linux-kernel - Re: [PATCH v4 2/5] riscv: Add checksum library

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Message-ID: <ZQOXlsCeA92mVSK9@ghost>
Date:   Thu, 14 Sep 2023 19:30:30 -0400
From:   Charlie Jenkins <charlie@...osinc.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     Conor Dooley <conor.dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Samuel Holland <samuel.holland@...ive.com>,
        David Laight <David.Laight@...lab.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH v4 2/5] riscv: Add checksum library

On Thu, Sep 14, 2023 at 07:02:29PM +0100, Conor Dooley wrote:
> > > > +#ifndef CONFIG_32BIT
> > > 
> > > These can also be moved to IS_ENABLED() FYI, since there's no 32-bit
> > > stuff here that'd break the build for 64-bit. Ditto elsewhere where
> > > you've got similar stuff.
> > > 
> > > Cheers,
> > > Conor.
> > This is an ifndef, so 32-bit compilation would throw a warning about
> > shifting by 32 bits if IS_ENABLED was used instead.
> 
>  Fair enough. I did accidentally invert things in my mail, I did notice
>  the n, I just thought it did the elimination beforehand those checks,
>  sorry for the noise.

It appears that LLVM is smart enough to not attempt to compile code that
will never execute but GCC is not. Pretty interesting because it allows the
".option arch" code to be encased in IS_ENABLED because it is only not
supported on LLVM, but the shifting code needs to be in ifdef because
otherwise gcc will complain.