linux-kernel - UML and fastcall/FASTCALL

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Date:	Fri, 8 Dec 2006 13:59:28 +0100
From:	Adrian Bunk <bunk@...sta.de>
To:	jdike@...aya.com
Cc:	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: UML and fastcall/FASTCALL

UML on i386 is now the only case where fastcall/FASTCALL is not a noop.

There are two use cases for fastcall/FASTCALL in UML on i386:

1. optimization for C code
A faster calling convention is used for the functions annotated this way.

2. interfacing with assembler code
But include/asm-um/linkage.h contains the following:

<--  snip  -->

#ifndef __ASM_UM_LINKAGE_H
#define __ASM_UM_LINKAGE_H

#include "asm/arch/linkage.h"

/* <linux/linkage.h> will pick sane defaults */
#ifdef CONFIG_GPROF
#undef FASTCALL
#undef fastcall
#endif

#endif

<--  snip  -->

E.g. if CONFIG_SMP was still available on UML, CONFIG_SMP=y, 
CONFIG_GPROF=y would have some horrible effects when calling the 
functions in arch/i386/lib/semaphore.S.

Are there any benchmark numbers that the existing fastcall/FASTCALL 
annotations in the kernel really make a measurable difference for
C code?

Otherwise, we could use it only for assembler code using this calling 
convention (if there is any used by UML) - and CONFIG_GPROF mustn't 
change this.

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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