds = %cond.true.i.i.i.i
call void @_ZSt17__throw_bad_allocv() noreturn nounwind
unreachable
_ZNSt12_Vector_baseIiSaIiEEC2EmRKS0_.exit.i.i: ; 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** %v2.sub, align 8, !tbaa !0
store i32* %0, i32** %tmp3.i.i.i.i.i, 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)
br label %_ZNSt6vectorIiSaIiEEC1EmRKiRKS0_.exit
This is just the handling the construction of the vector. Most surprising here
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.
//===---------------------------------------------------------------------===//