linux-kernel - Re: [PATCH] x86/crc32: use builtins to improve code generation

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Message-ID: <CAGG=3QX2WX5U-uNp3VWSbQCN9fPVAirOBn5KcaWn8m-87RhWfg@mail.gmail.com>
Date: Thu, 27 Feb 2025 04:17:24 -0800
From: Bill Wendling <morbo@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Eric Biggers <ebiggers@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>, Ard Biesheuvel <ardb@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Justin Stitt <justinstitt@...gle.com>, 
	LKML <linux-kernel@...r.kernel.org>, linux-crypto@...r.kernel.org, 
	clang-built-linux <llvm@...ts.linux.dev>
Subject: Re: [PATCH] x86/crc32: use builtins to improve code generation

On Thu, Feb 27, 2025 at 2:53 AM H. Peter Anvin <hpa@...or.com> wrote:
> On February 26, 2025 10:28:59 PM PST, Eric Biggers <ebiggers@...nel.org> wrote:
> >On Wed, Feb 26, 2025 at 10:12:47PM -0800, Bill Wendling wrote:
> >> For both gcc and clang, crc32 builtins generate better code than the
> >> inline asm. GCC improves, removing unneeded "mov" instructions. Clang
> >> does the same and unrolls the loops. GCC has no changes on i386, but
> >> Clang's code generation is vastly improved, due to Clang's "rm"
> >> constraint issue.
> >>
> >> The number of cycles improved by ~0.1% for GCC and ~1% for Clang, which
> >> is expected because of the "rm" issue. However, Clang's performance is
> >> better than GCC's by ~1.5%, most likely due to loop unrolling.
> >>
> >> Link: https://github.com/llvm/llvm-project/issues/20571#issuecomment-2649330009
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Cc: Borislav Petkov <bp@...en8.de>
> >> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> >> Cc: x86@...nel.org
> >> Cc: "H. Peter Anvin" <hpa@...or.com>
> >> Cc: Eric Biggers <ebiggers@...nel.org>
> >> Cc: Ard Biesheuvel <ardb@...nel.org>
> >> Cc: Nathan Chancellor <nathan@...nel.org>
> >> Cc: Nick Desaulniers <nick.desaulniers+lkml@...il.com>
> >> Cc: Justin Stitt <justinstitt@...gle.com>
> >> Cc: linux-kernel@...r.kernel.org
> >> Cc: linux-crypto@...r.kernel.org
> >> Cc: llvm@...ts.linux.dev
> >> Signed-off-by: Bill Wendling <morbo@...gle.com>
> >> ---
> >>  arch/x86/Makefile         | 3 +++
> >>  arch/x86/lib/crc32-glue.c | 8 ++++----
> >>  2 files changed, 7 insertions(+), 4 deletions(-)
> >
> >Thanks!  A couple concerns, though:
> >
> >> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> >> index 5b773b34768d..241436da1473 100644
> >> --- a/arch/x86/Makefile
> >> +++ b/arch/x86/Makefile
> >> @@ -114,6 +114,9 @@ else
> >>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
> >>  endif
> >>
> >> +# Enables the use of CRC32 builtins.
> >> +KBUILD_CFLAGS += -mcrc32
> >
> >Doesn't this technically allow the compiler to insert CRC32 instructions
> >anywhere in arch/x86/ without the needed runtime CPU feature check?  Normally
> >when using intrinsics it's necessary to limit the scope of the feature
> >enablement to match the runtime CPU feature check that is done, e.g. by using
> >the target function attribute.
> >
> >> diff --git a/arch/x86/lib/crc32-glue.c b/arch/x86/lib/crc32-glue.c
> >> index 2dd18a886ded..fdb94bff25f4 100644
> >> --- a/arch/x86/lib/crc32-glue.c
> >> +++ b/arch/x86/lib/crc32-glue.c
> >> @@ -48,9 +48,9 @@ u32 crc32_le_arch(u32 crc, const u8 *p, size_t len)
> >>  EXPORT_SYMBOL(crc32_le_arch);
> >>
> >>  #ifdef CONFIG_X86_64
> >> -#define CRC32_INST "crc32q %1, %q0"
> >> +#define CRC32_INST __builtin_ia32_crc32di
> >>  #else
> >> -#define CRC32_INST "crc32l %1, %0"
> >> +#define CRC32_INST __builtin_ia32_crc32si
> >>  #endif
> >
> >Do both gcc and clang consider these builtins to be a stable API, or do they
> >only guarantee the stability of _mm_crc32_*() from immintrin.h?  At least for
> >the rest of the SSE and AVX stuff, I thought that only the immintrin.h functions
> >are actually considered stable.
> >
> >- Eric
>
> There is that... also are there compiler versions that we support that do not have -mcrc32 support?
>
Checking GCC 5.1.0 and Clang 13.0.1, it seems that both support '-mcrc32'.

-bw