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Reassociate x + -0.1234 * y into x - 0.1234 * y

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This does not require -ffast-math, and it gives CSE/GVN more options to
eliminate duplicate expressions in, e.g.:

  return ((x + 0.1234 * y) * (x - 0.1234 * y));

Differential Revision: http://reviews.llvm.org/D4904

llvm-svn: 216169
This commit is contained in:
Erik Verbruggen 2014-08-21 10:45:30 +00:00
parent c0369ec35d
commit 3f6db9cd35
4 changed files with 119 additions and 41 deletions
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38
lib/Target/README.txt
View File

@ -93,44 +93,6 @@ This requires reassociating to forms of expressions that are already available,
something that reassoc doesn't think about yet.
//===---------------------------------------------------------------------===//
This function: (derived from GCC PR19988)
double foo(double x, double y) {
return ((x + 0.1234 * y) * (x + -0.1234 * y));
}
compiles to:
_foo:
movapd %xmm1, %xmm2
mulsd LCPI1_1(%rip), %xmm1
mulsd LCPI1_0(%rip), %xmm2
addsd %xmm0, %xmm1
addsd %xmm0, %xmm2
movapd %xmm1, %xmm0
mulsd %xmm2, %xmm0
ret
Reassociate should be able to turn it into:
double foo(double x, double y) {
return ((x + 0.1234 * y) * (x - 0.1234 * y));
}
Which allows the multiply by constant to be CSE'd, producing:
_foo:
mulsd LCPI1_0(%rip), %xmm1
movapd %xmm1, %xmm2
addsd %xmm0, %xmm2
subsd %xmm1, %xmm0
mulsd %xmm2, %xmm0
ret
This doesn't need -ffast-math support at all. This is particularly bad because
the llvm-gcc frontend is canonicalizing the later into the former, but clang
doesn't have this problem.
//===---------------------------------------------------------------------===//
These two functions should generate the same code on big-endian systems:

51
lib/Transforms/Scalar/Reassociate.cpp
View File

@ -193,6 +193,8 @@ namespace {
Value *OptimizeMul(BinaryOperator *I, SmallVectorImpl<ValueEntry> &Ops);
Value *RemoveFactorFromExpression(Value *V, Value *Factor);
void EraseInst(Instruction *I);
void optimizeFAddNegExpr(ConstantFP *ConstOperand, Instruction *I,
int OperandNr);
void OptimizeInst(Instruction *I);
};
}
@ -1914,6 +1916,33 @@ void Reassociate::EraseInst(Instruction *I) {
}
}
void Reassociate::optimizeFAddNegExpr(ConstantFP *ConstOperand, Instruction *I,
int OperandNr) {
// Change the sign of the constant.
APFloat Val = ConstOperand->getValueAPF();
Val.changeSign();
I->setOperand(0, ConstantFP::get(ConstOperand->getContext(), Val));
assert(I->hasOneUse() && "Only a single use can be replaced.");
Instruction *Parent = I->user_back();
Value *OtherOperand = Parent->getOperand(1 - OperandNr);
unsigned Opcode = Parent->getOpcode();
assert(Opcode == Instruction::FAdd ||
(Opcode == Instruction::FSub && Parent->getOperand(1) == I));
BinaryOperator *NI = Opcode == Instruction::FAdd
? BinaryOperator::CreateFSub(OtherOperand, I)
: BinaryOperator::CreateFAdd(OtherOperand, I);
NI->setFastMathFlags(cast<FPMathOperator>(Parent)->getFastMathFlags());
NI->insertBefore(Parent);
NI->setName(Parent->getName() + ".repl");
Parent->replaceAllUsesWith(NI);
NI->setDebugLoc(I->getDebugLoc());
MadeChange = true;
}
/// OptimizeInst - Inspect and optimize the given instruction. Note that erasing
/// instructions is not allowed.
void Reassociate::OptimizeInst(Instruction *I) {
@ -1940,8 +1969,8 @@ void Reassociate::OptimizeInst(Instruction *I) {
if (I->getType()->isFloatingPointTy() || I->getType()->isVectorTy()) {
// FAdd and FMul can be commuted.
if (I->getOpcode() == Instruction::FMul ||
I->getOpcode() == Instruction::FAdd) {
unsigned Opcode = I->getOpcode();
if (Opcode == Instruction::FMul || Opcode == Instruction::FAdd) {
Value *LHS = I->getOperand(0);
Value *RHS = I->getOperand(1);
unsigned LHSRank = getRank(LHS);
@ -1954,6 +1983,24 @@ void Reassociate::OptimizeInst(Instruction *I) {
}
}
// Reassociate: x + -ConstantFP * y -> x - ConstantFP * y
// The FMul can also be an FDiv, and FAdd can be a FSub.
if (Opcode == Instruction::FMul || Opcode == Instruction::FDiv) {
if (ConstantFP *LHSConst = dyn_cast<ConstantFP>(I->getOperand(0))) {
if (LHSConst->isNegative() && I->hasOneUse()) {
Instruction *Parent = I->user_back();
if (Parent->getOpcode() == Instruction::FAdd) {
if (Parent->getOperand(0) == I)
optimizeFAddNegExpr(LHSConst, I, 0);
else if (Parent->getOperand(1) == I)
optimizeFAddNegExpr(LHSConst, I, 1);
} else if (Parent->getOpcode() == Instruction::FSub)
if (Parent->getOperand(1) == I)
optimizeFAddNegExpr(LHSConst, I, 1);
}
}
}
// FIXME: We should commute vector instructions as well. However, this
// requires further analysis to determine the effect on later passes.

