https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345
ktkachov at gcc dot gnu.org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
CC| |ktkachov at gcc dot gnu.org
Last reconfirmed| |2023-01-17
--- Comment #12 from ktkachov at gcc dot gnu.org ---
(In reply to Kito Cheng from comment #7)
> We are hitting this issue on RISC-V, and got some complain from linux kernel
> developers, but in different form as the original report, we found cold
> function or any function is marked as cold by `-fguess-branch-probability`
> are all not honor to the -falign-functions=N setting, that become problem on
> some linux kernel feature since they want to control the minimal alignment
> to make sure they can atomically update the instruction which require align
> to 4 byte.
>
> However current GCC behavior can't guarantee that even -falign-functions=4
> is given, there is 3 option in my mind:
>
> 1. Fix -falign-functions=N, let it work as expect on -Os and all cold
> functions
> 2. Force align to 4 byte if -fpatchable-function-entry is given, that's
> should be doable by adjust RISC-V's FUNCTION_BOUNDARY
> 3. Adjust RISC-V's FUNCTION_BOUNDARY to let it honor to -falign-functions=N
> 4. Adding a -malign-functions=N...Okay, I know that suck idea, x86 already
> deprecated that.
>
> But I think ideally this should fixed by 1 option if possible.
>
> Testcase from RISC-V kernel guy:
> ```
> /* { dg-do compile } */
> /* { dg-options "-march=rv64gc -mabi=lp64d -O1 -falign-functions=128" } */
> /* { dg-final { scan-assembler-times ".align 7" 2 } } */
>
> // Using 128 byte align rather than 4 byte align since it easier to observe.
>
> __attribute__((__cold__)) void a() {} // This function isn't align to 128
> byte
> void b() {} // This function align to 128 byte.
> ```
>
> Proposed fix:
> ```
> diff --git a/gcc/varasm.c b/gcc/varasm.c
> index 49d5cda122f..6f8ed85fea9 100644
> --- a/gcc/varasm.c
> +++ b/gcc/varasm.c
> @@ -1907,8 +1907,7 @@ assemble_start_function (tree decl, const char *fnname)
> Note that we still need to align to DECL_ALIGN, as above,
> because ASM_OUTPUT_MAX_SKIP_ALIGN might not do any alignment at all.
> */
> if (! DECL_USER_ALIGN (decl)
> - && align_functions.levels[0].log > align
> - && optimize_function_for_speed_p (cfun))
> + && align_functions.levels[0].log > align)
> {
> #ifdef ASM_OUTPUT_MAX_SKIP_ALIGN
> int align_log = align_functions.levels[0].log;
>
> ```
I think this patch makes sense given the extra information you and Mark have
provided. Would you mind testing it and posting it to gcc-patches for review
please?
