plasmacel - 1 year ago 66

C++ Question

I'm experimenting with the generated assembly and found an interesting thing.

There are two function doing an identical computation. The only difference between them is the way how the results are summed together.

`#include <cmath>`

double func1(double x, double y)

{

double result1;

double result2;

if (x*x < 0.0) result1 = 0.0;

else

{

result1 = x*x+x+y;

}

if (y*y < 0.0) result2 = 0.0;

else

{

result2 = y*y+y+x;

}

return (result1 + result2) * 40.0;

}

double func2(double x, double y)

{

double result = 0.0;

if (x*x >= 0.0)

{

result += x*x+x+y;

}

if (y*y >= 0.0)

{

result += y*y+y+x;

}

return result * 40.0;

}

The assembly generated by x86 clang 3.7 with

`-O2`

`.LCPI0_0:`

.quad 4630826316843712512 # double 40

func1(double, double): # @func1(double, double)

movapd %xmm0, %xmm2

mulsd %xmm2, %xmm2

addsd %xmm0, %xmm2

addsd %xmm1, %xmm2

movapd %xmm1, %xmm3

mulsd %xmm3, %xmm3

addsd %xmm1, %xmm3

addsd %xmm0, %xmm3

addsd %xmm3, %xmm2

mulsd .LCPI0_0(%rip), %xmm2

movapd %xmm2, %xmm0

retq

.LCPI1_0:

.quad 4630826316843712512 # double 40

func2(double, double): # @func2(double, double)

movapd %xmm0, %xmm2

movapd %xmm2, %xmm4

mulsd %xmm4, %xmm4

xorps %xmm3, %xmm3

ucomisd %xmm3, %xmm4

xorpd %xmm0, %xmm0

jb .LBB1_2

addsd %xmm2, %xmm4

addsd %xmm1, %xmm4

xorpd %xmm0, %xmm0

addsd %xmm4, %xmm0

.LBB1_2:

movapd %xmm1, %xmm4

mulsd %xmm4, %xmm4

ucomisd %xmm3, %xmm4

jb .LBB1_4

addsd %xmm1, %xmm4

addsd %xmm2, %xmm4

addsd %xmm4, %xmm0

.LBB1_4:

mulsd .LCPI1_0(%rip), %xmm0

retq

`func1`

`func2`

`func2`

`func1`

Can someone explain this behavior?

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Answer Source

The reason for this behaviour of the comparison operators `<`

or `>=`

differs whether your `double`

is `NaN`

or not a `NaN`

. All comparisons where one of the operands is `NaN`

return `false`

. So your `x*x < 0.0`

will *always* be false regardless of whether `x`

is `NaN`

or not. So the compiler can safely optimize this away. However, the case of `x * x >= 0`

will behave differently for `NaN`

and non-`NaN`

values, thus the compiler leaves the conditional jumps in the assembly.

This is what cppreference says about comparing with NaNs involved:

the values of the operands after conversion are compared in the usual mathematical sense (except that positive and negative zeroes compare equal and any comparison involving a NaN value returns zero)

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