Re: quick question, probably of little importance...

[Cecil Ward via Digitalmars-d-learn]

On Wednesday, 26 April 2023 at 23:07:39 UTC, WhatMeWorry wrote:
On Wednesday, 26 April 2023 at 23:02:07 UTC, Richard (Rikki) 
Andrew Cattermole wrote:
Don't forget ``num % 2 == 0``.
None should matter, pretty much all production compilers 
within the last 30 years should recognize all forms of this 
and do the right thing.

Thanks. Fastest reply ever! And I believe across the world?   I 
suppose my examples required overhead of a function call. So 
maybe num % 2 == 0 is fastest?

I made a small change, making the retval a bool rather than an 
int. I got slightly better code generation with the int, as it 
seems that some of the compilers have not yet got all the good 
tricks they should be using when manipulating bool-typed 
expressions and also it can be one extra instruction converting 
values to bool strictly zero or one, not zero or any non-zero 
value. Here’s the D, enlarged a little so that we can see your 
routine in action, inlined. Your isEven boils down to two 
instructions with a seriously optimising compiler. I’ve included 
the x86-64 machine code generated by the GDC and LDC compilers so 
you can see how fast it is. GDC made a bit of a dog’s breakfast 
of my longer routine whereas LDC performed superbly. GDC 
generated twice as much code, but its excellent instruction 
scheduler and what looks like an awareness of ILP mean that the 
two streams of instructions will be carried out in parallel  so 
the two streams will only take three instruction times - ie 
whatever the total time is for those three instruction in the one 
stream - not six.

bool isEven( int num )
        {
        return ! ( num & 1 );
        }

bool AreBothEven( int a, int b )  // returns true if both 
arguments are even
        {
        return isEven( a )  &&  isEven( b );
        }

===
Compiler output:: GDC:: x86-64: -O3 -mcpu=native -frelease

bool isEven( int ):
        mov     eax, edi
        not             eax
        and             eax, 1
        ret

bool AreBothEven( int, int ):
        mov     eax, edi
        not             esi
        not             eax
        and             esi, 1
        and             eax, 1
        cmovne  eax, esi
        ret

===
Compiler LDC: x86-64: -O3 -mcpu=native -release

bool isEven( int ):
        test            dil, 1
        sete    al
        ret

bool AreBothEven( int, int ):
        or              edi, esi
        test            dil, 1
        sete    al
        ret