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
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"Only a promise," Lao Er said.
Pearl S. Buck - Dragon SeedQuoted 1 line · Adrian Bunk
UML on i386 is now the only case where fastcall/FASTCALL is not a noop.
If i386 doesn't use it any more, then UML shouldn't either. The only reason there's support for it in UML is that I was copying i386.
Jeff
Signature · 11 lines
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Quoted 5 lines · Jeff Dike
Quoted 1 line · Adrian Bunk
UML on i386 is now the only case where fastcall/FASTCALL is not a noop.
If i386 doesn't use it any more, then UML shouldn't either. The only reason there's support for it in UML is that I was copying i386.
It's a noop on i386 due to -mregparm=3 ...
Quoted 1 line
Jeff
cu
Adrian
Signature · 4 lines
"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