RE: [PATCH 6/6]AArch64: only emit mismatch error when features would be disabled.

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Earnshaw <Richard.Earnshaw@foss.arm.com>
> Sent: Thursday, November 16, 2023 9:27 AM
> To: Tamar Christina <Tamar.Christina@arm.com>; gcc-patches@gcc.gnu.org
> Cc: nd <nd@arm.com>; Richard Earnshaw <Richard.Earnshaw@arm.com>;
> Marcus Shawcroft <Marcus.Shawcroft@arm.com>; Kyrylo Tkachov
> <Kyrylo.Tkachov@arm.com>; Richard Sandiford
> <Richard.Sandiford@arm.com>
> Subject: Re: [PATCH 6/6]AArch64: only emit mismatch error when features
> would be disabled.
> 
> 
> 
> On 15/11/2023 17:08, Tamar Christina wrote:
> > Hi All,
> >
> > At the moment we emit a warning whenever you specify both -march and
> > -mcpu and the architecture of them differ.  The idea originally was
> > that the user may not be aware of this change.
> >
> > However this has a few problems:
> >
> > 1.  Architecture revisions is not an observable part of the architecture,
> >      extensions are.  Starting with GCC 14 we have therefore relaxed the rule
> that
> >      all extensions can be enabled at any architecture level.  Therefore it's
> >      incorrect, or at least not useful to keep the check on architecture.
> >
> > 2.  It's problematic in Makefiles and other build systems, where you want to
> >      for certain files enable CPU specific builds.  i.e. you may be by default
> >      building for -march=armv8-a but for some file for -mcpu=neoverse-n1.
> Since
> >      there's no easy way to remove the earlier options we end up warning and
> >      there's no way to disable just this warning.  Build systems compiling with
> >      -Werror face an issue in this case that compiling with GCC is needlessly
> >      hard.
> >
> > 3. It doesn't actually warn for cases that may lead to issues, so e.g.
> >     -march=armv8.2-a+sve -mcpu=neoverse-n1 does not give a warning that
> SVE would
> >     be disabled.
> >
> > For this reason I have one of two proposals:
> >
> > 1.  Just remove this warning all together.
> >
> > 2.  Rework the warning based on extensions and only warn when features
> would be
> >      disabled by the presence of the -mcpu.  This is the approach this patch has
> >      taken.
> 
> There's a third option here, which is what I plan to add for the Arm port:
> 
> 3. Add -mcpu=unset and -march=unset support in the driver, which has the
> effect of suppressing any earlier option that sets that flag.
> 
> [BTW: patch 5 seems to be missing so I'm holding off on approving this now.]
> 

Ah sorry, I should have re-numbered this series. Patch 5 was sent earlier to unblock
an internal team. It was https://gcc.gnu.org/pipermail/gcc-patches/2023-October/632802.html

Thanks,
Tamar
> R.
> 
> >
> > As examples:
> >
> >> aarch64-none-linux-gnu-gcc -march=armv8.2-a+sve -mcpu=neoverse-n1
> > cc1: warning: switch ‘-mcpu=neoverse-n1’ conflicts with ‘-march=armv8.2-
> a+sve’ switch and resulted in options +crc+sve+norcpc+nodotprod being
> added
> .arch armv8.2-a+crc+sve
> >
> >> aarch64-none-linux-gnu-gcc -march=armv8.2-a -mcpu=neoverse-n1
> >> aarch64-none-linux-gnu-gcc -march=armv8.2-a+dotprod -mcpu=neoverse-
> n1
> >> aarch64-none-linux-gnu-gcc -march=armv8.2-a+dotprod -mcpu=neoverse-
> n2
> > <no warning>
> >
> > The one remaining issue here is that if both -march and -mcpu are
> > specified we pick the -march.  This is not particularly obvious and
> > for the use case to be more useful I think it makes sense to pick the CPU's
> arch?
> >
> > I did not make that change in the patch as it changes semantics.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> >
> > Note that I can't write a test for this because dg-warning expects
> > warnings to be at a particular line and doesn't support warnings at the
> "global" level.
> >
> > Ok for master?
> >
> > Thanks,
> > Tamar
> >
> > gcc/ChangeLog:
> >
> > 	* config/aarch64/aarch64.cc (aarch64_override_options): Rework
> warnings.
> >
> > --- inline copy of patch --
> > diff --git a/gcc/config/aarch64/aarch64.cc
> > b/gcc/config/aarch64/aarch64.cc index
> >
> caf80d66b3a744cc93899645aa5f9374983cd3db..3afd222ad3bdcfb922cc01
> 0dcc0b
> > 138db29caf7f 100644
> > --- a/gcc/config/aarch64/aarch64.cc
> > +++ b/gcc/config/aarch64/aarch64.cc
> > @@ -16388,12 +16388,22 @@ aarch64_override_options (void)
> >     if (cpu && arch)
> >       {
> >         /* If both -mcpu and -march are specified, warn if they are not
> > -	 architecturally compatible and prefer the -march ISA flags.  */
> > -      if (arch->arch != cpu->arch)
> > -	{
> > -	  warning (0, "switch %<-mcpu=%s%> conflicts with %<-march=%s%>
> switch",
> > +	 feature compatible.  feature compatible means that the inclusion of
> the
> > +	 cpu features would end up disabling an achitecture feature.  In
> > +	 otherwords the cpu features need to be a strict superset of the arch
> > +	 features and if so prefer the -march ISA flags.  */
> > +      auto full_arch_flags = arch->flags | arch_isa;
> > +      auto full_cpu_flags = cpu->flags | cpu_isa;
> > +      if (~full_cpu_flags & full_arch_flags)
> > +	{
> > +	  std::string ext_diff
> > +	    = aarch64_get_extension_string_for_isa_flags (full_arch_flags,
> > +							  full_cpu_flags);
> > +	  warning (0, "switch %<-mcpu=%s%> conflicts with %<-march=%s%>
> switch "
> > +		      "and resulted in options %s being added",
> >   		       aarch64_cpu_string,
> > -		       aarch64_arch_string);
> > +		       aarch64_arch_string,
> > +		       ext_diff.c_str ());
> >   	}
> >
> >         selected_arch = arch->arch;
> >
> >
> >
> >