linux-kernel - Re: [PATCH 2/3] time: allow gcc to fold constants when using msecs_to

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Date:	Mon, 6 Apr 2015 09:21:50 +0200
From:	Nicholas Mc Guire <der.herr@...r.at>
To:	Joe Perches <joe@...ches.com>
Cc:	Nicholas Mc Guire <hofrat@...dl.org>,
	Michal Marek <mmarek@...e.cz>,
	Masahiro Yamada <yamada.m@...panasonic.com>,
	Sam Ravnborg <sam@...nborg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Alvin" <hpa@...or.com>,
	John Stultz <john.stultz@...aro.org>,
	Andrew Hunter <ahh@...gle.com>, Paul Turner <pjt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] time: allow gcc to fold constants when using
	msecs_to_jiffies

On Mon, 06 Apr 2015, Joe Perches wrote:

> On Mon, 2015-04-06 at 08:40 +0200, Nicholas Mc Guire wrote:
> > #define msecs_to_jiffies(m)                             \
> >   (__builtin_constant_p (m)                             \
> >   ? ((m) * HZ / MSECS_PER_SEC ) : __msecs_to_jiffies(m))
> []
> > main:
> > .LFB12:
> > 	.cfi_startproc
> > 	subq	$8, %rsp	#,
> > 	.cfi_def_cfa_offset 16
> > 	movl	$10, %esi	#,
> > 	movl	$.LC0, %edi	#,
> > 	xorl	%eax, %eax	#
> > 	call	printf	#
> 
> vs:
> 
> > static inline unsigned long msecs_to_jiffies(int m)
> > {
> >         return __builtin_constant_p (m) ?
> >                 (m) * HZ / MSECS_PER_SEC  : __msecs_to_jiffies(m);
> > }
> []
> > main:
> > .LFB13:
> > 	.cfi_startproc
> > 	subq	$8, %rsp	#,
> > 	.cfi_def_cfa_offset 16
> > 	xorl	%esi, %esi	#
> > 	movl	$.LC0, %edi	#,
> > 	xorl	%eax, %eax	#
> > 	call	printf	#
> > 
> > giving it another run from scratch somewhere I simply screwed up or 
> > overlooked some detail.
> 
> If the optimizer was doing it's job properly, wouldn't
> the macro and inline output object code be the same?
>
yes - and they are - that was my mistake I grabed the
wrong asm snippet - here is the complete test case 
also made a mess of the code while trimming down
the mail - so here is the single test case showing,
I think, that inline works as well and as expected.

testi.h:

#define HZ 100
#define MSECS_PER_SEC 1000
#define TIMEOUT 100

extern inline unsigned long __msecs_to_jiffies(int m);
unsigned long msecs_to_jiffies(int m)
{
	return __builtin_constant_p(m) ? ((m) * HZ / MSECS_PER_SEC ) : __msecs_to_jiffies(m);
}


test.c:

#include <stdio.h>
#include "testi.h"

unsigned long __msecs_to_jiffies(int m)
{
        return (m * HZ / MSECS_PER_SEC);
}

int main(int argc, char **argv) {
	//int m = atoi(argv[1]);
	int m = TIMEOUT;
	printf("%lu\n",msecs_to_jiffies(m));
	return 0;
}


compiled with:
gcc -O2 -S --verbose-asm test.c
<snip>
main:
.LFB13:
        .cfi_startproc
        subq    $8, %rsp        #,
        .cfi_def_cfa_offset 16
        movl    $10, %esi       #,
        movl    $.LC0, %edi     #,
        xorl    %eax, %eax      #
        call    printf  #
<snip>


need to cleanup here :)

thx!
hofrat
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