linux-kernel - Re: [RFC PATCH] x86: prevent gcc from emitting rep movsq/stosq for inlined ops

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Message-ID: <948fffdc-d0d8-49c4-90b6-b91f282f76c9@citrix.com>
Date: Wed, 2 Apr 2025 19:17:03 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: mjguzik@...il.com
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com,
 torvalds@...ux-foundation.org, x86@...nel.org,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [RFC PATCH] x86: prevent gcc from emitting rep movsq/stosq for
 inlined ops

> Please make this a gcc bug-report instead - I really don't want to
> have random compiler-specific tuning options in the kernel. Because
> that whole memcpy-strategy thing is something that gets tuned by a lot
> of other compiler options (ie -march and different versions).

I've discussed this with PeterZ in the past, although I can't for the
life of me find the bugzilla ticket I thought I opened on the matter. 
(Maybe I never got that far).

The behaviour wanted is:

1) Convert to plain plain accesses (so they can be merged/combined/etc), or
2) Emit a library call

because we do provide forms that are better than the GCC-chosen "REP
MOVSQ with manual alignment" in the general case.

Taking a leaf out of the repoline book, the ideal library call(s) would be:

    CALL __x86_thunk_rep_{mov,stos}sb

using the REP ABI (parameters in %rcx/%rdi/etc), rather than the SYSV ABI.

For current/future processors, which have fast reps of all short/zero
flavours, we can even inline the REP {MOV,STO}S instruction to avoid the
call.

For older microarchitectures, they can reuse the existing memcpy/memset
implementations, just with marginally less parameter shuffling.

How does this sound?

~Andrew