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Re: HAVE_FAST_UNALIGNED_ACCESS
From: |
Eli Zaretskii |
Subject: |
Re: HAVE_FAST_UNALIGNED_ACCESS |
Date: |
Thu, 30 Mar 2023 13:26:58 +0300 |
> From: Robert Pluim <rpluim@gmail.com>
> Cc: Po Lu <luangruo@yahoo.com>
> Date: Thu, 30 Mar 2023 11:34:42 +0200
>
> Fstring_lessp has:
>
> /* Check whether the platform allows access to unaligned addresses for
> size_t integers without trapping or undue penalty (a few cycles is OK).
>
> This whitelist is incomplete but since it is only used to improve
> performance, omitting cases is safe. */
> #if defined __x86_64__|| defined __amd64__ \
> || defined __i386__ || defined __i386 \
> || defined __arm64__ || defined __aarch64__ \
> || defined __powerpc__ || defined __powerpc \
> || defined __ppc__ || defined __ppc \
> || defined __s390__ || defined __s390x__
> #define HAVE_FAST_UNALIGNED_ACCESS 1
> #else
> #define HAVE_FAST_UNALIGNED_ACCESS 0
> #endif
>
> but even if unaligned access is normally permitted by a machine, it is
> still undefined behavior to dereference an unaligned pointer.
This is incorrect. There's nothing undefined about x86 unaligned
accesses. C standards can regard this as UB, but we are using
machine-specific knowledge here (and Emacs cannot be built with a
strict adherence to C standards anyway).
> Instead, HAVE_FAST_UNALIGNED_ACCESS and UNALIGNED_LOAD_SIZE should be
> removed and memcpy used instead:
>
> word_t a, c;
>
> memcpy (&a, w1 + b / ws, sizeof a);
> memcpy (&c, w2 + b / ws, sizeof c);
>
> doing so will make the compiler itself generate the right sequence of
> instructions for performing unaligned accesses, normally with only a few
> cycles penalty.
We don't want that penalty here, that's all.
> I would like to install such a change on emacs-29.
No, please don't.
> Emacs currently crashes when built with various compilers performing
> pointer alignment checks.
Details, please. Which compilers, on what platforms, for what target
architectures, etc. Unconditionally removing the fast copy there is a
non-starter.
Re: HAVE_FAST_UNALIGNED_ACCESS, Mattias Engdegård, 2023/03/30
Re: HAVE_FAST_UNALIGNED_ACCESS, Vibhav Pant, 2023/03/30