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Optimize square root squared (PR21126).

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When unsafe-fp-math is enabled, we can turn sqrt(X) * sqrt(X) into X.

This can happen in the real world when calculating x ** 3/2. This occurs
in test-suite/SingleSource/Benchmarks/BenchmarkGame/n-body.c.

Differential Revision: http://reviews.llvm.org/D5584



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218906 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2014-10-02 21:10:54 +00:00
parent 130d072eb7
commit 0ff4eb684f
2 changed files with 34 additions and 0 deletions
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5
lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -531,6 +531,11 @@ Instruction *InstCombiner::visitFMul(BinaryOperator &I) {
}
}
// sqrt(X) * sqrt(X) -> X
if (AllowReassociate && (Op0 == Op1))
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Op0))
if (II->getIntrinsicID() == Intrinsic::sqrt)
return ReplaceInstUsesWith(I, II->getOperand(0));
// Under unsafe algebra do:
// X * log2(0.5*Y) = X*log2(Y) - X

29
test/Transforms/InstCombine/fmul.ll
View File

@ -123,3 +123,32 @@ define float @test11(float %x, float %y) {
; CHECK-NOT: fadd float
; CHECK: fadd fast float
}
; PR21126: http://llvm.org/bugs/show_bug.cgi?id=21126
; With unsafe/fast math, sqrt(X) * sqrt(X) is just X.
declare double @llvm.sqrt.f64(double)
define double @sqrt_squared1(double %f) #0 {
%sqrt = call double @llvm.sqrt.f64(double %f)
%mul = fmul fast double %sqrt, %sqrt
ret double %mul
; CHECK-LABEL: @sqrt_squared1(
; CHECK-NEXT: ret double %f
}
; With unsafe/fast math, sqrt(X) * sqrt(X) is just X,
; but make sure another use of the sqrt is intact.
; Note that the remaining fmul is altered but is not 'fast'
; itself because it was not marked 'fast' originally.
; Thus, we have an overall fast result, but no more indication of
; 'fast'ness in the code.
define double @sqrt_squared2(double %f) #0 {
%sqrt = call double @llvm.sqrt.f64(double %f)
%mul1 = fmul fast double %sqrt, %sqrt
%mul2 = fmul double %mul1, %sqrt
ret double %mul2
; CHECK-LABEL: @sqrt_squared2(
; CHECK-NEXT: %sqrt = call double @llvm.sqrt.f64(double %f)
; CHECK-NEXT: %mul2 = fmul double %sqrt, %f
; CHECK-NEXT: ret double %mul2
}