Re: [PATCH v2 4/6] bswap.h: Always overwrite ntohl/ ntohll macros

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 12, 2025, 20:16 UTC
Message-ID: <xmqqmsacd8cc.fsf@gitster.g>
In-Reply-To: <aEob-FXAvc2Wqmlr@fruit.crustytoothpaste.net>

"brian m. carlson" <sandals@crustytoothpaste.net> writes:

Show 13 quoted lines

> 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.