Re: [PATCH] powerpc/boot: Fix build failure since GCC 4.9 removal

[Guenter Roeck]

On Tue, Sep 14, 2021 at 10:17:23PM +1000, Michael Ellerman wrote:
> Stephen reported that the build was broken since commit
> 6d2ef226f2f1 ("compiler_attributes.h: drop __has_attribute() support for
> gcc4"), with errors such as:
> 
>   include/linux/compiler_attributes.h:296:5: warning: "__has_attribute" is not defined, evaluates to 0 [-Wundef]
>     296 | #if __has_attribute(__warning__)
>         |     ^~~~~~~~~~~~~~~
>   make[2]: *** [arch/powerpc/boot/Makefile:225: arch/powerpc/boot/crt0.o] Error 1
> 
> But we expect __has_attribute() to always be defined now that we've
> stopped using GCC 4.
> 
> Linus debugged it to the point of reading the GCC sources, and noticing
> that the problem is that __has_attribute() is not defined when
> preprocessing assembly files, which is what we're doing here.
> 
> Our assembly files don't include, or need, compiler_attributes.h, but
> they are getting it unconditionally from the -include in BOOT_CFLAGS,
> which is then added in its entirety to BOOT_AFLAGS.
> 
> That -include was added in commit 77433830ed16 ("powerpc: boot: include
> compiler_attributes.h") so that we'd have "fallthrough" and other
> attributes defined for the C files in arch/powerpc/boot. But it's not
> needed for assembly files.
> 
> The minimal fix is to move the addition to BOOT_CFLAGS of -include
> compiler_attributes.h until after we've copied BOOT_CFLAGS into
> BOOT_AFLAGS. That avoids including compiler_attributes.h for asm files,
> but makes no other change to BOOT_CFLAGS or BOOT_AFLAGS.
> 
> Reported-by: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Debugged-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>

Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>

> ---
>  arch/powerpc/boot/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> 
> This seemed safer as a minimal fix, rather than doing a more
> comprehensive separation of CFLAGS/AFLAGS. We can do that in a future
> patch.
> 
> It passed my usual build/boot tests, including booting the built zImage
> on some real hardware, so this is good to go from my POV.
> 
> cheers
> 
> diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
> index 6900d0ac2421..089ee3ea55c8 100644
> --- a/arch/powerpc/boot/Makefile
> +++ b/arch/powerpc/boot/Makefile
> @@ -35,7 +35,6 @@ endif
>  BOOTCFLAGS    := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
>  		 -fno-strict-aliasing -O2 -msoft-float -mno-altivec -mno-vsx \
>  		 -pipe -fomit-frame-pointer -fno-builtin -fPIC -nostdinc \
> -		 -include $(srctree)/include/linux/compiler_attributes.h \
>  		 $(LINUXINCLUDE)
>  
>  ifdef CONFIG_PPC64_BOOT_WRAPPER
> @@ -70,6 +69,7 @@ ifeq ($(call cc-option-yn, -fstack-protector),y)
>  BOOTCFLAGS	+= -fno-stack-protector
>  endif
>  
> +BOOTCFLAGS	+= -include $(srctree)/include/linux/compiler_attributes.h
>  BOOTCFLAGS	+= -I$(objtree)/$(obj) -I$(srctree)/$(obj)
>  
>  DTC_FLAGS	?= -p 1024
> -- 
> 2.25.1
>