From: Junio C Hamano <gitster@pobox•com>
To: "brian m. carlson" <sandals@crustytoothpaste•net>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint•cc>,
git@vger•kernel.org
Subject: Re: [PATCH v2 4/6] bswap.h: Always overwrite ntohl/ ntohll macros
Date: Thu, 12 Jun 2025 13:16:03 -0700 [thread overview]
Message-ID: <xmqqmsacd8cc.fsf@gitster.g> (raw)
In-Reply-To: <aEob-FXAvc2Wqmlr@fruit.crustytoothpaste.net> (brian m. carlson's message of "Thu, 12 Jun 2025 00:14:48 +0000")
"brian m. carlson" <sandals@crustytoothpaste•net> writes:
> On 2025-06-11 at 22:14:40, Sebastian Andrzej Siewior wrote:
>> The ntohl and htonl macros are redefined because the provided macros were
>> not always optimal. Sometimes it was a function call, sometimes it was a
>> macro which did the shifting. Using the 'bswap' opcode on x86 provides
>> probably better performance than performing the shifting.
>
> I believe that the peephole optimizer will almost always optimize them
> to the bswap or equivalent opcode, much like it recognizes how to
> generate rotate opcodes from two shifts and an or, so they should
> actually be equivalent.
>
> GCC and clang both emit simple bswap instructions with `-O2`, which is
> the optimization level we use: https://godbolt.org/z/1r8P1Pqo7.
Good observation. In short, we do not have to redefine these in
terms of bswap32/64 for performance as the compilers should do a
reasonable job.
The updated organization to separate the two concerns in this file,
namely, (1) figure out the best way to write bswap32/64, and (2)
override (or supply on platforms that do not offer) host-network
byte order helpers, does clean things up a lot, so I still like to
see it, though.
next prev parent reply other threads:[~2025-06-12 20:16 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-06-11 22:14 [PATCH v2 0/6] bswap.h: Rework ntohl handling Sebastian Andrzej Siewior
2025-06-11 22:14 ` [PATCH v2 1/6] Revert "bswap.h: add support for built-in bswap functions" Sebastian Andrzej Siewior
2025-06-12 20:04 ` Junio C Hamano
2025-06-11 22:14 ` [PATCH v2 2/6] bswap.h: Add support for __BYTE_ORDER__ Sebastian Andrzej Siewior
2025-06-12 0:24 ` brian m. carlson
2025-06-12 0:43 ` Collin Funk
2025-06-12 20:10 ` Junio C Hamano
2025-06-11 22:14 ` [PATCH v2 3/6] bswap.h: Define GIT_LITTLE_ENDIAN on msvc as little endian Sebastian Andrzej Siewior
2025-06-12 20:11 ` Junio C Hamano
2025-06-11 22:14 ` [PATCH v2 4/6] bswap.h: Always overwrite ntohl/ ntohll macros Sebastian Andrzej Siewior
2025-06-12 0:14 ` brian m. carlson
2025-06-12 20:16 ` Junio C Hamano [this message]
2025-06-11 22:14 ` [PATCH v2 5/6] bswap.h: Remove optimized x86 version of bswap32/64 Sebastian Andrzej Siewior
2025-06-26 15:55 ` Kristoffer Haugsbakk
2025-07-15 19:02 ` Sebastian Andrzej Siewior
2025-06-11 22:14 ` [PATCH v2 5/6] bswap: " Sebastian Andrzej Siewior
2025-06-11 22:14 ` [PATCH v2 6/6] bswap.h: Provide a built-in based version of bswap32/64 if possible Sebastian Andrzej Siewior
2025-06-12 20:21 ` [PATCH v2 0/6] bswap.h: Rework ntohl handling Junio C Hamano
2025-07-07 22:43 ` Junio C Hamano
2025-07-09 6:04 ` Sebastian Andrzej Siewior
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