linux-kernel - RE: [PATCH 0/7] Add support to handle misaligned accesses in S-mode

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Date:   Wed, 4 Oct 2023 14:10:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Clément Léger' <cleger@...osinc.com>,
        Conor Dooley <conor@...nel.org>
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Atish Patra <atishp@...osinc.com>,
        Andrew Jones <ajones@...tanamicro.com>,
        Evan Green <evan@...osinc.com>,
        Björn Topel <bjorn@...osinc.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ron Minnich <rminnich@...il.com>,
        Daniel Maslowski <cyrevolt@...glemail.com>
Subject: RE: [PATCH 0/7] Add support to handle misaligned accesses in S-mode

...
> Saying that you support misaligned access does not mean that they are
> going to be efficient, just that they are supported (in fact, the uABI
> state that they may perform poorly). The compiler is actually not so
> stupid and will try to do as much aligned access as possible in what it
> generates (unless forced by some assembly, cast or whatever that can
> screw up alignment accesses). This is already the case and it will most
> probably not change.

I did a quick check.

https://godbolt.org/z/j3e9drv4e

The code generated by both clang and gcc for misaligned reads is horrid.
Gcc does a better job if the alignment is known but generates much
the same as the clang code when it isn't.

The C code is much shorter.
Even though both gcc and clang add a (different) instruction to it

	David

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