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[InstCombine] Remove hasNoInfs check for pow(C,y) -> exp2(log2(C)*y)

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We already check hasNoNaNs and that x is finite and strictly positive.
That only leaves the following special cases (taken from the Linux man
page for pow):

If x is +1, the result is 1.0 (even if y is a NaN).
If the absolute value of x is less than 1, and y is negative infinity, the result is positive infinity.
If the absolute value of x is greater than 1, and y is negative infinity, the result is +0.
If the absolute value of x is less than 1, and y is positive infinity, the result is +0.
If the absolute value of x is greater than 1, and y is positive infinity, the result is positive infinity.

The first case is handled elsewhere, and this transformation preserves
all the others, so there is no need to limit it to hasNoInfs.

Differential Revision: https://reviews.llvm.org/D79409
This commit is contained in:
Jay Foad 2020-05-05 13:00:48 +01:00
parent 4dad997477
commit ed678c9f29
2 changed files with 11 additions and 5 deletions
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11
lib/Transforms/Utils/SimplifyLibCalls.cpp
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@ -1563,9 +1563,14 @@ Value *LibCallSimplifier::replacePowWithExp(CallInst *Pow, IRBuilderBase &B) {
return emitUnaryFloatFnCall(Expo, TLI, LibFunc_exp10, LibFunc_exp10f,
LibFunc_exp10l, B, Attrs);
// pow(n, x) -> exp2(log2(n) * x)
if (Pow->hasApproxFunc() && Pow->hasNoNaNs() && Pow->hasNoInfs() &&
BaseF->isFiniteNonZero() && !BaseF->isNegative()) {
// pow(x, y) -> exp2(log2(x) * y)
if (Pow->hasApproxFunc() && Pow->hasNoNaNs() && BaseF->isFiniteNonZero() &&
!BaseF->isNegative()) {
// pow(1, inf) is defined to be 1 but exp2(log2(1) * inf) evaluates to NaN.
// Luckily optimizePow has already handled the x == 1 case.
assert(!match(Base, m_FPOne()) &&
"pow(1.0, y) should have been simplified earlier!");
Value *Log = nullptr;
if (Ty->isFloatTy())
Log = ConstantFP::get(Ty, std::log2(BaseF->convertToFloat()));

5
test/Transforms/InstCombine/pow-exp.ll
View File

@ -400,8 +400,9 @@ define double @pow_ok_base_no_nnan(double %e) {
define double @pow_ok_base_no_ninf(double %e) {
; CHECK-LABEL: @pow_ok_base_no_ninf(
; CHECK-NEXT: [[CALL:%.*]] = tail call nnan afn double @pow(double 0x3FE6666666666666, double [[E:%.*]])
; CHECK-NEXT: ret double [[CALL]]
; CHECK-NEXT: [[MUL:%.*]] = fmul nnan afn double [[E:%.*]], 0xBFE0776{{.*}}
; CHECK-NEXT: [[EXP2:%.*]] = call nnan afn double @exp2(double [[MUL]])
; CHECK-NEXT: ret double [[EXP2]]
;
%call = tail call afn nnan double @pow(double 0x3FE6666666666666, double %e)
ret double %call