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InstCombine: fix fold "fcmp x, undef" to account for NaN

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Summary:
See the two test cases.

; Can fold fcmp with undef on one side by choosing NaN for the undef

; Can fold fcmp with undef on both side
;   fcmp u_pred undef, undef -> true
;   fcmp o_pred undef, undef -> false
; because whatever you choose for the first undef
; you can choose NaN for the other undef

Reviewers: hfinkel, chandlerc, majnemer

Reviewed By: majnemer

Subscribers: majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D7617

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 231626
This commit is contained in:
Mehdi Amini 2015-03-09 03:20:25 +00:00
parent 112fafc026
commit 201b59d62e
4 changed files with 67 additions and 13 deletions
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10
include/llvm/IR/Instructions.h
View File

@ -1201,11 +1201,15 @@ public:
/// @returns true if the predicate of this instruction is EQ or NE.
/// \brief Determine if this is an equality predicate.
bool isEquality() const {
return getPredicate() == FCMP_OEQ || getPredicate() == FCMP_ONE ||
getPredicate() == FCMP_UEQ || getPredicate() == FCMP_UNE;
static bool isEquality(Predicate Pred) {
return Pred == FCMP_OEQ || Pred == FCMP_ONE || Pred == FCMP_UEQ ||
Pred == FCMP_UNE;
}
/// @returns true if the predicate of this instruction is EQ or NE.
/// \brief Determine if this is an equality predicate.
bool isEquality() const { return isEquality(getPredicate()); }
/// @returns true if the predicate of this instruction is commutative.
/// \brief Determine if this is a commutative predicate.
bool isCommutative() const {

9
lib/Analysis/InstructionSimplify.cpp
View File

@ -3054,8 +3054,13 @@ static Value *SimplifyFCmpInst(unsigned Predicate, Value *LHS, Value *RHS,
if (Pred == FCmpInst::FCMP_TRUE)
return ConstantInt::get(GetCompareTy(LHS), 1);
if (isa<UndefValue>(RHS)) // fcmp pred X, undef -> undef
return UndefValue::get(GetCompareTy(LHS));
// fcmp pred x, undef and fcmp pred undef, x
// fold to true if unordered, false if ordered
if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS)) {
// Choosing NaN for the undef will always make unordered comparison succeed
// and ordered comparison fail.
return ConstantInt::get(GetCompareTy(LHS), CmpInst::isUnordered(Pred));
}
// fcmp x,x -> true/false. Not all compares are foldable.
if (LHS == RHS) {

26
lib/IR/ConstantFold.cpp
View File

@ -1327,7 +1327,7 @@ static FCmpInst::Predicate evaluateFCmpRelation(Constant *V1, Constant *V2) {
if (!isa<ConstantExpr>(V1)) {
if (!isa<ConstantExpr>(V2)) {
// We distilled thisUse the standard constant folder for a few cases
// Simple case, use the standard constant folder.
ConstantInt *R = nullptr;
R = dyn_cast<ConstantInt>(
ConstantExpr::getFCmp(FCmpInst::FCMP_OEQ, V1, V2));
@ -1665,15 +1665,22 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
// Handle some degenerate cases first
if (isa<UndefValue>(C1) || isa<UndefValue>(C2)) {
CmpInst::Predicate Predicate = CmpInst::Predicate(pred);
bool isIntegerPredicate = ICmpInst::isIntPredicate(Predicate);
// For EQ and NE, we can always pick a value for the undef to make the
// predicate pass or fail, so we can return undef.
// Also, if both operands are undef, we can return undef.
if (ICmpInst::isEquality(ICmpInst::Predicate(pred)) ||
(isa<UndefValue>(C1) && isa<UndefValue>(C2)))
// Also, if both operands are undef, we can return undef for int comparison.
if (ICmpInst::isEquality(Predicate) || (isIntegerPredicate && C1 == C2))
return UndefValue::get(ResultTy);
// Otherwise, pick the same value as the non-undef operand, and fold
// it to true or false.
return ConstantInt::get(ResultTy, CmpInst::isTrueWhenEqual(pred));
// Otherwise, for integer compare, pick the same value as the non-undef
// operand, and fold it to true or false.
if (isIntegerPredicate)
return ConstantInt::get(ResultTy, CmpInst::isTrueWhenEqual(pred));
// Choosing NaN for the undef will always make unordered comparison succeed
// and ordered comparison fails.
return ConstantInt::get(ResultTy, CmpInst::isUnordered(Predicate));
}
// icmp eq/ne(null,GV) -> false/true
@ -1789,7 +1796,10 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
return ConstantVector::get(ResElts);
}
if (C1->getType()->isFloatingPointTy()) {
if (C1->getType()->isFloatingPointTy() &&
// Only call evaluateFCmpRelation if we have a constant expr to avoid
// infinite recursive loop
(isa<ConstantExpr>(C1) || isa<ConstantExpr>(C2))) {
int Result = -1; // -1 = unknown, 0 = known false, 1 = known true.
switch (evaluateFCmpRelation(C1, C2)) {
default: llvm_unreachable("Unknown relation!");

35
test/Transforms/InstCombine/fcmp.ll
View File

@ -240,3 +240,38 @@ define i32 @test17(double %a, double (double)* %p) nounwind {
%conv = zext i1 %cmp to i32
ret i32 %conv
}
; Can fold fcmp with undef on one side by choosing NaN for the undef
define i32 @test18_undef_unordered(float %a) nounwind {
; CHECK-LABEL: @test18_undef_unordered
; CHECK: ret i32 1
%cmp = fcmp ueq float %a, undef
%conv = zext i1 %cmp to i32
ret i32 %conv
}
; Can fold fcmp with undef on one side by choosing NaN for the undef
define i32 @test18_undef_ordered(float %a) nounwind {
; CHECK-LABEL: @test18_undef_ordered
; CHECK: ret i32 0
%cmp = fcmp oeq float %a, undef
%conv = zext i1 %cmp to i32
ret i32 %conv
}
; Can fold fcmp with undef on both side
; fcmp u_pred undef, undef -> true
; fcmp o_pred undef, undef -> false
; because whatever you choose for the first undef
; you can choose NaN for the other undef
define i1 @test19_undef_unordered() nounwind {
; CHECK-LABEL: @test19_undef
; CHECK: ret i1 true
%cmp = fcmp ueq float undef, undef
ret i1 %cmp
}
define i1 @test19_undef_ordered() nounwind {
; CHECK-LABEL: @test19_undef
; CHECK: ret i1 false
%cmp = fcmp oeq float undef, undef
ret i1 %cmp
}