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[GVN] don't propagate equality comparisons of FP zero (PR22376)

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In http://reviews.llvm.org/D6911, we allowed GVN to propagate FP equalities
to allow some simple value range optimizations. But that introduced a bug
when comparing to -0.0 or 0.0: these compare equal even though they are not
bitwise identical.

This patch disallows propagating zero constants in equality comparisons. 
Fixes: http://llvm.org/bugs/show_bug.cgi?id=22376

Differential Revision: http://reviews.llvm.org/D7257



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227491 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2015-01-29 20:51:49 +00:00
parent 1b7b2597d5
commit 26c81cc870
2 changed files with 52 additions and 9 deletions
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13
lib/Transforms/Scalar/GVN.cpp
View File

@ -2182,9 +2182,16 @@ bool GVN::propagateEquality(Value *LHS, Value *RHS,
// Handle the floating point versions of equality comparisons too. // Handle the floating point versions of equality comparisons too.
if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) || if ((isKnownTrue && Cmp->getPredicate() == CmpInst::FCMP_OEQ) ||
(isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) (isKnownFalse && Cmp->getPredicate() == CmpInst::FCMP_UNE)) {
Worklist.push_back(std::make_pair(Op0, Op1)); // Floating point -0.0 and 0.0 compare equal, so we can't
// propagate a constant based on that comparison.
// FIXME: We should do this optimization if 'no signed zeros' is
// applicable via an instruction-level fast-math-flag or some other
// indicator that relaxed FP semantics are being used.
if (!isa<ConstantFP>(Op1) || !cast<ConstantFP>(Op1)->isZero())
Worklist.push_back(std::make_pair(Op0, Op1));
}
// If "A >= B" is known true, replace "A < B" with false everywhere. // If "A >= B" is known true, replace "A < B" with false everywhere.
CmpInst::Predicate NotPred = Cmp->getInversePredicate(); CmpInst::Predicate NotPred = Cmp->getInversePredicate();
Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse); Constant *NotVal = ConstantInt::get(Cmp->getType(), isKnownFalse);

48
test/Transforms/GVN/edge.ll
View File

@ -69,11 +69,11 @@ if:
br label %return br label %return
return: return:
%retval.0 = phi double [ %div, %if ], [ %x, %entry ] %retval = phi double [ %div, %if ], [ %x, %entry ]
ret double %retval.0 ret double %retval
; CHECK-LABEL: define double @fcmp_oeq( ; CHECK-LABEL: define double @fcmp_oeq(
; CHECK: %div = fdiv double %x, 2.000000e+00 ; CHECK: %div = fdiv double %x, 2.0
} }
define double @fcmp_une(double %x, double %y) { define double @fcmp_une(double %x, double %y) {
@ -86,10 +86,46 @@ else:
br label %return br label %return
return: return:
%retval.0 = phi double [ %div, %else ], [ %x, %entry ] %retval = phi double [ %div, %else ], [ %x, %entry ]
ret double %retval.0 ret double %retval
; CHECK-LABEL: define double @fcmp_une( ; CHECK-LABEL: define double @fcmp_une(
; CHECK: %div = fdiv double %x, 2.000000e+00 ; CHECK: %div = fdiv double %x, 2.0
} }
; PR22376 - We can't propagate zero constants because -0.0
; compares equal to 0.0. If %y is -0.0 in this test case,
; we would produce the wrong sign on the infinity return value.
define double @fcmp_oeq_zero(double %x, double %y) {
entry:
%cmp = fcmp oeq double %y, 0.0
br i1 %cmp, label %if, label %return
if:
%div = fdiv double %x, %y
br label %return
return:
%retval = phi double [ %div, %if ], [ %x, %entry ]
ret double %retval
; CHECK-LABEL: define double @fcmp_oeq_zero(
; CHECK: %div = fdiv double %x, %y
}
define double @fcmp_une_zero(double %x, double %y) {
entry:
%cmp = fcmp une double %y, -0.0
br i1 %cmp, label %return, label %else
else:
%div = fdiv double %x, %y
br label %return
return:
%retval = phi double [ %div, %else ], [ %x, %entry ]
ret double %retval
; CHECK-LABEL: define double @fcmp_une_zero(
; CHECK: %div = fdiv double %x, %y
}