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

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Date:	Mon, 06 Apr 2015 00:12:46 -0700
From:	Joe Perches <joe@...ches.com>
To:	Nicholas Mc Guire <der.herr@...r.at>
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, 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?


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