From: Eric Biggers <ebiggers@kernel.org>
To: Bill Wendling <morbo@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Ard Biesheuvel <ardb@kernel.org>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <nick.desaulniers+lkml@gmail.com>,
Justin Stitt <justinstitt@google.com>,
LKML <linux-kernel@vger.kernel.org>,
linux-crypto@vger.kernel.org,
clang-built-linux <llvm@lists.linux.dev>
Subject: Re: [PATCH v2] x86/crc32: use builtins to improve code generation
Date: Fri, 28 Feb 2025 21:20:48 +0000 [thread overview]
Message-ID: <20250228212048.GA2812743@google.com> (raw)
In-Reply-To: <CAGG=3QVkd9Vb9a=pQ=KwhKzGJXaS+6Mk5K+JtBqamj15MzT9mQ@mail.gmail.com>
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?
- Eric
next prev parent reply other threads:[~2025-02-28 21:20 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-27 6:12 [PATCH] x86/crc32: use builtins to improve code generation Bill Wendling
2025-02-27 6:28 ` Eric Biggers
2025-02-27 7:08 ` Bill Wendling
2025-02-28 2:08 ` Eric Biggers
2025-02-27 10:52 ` H. Peter Anvin
2025-02-27 12:17 ` Bill Wendling
2025-02-27 20:56 ` Bill Wendling
2025-02-27 16:26 ` Dave Hansen
2025-02-27 20:57 ` Bill Wendling
2025-02-27 21:03 ` Dave Hansen
2025-02-27 23:47 ` [PATCH v2] " Bill Wendling
2025-02-28 21:20 ` Eric Biggers [this message]
2025-02-28 21:29 ` Bill Wendling
2025-03-03 20:15 ` David Laight
2025-03-03 20:27 ` Bill Wendling
2025-03-03 22:42 ` David Laight
2025-03-03 23:57 ` H. Peter Anvin
2025-03-04 0:16 ` Bill Wendling
2025-03-04 0:43 ` H. Peter Anvin
2025-03-04 4:32 ` David Laight
2025-03-04 20:52 ` David Laight
2025-03-04 21:52 ` Eric Biggers
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