linux-kernel - Re: [PATCH v2] x86/crc32: use builtins to improve code generation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAGG=3QXR8WNZhYTRT_cF76P+NkRNiK7Wd+8rpVc=2JVNrhXZEw@mail.gmail.com>
Date: Fri, 28 Feb 2025 13:29:06 -0800
From: Bill Wendling <morbo@...gle.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: 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>, "H. Peter Anvin" <hpa@...or.com>, 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 v2] x86/crc32: use builtins to improve code generation

On Fri, Feb 28, 2025 at 1:20 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Thu, Feb 27, 2025 at 03:47:03PM -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.
>
> Also note that the patch
> https://lore.kernel.org/r/20250210210741.471725-1-ebiggers@kernel.org/ (which is
> already enqueued in the crc tree for 6.15) changes "rm" to "r" when the compiler
> is clang, to improve clang's code generation.  The numbers you quote are against
> the original version, right?
>
Yeah, they were against top-of-tree.

-bw