Missed optimization in math expression: sqrt(a) * sqrt(b) == sqrt(a*b)

[zamazan4ik]

Bugzilla Link	35604
Resolution	FIXED
Resolved on	Feb 03, 2019 23:23
Version	trunk
OS	All
Blocks	#34959
CC	@hfinkel,@rotateright

Extended Description

clang(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags for this code:

#include <cmath>

double test(double a, double b)
{
    return sqrt(a) * sqrt(b);
}

generates this assembly:

test(double, double, double): # @test(double, double, double)
  vsqrtsd xmm0, xmm0, xmm0
  vsqrtsd xmm1, xmm1, xmm1
  vmulsd xmm0, xmm1, xmm0
  ret

gcc(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags generates this:

test(double, double, double):
        vmulsd  xmm0, xmm0, xmm1
        vsqrtsd xmm0, xmm0, xmm0
        ret

Also if you will increase count of multiplication on sqrt(varible_name), gcc continues generate more optimal code:

#include <cmath>

double test(double a, double b, double c)
{
    return sqrt(a) * sqrt(b) * sqrt(c);
}

clang:

test(double, double, double): # @test(double, double, double)
  vsqrtsd xmm0, xmm0, xmm0
  vsqrtsd xmm1, xmm1, xmm1
  vmulsd xmm0, xmm1, xmm0
  vsqrtsd xmm1, xmm2, xmm2
  vmulsd xmm0, xmm0, xmm1
  ret

gcc:

test(double, double, double):
        vmulsd  xmm2, xmm1, xmm2
        vmulsd  xmm0, xmm2, xmm0
        vsqrtsd xmm0, xmm0, xmm0
        ret

[zamazan4ik]

See a lot of optimizations here: https://github.com/gcc-mirror/gcc/blob/07b69d3f1cd3dd8ebb0af1fbff95914daee477d2/gcc/match.pd

[llvmbot]

Thanks for all these reports. May I ask you to open a meta/umbrella ticket, something like "missing fast-math optimizations" and link all them there? I think I might consider picking up some. Thanks!

[hfinkel]

Thanks for all these reports. May I ask you to open a meta/umbrella ticket,
something like "missing fast-math optimizations" and link all them there? I
think I might consider picking up some. Thanks!

#34959

[llvmbot]

Fixed in https://reviews.llvm.org/D41322. Commit: rL321637. See: https://godbolt.org/z/Ffks6m

[RKSimon]

mentioned in issue #34959