Allow FAST_FUNC to be overridden at build time

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>

1 files changed, 10 insertions, 5 deletions

diff --git a/include/platform.h b/include/platform.h
index c987d418c..6c7d03dc7 100644
--- a/include/platform.h
+++ b/include/platform.h
@@ -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) */