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[CodeGenPrepare] don't convert an unpredictable select into control flow

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Suggested in the review of D19488:
http://reviews.llvm.org/D19488

llvm-svn: 267504
This commit is contained in:
Sanjay Patel 2016-04-26 00:47:39 +00:00
parent 69d7e17058
commit b5dda51232
2 changed files with 26 additions and 6 deletions
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3
lib/CodeGen/CodeGenPrepare.cpp
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@ -4568,7 +4568,8 @@ bool CodeGenPrepare::optimizeSelectInst(SelectInst *SI) {
bool VectorCond = !SI->getCondition()->getType()->isIntegerTy(1);
// Can we convert the 'select' to CF ?
if (DisableSelectToBranch || OptSize || !TLI || VectorCond)
if (DisableSelectToBranch || OptSize || !TLI || VectorCond ||
SI->getMetadata(LLVMContext::MD_unpredictable))
return false;
TargetLowering::SelectSupportKind SelectKind;

29
test/Transforms/CodeGenPrepare/X86/select.ll
View File

@ -53,7 +53,7 @@ entry:
; CHECK: %div = fdiv float %a, %b
; CHECK: br label %select.end
; CHECK: select.end:
; CHECK: %sel = phi float [ 4.000000e+00, %entry ], [ %div, %select.false.sink ]
; CHECK: %sel = phi float [ 4.000000e+00, %entry ], [ %div, %select.false.sink ]
; CHECK: ret float %sel
}
@ -75,20 +75,39 @@ entry:
; CHECK: %div2 = fdiv float %b, %a
; CHECK: br label %select.end
; CHECK: select.end:
; CHECK: %sel = phi float [ %div1, %select.true.sink ], [ %div2, %select.false.sink ]
; CHECK: %sel = phi float [ %div1, %select.true.sink ], [ %div2, %select.false.sink ]
; CHECK: ret float %sel
}
; But if the select is marked unpredictable, then don't turn it into a branch.
define float @unpredictable_select(float %a, float %b) {
; CHECK-LABEL: @unpredictable_select(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[DIV:%.*]] = fdiv float %a, %b
; CHECK-NEXT: [[CMP:%.*]] = fcmp ogt float %a, 1.000000e+00
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], float [[DIV]], float 2.000000e+00, !unpredictable !0
; CHECK-NEXT: ret float [[SEL]]
;
entry:
%div = fdiv float %a, %b
%cmp = fcmp ogt float %a, 1.0
%sel = select i1 %cmp, float %div, float 2.0, !unpredictable !0
ret float %sel
}
!0 = !{}
; An 'fadd' is not too expensive, so it's ok to speculate.
define float @fadd_no_sink(float %a, float %b) {
%add = fadd float %a, %b
%cmp = fcmp ogt float 6.0, %a
%sel = select i1 %cmp, float %add, float 7.0
%sel = select i1 %cmp, float %add, float 7.0
ret float %sel
; CHECK-LABEL: @fadd_no_sink(
; CHECK: %sel = select i1 %cmp, float %add, float 7.0
; CHECK: %sel = select i1 %cmp, float %add, float 7.0
}
; Possible enhancement: sinkability is only calculated with the direct
@ -104,7 +123,7 @@ entry:
ret float %sel
; CHECK-LABEL: @fdiv_no_sink(
; CHECK: %sel = select i1 %cmp, float %add, float 8.0
; CHECK: %sel = select i1 %cmp, float %add, float 8.0
}
; Do not transform the CFG if the select operands may have side effects.