> -----Original Message----- > From: Richard Earnshaw <richard.earns...@foss.arm.com> > Sent: Thursday, November 16, 2023 9:42 AM > To: Tamar Christina <tamar.christ...@arm.com>; gcc-patches@gcc.gnu.org > Cc: nd <n...@arm.com>; Richard Earnshaw <richard.earns...@arm.com>; > Marcus Shawcroft <marcus.shawcr...@arm.com>; Kyrylo Tkachov > <kyrylo.tkac...@arm.com>; Richard Sandiford > <richard.sandif...@arm.com> > Subject: Re: [PATCH 6/6]AArch64: only emit mismatch error when features > would be disabled. > > > > On 16/11/2023 09:33, Tamar Christina wrote: > >> -----Original Message----- > >> From: Richard Earnshaw <richard.earns...@foss.arm.com> > >> Sent: Thursday, November 16, 2023 9:27 AM > >> To: Tamar Christina <tamar.christ...@arm.com>; > >> gcc-patches@gcc.gnu.org > >> Cc: nd <n...@arm.com>; Richard Earnshaw <richard.earns...@arm.com>; > >> Marcus Shawcroft <marcus.shawcr...@arm.com>; Kyrylo Tkachov > >> <kyrylo.tkac...@arm.com>; Richard Sandiford > >> <richard.sandif...@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 > > Ah, OK. > > So going back to your option 2. What should happen if the user specified - > mcpu=cortex-r82 and then specifies an extension that doesn't exist in the R > profile? >
AArch64 in general does not validate extensions to architectures. So basically we would allow it. e.g. > aarch64-none-elf-gcc -O3 ./gcc/testsuite/gcc.dg/tree-ssa/slsr-20.c -S -o - > -march=armv8.2-a+sve -mcpu=cortex-r82 cc1: warning: switch '-mcpu=cortex-r82' conflicts with '-march=armv8.2-a+sve' switch and would result in options +sve+norcpc+nodotprod+nofp16fml+noflagm+nopauth being added .arch armv8.2-a+crc+sve The new warning only tells you exactly what the compiler will be doing to your options, but doesn't change the behavior the compiler exhibits today since we always take -march over -mcpu. The difference is today we just say "there's a conflict" and don't specify what the conflict it. Regards, Tamar > R. > > > > > 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; > >>> > >>> > >>> > >>>