2
test/Transforms/Reassociate/fast-basictest.ll
View File

@ -193,7 +193,7 @@ define float @test13(float %X1, float %X2, float %X3) {
define float @test14(float %X1, float %X2) {
; CHECK-LABEL: @test14
; CHECK-NEXT: fsub fast float %X1, %X2
; CHECK-NEXT: fmul fast float %tmp, 4.700000e+01
; CHECK-NEXT: fmul fast float %1, 4.700000e+01
; CHECK-NEXT: ret float
%B = fmul fast float %X1, 47. ; X1*47

69
test/Transforms/Reassociate/liftsign.ll Normal file
View File

@ -0,0 +1,69 @@
; RUN: opt -reassociate -gvn -S < %s | FileCheck %s
; (x + 0.1234 * y) * (x + -0.1234 * y) -> (x + 0.1234 * y) * (x - 0.1234 * y)
; so CSE can simplify it further
define double @lift_sign1(double %x, double %y) nounwind readnone ssp uwtable {
; CHECK-LABEL: @lift_sign1(
%mul = fmul double 1.234000e-01, %y
%add = fadd double %mul, %x
%mul1 = fmul double -1.234000e-01, %y
%add2 = fadd double %mul1, %x
%mul3 = fmul double %add, %add2
; CHECK-NOT: %mul1 = fmul double -1.234000e-01, %y
; CHECK-NOT: %add2 = fadd %mul1, %x
; CHECK: %add2.repl = fsub double %x, %mul
; CHECK: %mul3 = fmul double %add, %add2
ret double %mul3
}
; (x + -0.1234 * y) * (x + -0.1234 * y) -> (x - 0.1234 * y) * (x - 0.1234 * y)
; GVN can then rewrite it even further
define double @lift_sign2(double %x, double %y) nounwind readnone ssp uwtable {
; CHECK-LABEL: @lift_sign2(
%mul = fmul double %y, -1.234000e-01
%add = fadd double %mul, %x
%mul1 = fmul double %y, -1.234000e-01
%add2 = fadd double %mul1, %x
%mul3 = fmul double %add, %add2
; CHECK-NOT: %mul = fmul double %y, -1.234000e-01
; CHECK-NOT: %add = fadd double %mul, %x
; CHECK-NOT: %mul1 = fmul double %y, -1.234000e-01
; CHECK-NOT: %add2 = fadd double %mul1, %x
; CHECK-NOT: %mul3 = fmul double %add, %add2
; CHECK: %mul = fmul double 1.234000e-01, %y
; CHECK: %add.repl = fsub double %x, %mul
; CHECK: %mul3 = fmul double %add.repl, %add.repl
ret double %mul3
}
; (x + 0.1234 * y) * (x - -0.1234 * y) -> (x + 0.1234 * y) * (x + 0.1234 * y)
define double @lift_sign3(double %x, double %y) nounwind readnone ssp uwtable {
; CHECK-LABEL: @lift_sign3(
%mul = fmul double %y, 1.234000e-01
%add = fadd double %mul, %x
%mul1 = fmul double %y, -1.234000e-01
%add2 = fsub double %x, %mul1
%mul3 = fmul double %add, %add2
; CHECK-NOT: %mul1 = fmul double %y, -1.234000e-01
; CHECK-NOT: %add2 = fsub double %x, %mul1
; CHECK-NOT: %mul3 = fmul double %add, %add2
; CHECK: %mul3 = fmul double %add, %add
ret double %mul3
}
; (x + 0.1234 / y) * (x + -0.1234 / y) -> (x + 0.1234 / y) * (x - 0.1234 / y)
; so CSE can simplify it further
define double @lift_sign4(double %x, double %y) nounwind readnone ssp uwtable {
; CHECK-LABEL: @lift_sign4(
%div = fdiv double 1.234000e-01, %y
%add = fadd double %div, %x
%div1 = fdiv double -1.234000e-01, %y
%add2 = fadd double %div1, %x
%mul3 = fmul double %add, %add2
; CHECK-NOT: %div1 = fdiv double -1.234000e-01, %y
; CHECK-NOT: %add2 = fadd double %div1, %x
; CHECK-NOT: %mul3 = fmul double %add, %add2
; CHECK: %add2.repl = fsub double %x, %div
; CHECK: %mul3 = fmul double %add, %add2.repl
ret double %mul3
}