The 04/30/2020 12:26, Kyrylo Tkachov wrote: > > > From: Gcc <gcc-boun...@gcc.gnu.org> On Behalf Of Andrew Pinski via Gcc > > > On Wed, Apr 29, 2020 at 6:25 AM Florian Weimer via Gcc <g...@gcc.gnu.org> > > > wrote: > > > > Distributions are receiving requests to build things with > > > > -moutline-atomics: > > > > > > > > <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=956418> > > > > > > > > Should this be reflected in the GCC upstream defaults for ARMv8-A > > > > generic tuning? It does not make much sense to me if every distribution > > > > has to overide these flags, either in their build system or by patching > > > > GCC. > > > > > > At least we should make it a configure option. > > > I do want the ability to default it for our (Marvell) toolchain for > > > Linux (our bare metal toolchain will be defaulting to ARMv8.2-a > > > anyways). > > > > After some internal discussions, I am open to having it on as a default. > > Here are two versions. One has it as a tuning setting that CPUs can > > override, > > the other just switches it on by default always unless overridden by -mno- > > outline-atomics. > > I slightly prefer the second one as it's cleaner and simpler, but happy to > > take > > either. > > Any preferences? > > Thanks, > > Kyrill > > > > ChangeLogs: > > > > 2020-04-30 Kyrylo Tkachov <kyrylo.tkac...@arm.com> > > > > * config/aarch64/aarch64-tuning-flags.def (no_outline_atomics): > > Declare. > > * config/aarch64/aarch64.h (TARGET_OUTLINE_ATOMICS): Define. > > * config/aarch64/aarch64.opt (moutline-atomics): Change to Int > > variable. > > > > 2020-04-30 Kyrylo Tkachov <kyrylo.tkac...@arm.com> > > > > * config/aarch64/aarch64.h (TARGET_OUTLINE_ATOMICS): Define. > > * config/aarch64/aarch64.opt (moutline-atomics): Change to Int > > variable. > > * doc/invoke.texi (moutline-atomics): Document as on by default. > > I've pushed this second variant after bootstrap and testing on > aarch64-none-linux-gnu. > Compiled a simple atomic-using testcase with all relevant combinations of > -moutline-atomics and LSE and no-LSE -march options. > Before the results were (as expected): > |-moutline-atomics | -mno-outline-atomics | <no outline-atomics > option > -------------------------------------------------------------------------------- > LSE | inline LSE | inline LSE | inline LSE > no-LSE | outline | inline LDXR/STXR | inline LDX/STXR > > > with this patch they are: > -moutline-atomics | -mno-outline-atomics | <no outline-atomics > option> > -------------------------------------------------------------------------------- > LSE | inline LSE | inline LSE | inline LSE > no-LSE | outline | inline LDXR/STXR | outline
note that this change affects all aarch64 targets not just *-gnu, (baremetal, freebsd, musl,..), however on non-gnu targets the initializer with __getauxval is missing so the outlined calls always use baseline LDXR/STXR. in a given elf module the libgcc initializer may not run first (e.g. it runs after ifunc resolvers, other initializers or signal handler) and different objects may be compiled with different outline-atomics setting so the outlined calls must be abi compatible with LDXR/STXR atomics (i.e. outlining is not an atomics abi change). so presumably non-gnu targets want to build gcc with outline atomics turned off by default or want a mechanism to propagate the lse hwcap to libgcc that works other than in glibc. I opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95128 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95129
