Re: [PATCH v2 1/2] bpf: don't rely on GCC __attribute__((optimize)) to disable GCSE

[Alexei Starovoitov]

On Wed, Oct 28, 2020 at 06:15:05PM +0100, Ard Biesheuvel wrote:
> Commit 3193c0836 ("bpf: Disable GCC -fgcse optimization for
> ___bpf_prog_run()") introduced a __no_fgcse macro that expands to a
> function scope __attribute__((optimize("-fno-gcse"))), to disable a
> GCC specific optimization that was causing trouble on x86 builds, and
> was not expected to have any positive effect in the first place.
> 
> However, as the GCC manual documents, __attribute__((optimize))
> is not for production use, and results in all other optimization
> options to be forgotten for the function in question. This can
> cause all kinds of trouble, but in one particular reported case,
> it causes -fno-asynchronous-unwind-tables to be disregarded,
> resulting in .eh_frame info to be emitted for the function.
> 
> This reverts commit 3193c0836, and instead, it disables the -fgcse
> optimization for the entire source file, but only when building for
> X86 using GCC with CONFIG_BPF_JIT_ALWAYS_ON disabled. Note that the
> original commit states that CONFIG_RETPOLINE=n triggers the issue,
> whereas CONFIG_RETPOLINE=y performs better without the optimization,
> so it is kept disabled in both cases.
> 
> Fixes: 3193c0836 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")
> Link: https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@xxxxxxxxxxxxxx/
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  include/linux/compiler-gcc.h   | 2 --
>  include/linux/compiler_types.h | 4 ----
>  kernel/bpf/Makefile            | 6 +++++-
>  kernel/bpf/core.c              | 2 +-
>  4 files changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d1e3c6896b71..5deb37024574 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -175,5 +175,3 @@
>  #else
>  #define __diag_GCC_8(s)
>  #endif
> -
> -#define __no_fgcse __attribute__((optimize("-fno-gcse")))

See my reply in the other thread.
I prefer
-#define __no_fgcse __attribute__((optimize("-fno-gcse")))
+#define __no_fgcse __attribute__((optimize("-fno-gcse,-fno-omit-frame-pointer")))

Potentially with -fno-asynchronous-unwind-tables.

__attribute__((optimize("")) is not as broken as you're claiming to be.
It has quirky gcc internal logic, but it's still widely used
in many software projects.