powerpc: Avoid load of static chain register when calling nested functions through...

The ppc64 ABI has a static chain register (r11) which is only used
when calling nested functions through a pointer. Considering that
we take a dim view of nested functions in the kernel, we have a lot
of unnecessary overhead here.

gcc 4.7 has an option to disable loading of r11 so lets use it.

If hell freezes over and hipsters manage to litter the kernel
with nested functions, gcc will give us an error message and
won't simply compile bad code:

    You cannot take the address of a nested function if you use
    the -mno-pointers-to-nested-functions option.

Furthermore our kernel module trampolines don't setup the static
chain register so adding this option and forcing gcc to error out
makes even more sense.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 31d8965e08478d9b1a7c6dc331ac67b191d69d0a..967fd23ace78bd5886e7a8c8b417afa58915b433 100644 (file)
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -85,6 +85,7 @@ endif
 
 CFLAGS-$(CONFIG_PPC64) := -mtraceback=no -mcall-aixdesc
 CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mcmodel=medium,-mminimal-toc)
+CFLAGS-$(CONFIG_PPC64) += $(call cc-option,-mno-pointers-to-nested-functions)
 CFLAGS-$(CONFIG_PPC32) := -ffixed-r2 -mmultiple
 
 CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=power7,-mtune=power4)