Re: [v3] powerpc/Makefile: Fix PPC_BOOK3S_64 ASFLAGS

[Michael Ellerman <patch-notifications@ellerman.id.au>]

On Thu, 2018-10-11 at 02:43:03 UTC, Joel Stanley wrote:
> Ever since commit 15a3204d24a3 ("powerpc/64s: Set assembler machine type
> to POWER4") we force -mpower4 to be passed to the assembler irrespective
> of the CFLAGS used.
> 
> When building a powerpc64 kernel with clang, clang will not add -many to
> the assembler flags, so any instructions that the compiler has generated
> that are not available on power4 will cause an error:
> 
>  /usr/bin/as -a64 -mppc64 -mlittle-endian -mpower8 \
>   -I ./arch/powerpc/include -I ./arch/powerpc/include/generated \
>   -I ./include -I ./arch/powerpc/include/uapi \
>   -I ./arch/powerpc/include/generated/uapi -I ./include/uapi \
>   -I ./include/generated/uapi -I arch/powerpc -I arch/powerpc \
>   -maltivec -mpower4 -o init/do_mounts.o /tmp/do_mounts-3b0a3d.s
>  /tmp/do_mounts-51ce54.s:748: Error: unrecognized opcode: `isel'
> 
> GCC does include -many, so the GCC driven gas call will succeed:
> 
>   as -v -I ./arch/powerpc/include -I ./arch/powerpc/include/generated -I
>   ./include -I ./arch/powerpc/include/uapi
>   -I ./arch/powerpc/include/generated/uapi -I ./include/uapi
>   -I ./include/generated/uapi -I arch/powerpc -I arch/powerpc
>    -a64 -mpower8 -many -mlittle -maltivec -mpower4 -o init/do_mounts.o
> 
> Note that isel is power7 and above for IBM CPUs. GCC only generates it
> for Power9 and above, but the above test was run against the clang
> generated assembly.
> 
> Peter Bergner explains:
> 
>  > When using -many -mpower4, gas will first try and find a matching
>  > power4 mnemonic and failing that, it will then allow any valid mnemonic
>  > that gas knows about.  GCC's use of -many predates me though.
>  >
>  > IIRC, Alan looked at trying to remove it, but I forget why he didn't.
>  > Could be either a gcc or gas issue at the time.  I'm not sure whether
>  > issue still exists or not.  He and I have modified how gas works
>  > internally a fair amount since he tried removing gcc use of -many
>  >
>  > I will also note that when using -many, gas will choose the first
>  > mnemonic that matches in the mnemonic table and we have (mostly) sorted
>  > the table so that server mnemonics show up earlier in the table than
>  > other mnemonics, so they'll be seen/chosen first
> 
> By explicitly setting -many we can build with Clang and GCC while
> retaining the -mpower4 option.
> 
> Signed-off-by: Joel Stanley <joel@jms.id.au>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/960e30029863db95ec79a71009272d

cheers