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Allow FAST_FUNC to be overridden at build time

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Busybox uses FAST_FUNC macro to tweak with IA-32 calling conventions in
order to make the function call slightly smaller or slightly faster.
However, when I experiment with GCC's LTO (Link Time Optimization), I
discovered that FAST_FUNC could hinder LTO's optimization so that the
resulting executable become a few bytes larger (than what is compiled
without FAST_FUNC).

This change allows to specify e.g.
	CONFIG_EXTRA_CFLAGS="-DFAST_FUNC= -flto"
and compile with LTO without a source code hack.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Kang-Che Sung 2017-01-05 09:25:03 +08:00 committed by Denys Vlasenko
parent 61a91af63d
commit f10f7a21d4
1 changed files with 10 additions and 5 deletions
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15
include/platform.h
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@ -108,13 +108,18 @@
* and/or smaller by using modified ABI. It is usually only needed
* on non-static, busybox internal functions. Recent versions of gcc
* optimize statics automatically. FAST_FUNC on static is required
* only if you need to match a function pointer's type */
#if __GNUC_PREREQ(3,0) && defined(i386) /* || defined(__x86_64__)? */
* only if you need to match a function pointer's type.
* FAST_FUNC may not work well with -flto so allow user to disable this.
* (-DFAST_FUNC= )
*/
#ifndef FAST_FUNC
# if __GNUC_PREREQ(3,0) && defined(i386)
/* stdcall makes callee to pop arguments from stack, not caller */
# define FAST_FUNC __attribute__((regparm(3),stdcall))
# define FAST_FUNC __attribute__((regparm(3),stdcall))
/* #elif ... - add your favorite arch today! */
#else
# define FAST_FUNC
# else
# define FAST_FUNC
# endif
#endif
/* Make all declarations hidden (-fvisibility flag only affects definitions) */