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 08:40:05 +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 Sun, 05 Apr 2015, Joe Perches wrote:

> On Mon, 2015-04-06 at 06:26 +0200, Nicholas Mc Guire wrote:
> > On Sun, 05 Apr 2015, Joe Perches wrote:
> > > Try it and look at the generated .lst files with and
> > > without the patch I sent.
> []
> > from all that I understood it should
> > be doable both as macro and inline.
> 
> I think it _should_ be doable too but I also think
> the only reason gcc doesn't optimize the inline
> is because gcc's optimizer isn't good enough yet.
> 

"unfortunately" I can't blame it on gcc - here is the initial toy-case 
- test.c and either testi.h or testm.h included
- m = TIMEOUT or m = atoi(argv[1]);
both in the inline and the macro case gcc reduced the code to a single 
load mediate or register instruction for the constant - so the optimizer
is doing its job.

test.c:
#include <stdio.h>
#define HZ 100
#define MSECS_PER_SEC 1000
#define TIMEOUT 100
#include "testi.h"	/* inline msecs_to_jiffies */
//#include "testm.h"	/* macro versions */

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

testm.h:

#define msecs_to_jiffies(m)                             \
  (__builtin_constant_p (m)                             \
  ? ((m) * HZ / MSECS_PER_SEC ) : __msecs_to_jiffies(m))

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

first case with a non-const
main:
.LFB12:
        .cfi_startproc
        subq    $8, %rsp        #,
        .cfi_def_cfa_offset 16
        movq    8(%rsi), %rdi   # MEM[(char * *)argv_2(D) + 8B], MEM[(char * *)argv_2(D) + 8B]
        xorl    %eax, %eax      #
        call    atoi    #
        movl    $1717986919, %edx       #, tmp69
        movl    %eax, %ecx      #, m
        movl    $.LC0, %edi     #,
        imull   %edx    # tmp69
        sarl    $31, %ecx       #, tmp71
        xorl    %eax, %eax      #
        sarl    $2, %edx        #, tmp67
        subl    %ecx, %edx      # tmp71, tmp67
        movslq  %edx, %rsi      # tmp67, tmp72
        call    printf  #

o
second with a constant:
main:
.LFB12:
	.cfi_startproc
	subq	$8, %rsp	#,
	.cfi_def_cfa_offset 16
	movl	$10, %esi	#,
	movl	$.LC0, %edi	#,
	xorl	%eax, %eax	#
	call	printf	#


inline:
-------

testi.h:
static inline unsigned long __msecs_to_jiffies(int m)
{
        return m * HZ / MSECS_PER_SEC;
}

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

first case with a non-const
main:
.LFB13:
	.cfi_startproc
	subq	$8, %rsp	#,
	.cfi_def_cfa_offset 16
	movq	(%rsi), %rdi	# *argv_1(D),
	xorl	%eax, %eax	#
	call	atoi	#
	movl	$1717986919, %edx	#, tmp68
	movl	%eax, %ecx	#, m
	movl	$.LC0, %edi	#,
	imull	%edx	# tmp68
	sarl	$31, %ecx	#, tmp70
	xorl	%eax, %eax	#
	sarl	$2, %edx	#, tmp66
	subl	%ecx, %edx	# tmp70, tmp66
	movslq	%edx, %rsi	# tmp66, tmp71
	call	printf	#

second with a constant:
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.

thx!
hofrat
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