quick question, probably of little importance...

[Cecil Ward]

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