Re: [PATCH 06/14] powerpc/vdso: Remove unused '-s' flag from ASFLAGS

[Segher Boessenkool <segher@kernel.crashing.org>]

On Mon, Jan 09, 2023 at 03:37:53PM -0700, Nathan Chancellor wrote:
> On Mon, Jan 09, 2023 at 04:23:37PM -0600, Segher Boessenkool wrote:
> > Hi!  Happy new year all.
> > 
> > On Mon, Jan 09, 2023 at 01:58:32PM -0800, Nick Desaulniers wrote:
> > > On Wed, Jan 4, 2023 at 11:55 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > > >
> > > > When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, it
> > > > warns that ASFLAGS contains '-s', which is a linking phase option, so it
> > > > is unused.
> > > >
> > > >   clang-16: error: argument unused during compilation: '-s' [-Werror,-Wunused-command-line-argument]
> > > >
> > > > Looking at the GAS sources, '-s' is only useful when targeting Solaris
> > > > and it is ignored for the powerpc target so just drop the flag
> > > > altogether, as it is not needed.
> > > 
> > > Do you have any more info where you found this?  I don't see -s
> > > documented as an assembler flag.
> > > https://sourceware.org/binutils/docs/as/PowerPC_002dOpts.html
> > > https://sourceware.org/binutils/docs/as/Invoking.html
> > 
> > It is required by POSIX (for the c99 command, anyway).  It *also* is
> > required to be supported when producing object files (so when no linking
> > is done).
> > 
> > It is a GCC flag, and documented just fine:
> > https://gcc.gnu.org/onlinedocs/gcc/Link-Options.html#index-s
> > 
> > (Yes, that says it is for linking; but the option is allowed without
> > error of any kind always).
> > 
> > (ASFLAGS sounds like it is for assembler commands, but it really is
> > for compiler commands that just happen to get .S input files).
> > 
> > > The patch seems fine to me, but what was this ever supposed to be?
> > > FWICT it predates git history (looking at
> > > arch/powerpc/kernel/vdso32/Makefile at fc15351d9d63)
> > 
> > Yeah, good question.  This compiler flag does the moral equivalent of
> > strip -s (aka --strip-all).  Maybe this was needed at some point, or
> > the symbol or debug info was just annoying (during bringup or similar)?
> > 
> > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
> 
> Thank you for the review and extra explanation! I had kind of expanded
> on this in the s390 version of this patch [1], I will go ahead and do
> the same thing for the powerpc version too.
> 
> [1]: https://lore.kernel.org/llvm/20221228-drop-qunused-arguments-v1-9-658cbc8fc592@kernel.org/

The underwhelming GCC source code for this is
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/gcc.cc;h=d629ca5e424ad3120be13e82b9f38b7bf8f1cdf2;hb=HEAD#l1148
which says that the -s flag is passed through unmodified to the linker
when invoking the linker.  See #l603 for the %{s} specs syntax.

This is not a flag to the assembler at all.  It is a flag to the
compiler, which passes it on to the linker :-)

> > (ASFLAGS sounds like it is for assembler commands, but it really is
> > for compiler commands that just happen to get .S input files).


Segher