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LoopVectorize: External uses must use the last value in a reduction cycle

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Otherwise, we don't perform operations that would have been performed on
the scalar version.

Fixes PR17498.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192133 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Arnold Schwaighofer 2013-10-07 21:05:43 +00:00
parent 42be15fcbe
commit 1ee3c0008b
2 changed files with 33 additions and 0 deletions
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6
lib/Transforms/Vectorize/LoopVectorize.cpp
View File

@ -4015,6 +4015,12 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
if (ExitInstruction != 0 || Cur == Phi) if (ExitInstruction != 0 || Cur == Phi)
return false; return false;
// The instruction used by an outside user must be the last instruction
// before we feed back to the reduction phi. Otherwise, we loose VF-1
// operations on the value.
if (std::find(Phi->op_begin(), Phi->op_end(), Cur) == Phi->op_end())
return false;
ExitInstruction = Cur; ExitInstruction = Cur;
continue; continue;
} }

27
test/Transforms/LoopVectorize/reduction.ll
View File

@ -467,3 +467,30 @@ for.body:
for.end: for.end:
ret i32 %p.addr.02 ret i32 %p.addr.02
} }
; Don't vectorize a reduction value that is not the last in a reduction cyle. We
; would loose iterations (VF-1) on the operations after that use.
; PR17498
; CHECK-LABEL: not_last_operation
; CHECK-NOT: x i32>
define i32 @not_last_operation(i32 %p, i32 %val) {
entry:
%tobool = icmp eq i32 %p, 0
br label %for.body
for.body:
%inc613.1 = phi i32 [ 0, %entry ], [ %inc6.1, %for.body ]
%inc511.1 = phi i32 [ %val, %entry ], [ %inc5.1, %for.body ]
%0 = zext i1 %tobool to i32
%inc4.1 = xor i32 %0, 1
%inc511.1.inc4.1 = add nsw i32 %inc511.1, %inc4.1
%inc5.1 = add nsw i32 %inc511.1.inc4.1, 1
%inc6.1 = add nsw i32 %inc613.1, 1
%exitcond.1 = icmp eq i32 %inc6.1, 22
br i1 %exitcond.1, label %exit, label %for.body
exit:
%inc.2 = add nsw i32 %inc511.1.inc4.1, 2
ret i32 %inc.2
}