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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