Do not check for equality

[pavlinux]

Do not check for equality a and b,
if ( a = b )

then

return (a - b) # a - b = 0 :)

else

return 0 # same

a - b = 0 if a = b :)

[pavlinux]

if (a >= b )

0: 89 fa mov %edi,%edx
2: 31 c0 xor %eax,%eax
4: 29 f2 sub %esi,%edx
6: 48 39 f7 cmp %rsi,%rdi
9: 0f 4f c2 cmovge %edx,%eax
c: c3 retq

if ( b < a )

10: 89 fa mov %edi,%edx
12: 31 c0 xor %eax,%eax
14: 29 f2 sub %esi,%edx
16: 48 39 f7 cmp %rsi,%rdi
19: 0f 4d c2 cmovl %edx,%eax
1c: c3 retq

Move condition

CMOVGE = ˜(SF ˆ OF) & ˜ZF == NOT ( SF XOR OF ) AND ( NOT ZF)
CMOVL = SF ˆ OF == SF XOR OF

[zaytsev]

What is the difference?

[zaytsev]

I mean it's still one command?

[pavlinux]

Replying to zaytsev:

I mean it's still one command?

CMOVGE = ˜(SF ˆ OF) & ˜ZF
CMOVL = SF ˆ OF

CMOVL not use "ZF" flag register, minus 3 logic operations, at same С logic

if ( a > b ) == if ( b < a) ?

It is ASSEMBLER's OPTIMIZATION =)

[zaytsev]

I understand that it does not use ZF, but my question was is it still executed in one (or the same amount of) clock tick(s) or not?

If yes, then why even care about that?

[pavlinux]

Replying to zaytsev:

I understand that it does not use ZF, but my question was is it still executed in one (or the same amount of) clock tick(s) or not?

If yes, then why even care about that?

CMOVGE - performed for 450 CPU cycles
CMOVL - about 100

The probability of coincidence of 100 cycles of 1 clock tick, 4 times greater than 450 cycles.

[zaytsev]

Wow! is all I can say...

[pavlinux]

Software Optimization Guide for AMD64 Processors

​http://support.amd.com/us/Processor_TechDocs/25112.PDF

page 130

[pavlinux]

Replying to zaytsev:

Wow! is all I can say...

~ 350 nanosec faster :)

[ossi]

Replying to pavlinux:

Replying to zaytsev:

I understand that it does not use ZF, but my question was is it still executed in one (or the same amount of) clock tick(s) or not?

If yes, then why even care about that?

CMOVGE - performed for 450 CPU cycles
CMOVL - about 100

The probability of coincidence of 100 cycles of 1 clock tick, 4 times greater than 450 cycles.

it's beyond me what you mean by that ...

[pavlinux]

it's beyond me what you mean by that ...

Забей... Не нужно это людям... cmovge, cmovle... придумают тоже...
галактико в опасности, а тут наносекунды экономить...

[ossi]

sounds terribly philosophical to me. mind a somewhat more technical explanation? :D
(i find it interesting irrespective of the total irrelevance to mc).

[andrew_b]

Strict inequality (a<=b) is slower than non-strict one (a>b). :)

[andrew_b]

Replying to andrew_b:

Strict inequality (a<=b) is slower than non-strict one (a>b). :)

Oh, no-no-no. :) Forget this. :) Of cause, I mean that non-strict inequality (a<=b) is slower than strict one (a>b).

[slyfox]

You seem to hit wrong bugzilla: ​http://gcc.gnu.org/bugzilla/
Pipe stall graphs are "welcome" though ;]

[pavlinux]

Replying to andrew_b:

Replying to andrew_b:

Strict inequality (a<=b) is slower than non-strict one (a>b). :)

Oh, no-no-no. :) Forget this. :) Of cause, I mean that non-strict inequality (a<=b) is slower than strict one (a>b).

Not ( a <= b ) faster then ( a > b ), but ( b < a ) faster then ( a > b ) :)