linux-kernel - Re: [PATCH v9 06/12] riscv: prevent compressed instructions in alternatives

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Message-ID: <CAAeLtUCTpfnVM_e4vgYwpBDjbN24Vj-iZY8Nne6qLNTy9UeAEA@mail.gmail.com>
Date:   Wed, 13 Apr 2022 11:09:07 +0200
From:   Philipp Tomsich <philipp.tomsich@...ll.eu>
To:     Heiko Stuebner <heiko@...ech.de>
Cc:     palmer@...belt.com, paul.walmsley@...ive.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, wefu@...hat.com,
        liush@...winnertech.com, guoren@...nel.org, atishp@...shpatra.org,
        anup@...infault.org, drew@...gleboard.org, hch@....de,
        arnd@...db.de, wens@...e.org, maxime@...no.tech,
        gfavor@...tanamicro.com, andrea.mondelli@...wei.com,
        behrensj@....edu, xinhaoqu@...wei.com, mick@....forth.gr,
        allen.baum@...erantotech.com, jscheid@...tanamicro.com,
        rtrauben@...il.com, samuel@...lland.org, cmuellner@...ux.com
Subject: Re: [PATCH v9 06/12] riscv: prevent compressed instructions in alternatives

On Wed, 13 Apr 2022 at 05:03, Heiko Stuebner <heiko@...ech.de> wrote:
>
> Instructions are opportunistically compressed by the RISC-V assembler
> when possible, but in alternatives-blocks both the old and new content
> need to be the same size, so having the toolchain do somewhat random
> optimizations will cause strange side-effects like
> "attempt to move .org backwards" compile-time errors.
>
> Already a simple "and" used in alternatives assembly will cause these
> mismatched code sizes.
>
> So prevent compressed instructions to be generated in alternatives-
> code and use option-push and -pop to only limit this to the relevant
> code blocks
>
> Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> Reviewed-by: Christoph Hellwig <hch@....de>

Reviewed-by: Philipp Tomsich <philipp.tomsich@...ll.eu>