On Mon, Jan 09, 2023 at 03:43:16PM +0100, Miguel Ojeda wrote: > On Mon, Jan 9, 2023 at 2:58 PM Mark Rutland <mark.rutland@xxxxxxx> wrote: > > > > As far as I can tell, GCC doesn't respect '-falign-functions=N': > > > > * When the __weak__ attribute is used > > > > GCC seems to forget the alignment specified by '-falign-functions=N', > > but will respect the '__aligned__(N)' function attribute. Thus, we can > > work around this by explciitly setting the alignment for weak Whoops: s/explciitly/explicitly/ here too; I'll go re-proofread the series. > > functions. > > > > * When the __cold__ attribute is used > > > > GCC seems to forget the alignment specified by '-falign-functions=N', > > and also doesn't seem to respect the '__aligned__(N)' function > > attribute. The only way to work around this is to not use the __cold__ > > attibute. Whoops: s/attibute/attribute/ > If you happen to have a reduced case, then it would be nice to link it > in the commit. A bug report to GCC would also be nice. > > I gave it a very quick try in Compiler Explorer, but I couldn't > reproduce it, so I guess it depends on flags, non-trivial functions or > something else. Sorry, that is something I had intendeed to do but I hadn't extracted a reproducer yet. I'll try to come up with something that can be included in the commit message and reported to GCC folk (and double-check at the same time that there's not another hidden cause) With this series applied and this patch reverted, it's possible to see when building defconfig + CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y, where scanning /proc/kallsyms with: $ grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' ... will show a bunch of cold functions (and their callees/callers), largely init/exit functions (so I'll double-check whether section handling as an effect), e.g. ffffdf08be173b8c t snd_soc_exit ffffdf08be173bc4 t apple_mca_driver_exit ffffdf08be173be8 t failover_exit ffffdf08be173c10 t inet_diag_exit ffffdf08be173c60 t tcp_diag_exit ffffdf08be173c84 t cubictcp_unregister ffffdf08be173cac t af_unix_exit ffffdf08be173cf4 t packet_exit ffffdf08be173d3c t cleanup_sunrpc ffffdf08be173d8c t exit_rpcsec_gss ffffdf08be173dc4 t exit_p9 ffffdf08be173dec T p9_client_exit ffffdf08be173e10 t p9_trans_fd_exit ffffdf08be173e58 t p9_virtio_cleanup ffffdf08be173e90 t exit_dns_resolver > > + * '-falign-functions=N', and require alignment to be specificed via a function > > Nit: specificed -> specified Thanks, fixed > > +#if CONFIG_FUNCTION_ALIGNMENT > 0 > > +#define __function_aligned __aligned(CONFIG_FUNCTION_ALIGNMENT) > > +#else > > +#define __function_aligned > > +#endif > > Currently, the file is intended for attributes that do not depend on > `CONFIG_*` options. > > What I usually mention is that we could change that policy, but > otherwise these would go into e.g. `compiler_types.h`. I'm happy to move these, I just wasn't sure what the policy would be w.r.t. the existing __weak and __cold defitions since those end up depending upon __function_aligned. I assume I should move them all? i.e. move __weak as well? > > +#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0) > > #define __cold __attribute__((__cold__)) > > +#else > > +#define __cold > > +#endif > > Similarly, in this case this could go into `compiler-gcc.h` / > `compiler-clang.h` etc., since the definition will be different for > each. Sure, can do. Thanks, Mark.
