On Wed, Aug 22, 2007 at 11:10:29AM -0700, Andrew Morton wrote:
[...]
> > CC arch/x86_64/kernel/asm-offsets.s
> > arch/x86_64/kernel/asm-offsets.c:1: error: -mpreferred-stack-boundary=3
> > is not between 4 and 12
> > make[1]: *** [arch/x86_64/kernel/asm-offsets.s] Error 1
[...]
> x86_64-mm-less-stack-alignment.patch has
>
> cflags-y += $(call cc-option,-mpreferred-stack-boundary=3)
>
> So we _should_ have detected that gcc didn't like =3, so it
> should not have been used.
>
> I am suspecting a kbuild glitch: asm-offsets.c tends to be handled
> in special ways (ie: it's usually the thing which blows up first)
> so perhaps it is somehow avoiding the above does-gcc-support-this test.
>
> Suitable cc's added ;)
It seems that this is a problem caused by the way we check for
compiler options in x86_64. Each compiler flag is checked for
individually and if available added to cflags-y, later that is
added to CFLAGS. However, this means that each flag is checked
in total isolation. On x86_64 (on this compiler at least) the
-mpreferred-stack-boundary and -m{32,64} flags are actually mutually
dependant, the alignment constraints vary based on the word size.
This leads to the compile failure:
# gcc -mpreferred-stack-boundary=3 -S -xc /dev/null -o FOO
# echo $?
0
# gcc -m64 -mpreferred-stack-boundary=3 -S -xc /dev/null -o FOO
/dev/null:1: error: -mpreferred-stack-boundary=3 is not between 4 and 12
# echo $?
1
In the main Makefile we always add each flag directly to CFLAGS
which means we check them all in combination, perhaps this is
prudent here also? Either way I suspect that changing the -m64
check to add itself directly to CFLAGS will fix this us.
-apw
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