linux-kernel - Re: [PATCH] [v2] dma-mapping: work around clang bug

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Date:   Thu, 7 Mar 2019 10:28:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Paul Burton <paul.burton@...s.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [v2] dma-mapping: work around clang bug

On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@....com> wrote:
> On 2019-03-07 8:52 am, Arnd Bergmann wrote:
> >
> > -#define DMA_BIT_MASK(n)      (((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
> > +/* double shift to work around https://bugs.llvm.org/show_bug.cgi?id=38789 */
> > +#define DMA_BIT_MASK(n)      (((n) == 64) ? ~0ULL : ((1ULL<<((n)-1))<<1)-1)
>
> I think that now makes DMA_BIT_MASK(0) undefined - that shouldn't matter
> in most cases, but it could potentially happen at runtime where callers
> use a non-constant argument. However, it also means we don't need to
> special-case 64 any more (since that's there to avoid the same thing
> anyway), so we could simply flip that to handle 0 instead.

Yes, good idea.

> FWIW I'd be very tempted to fold in the second shift as "2ULL<<((n)-1)",
> but that may not be to everyone's taste.

I like that. So shall we do this?

/*
 * Shifting '2' instead of '1' because of
 * https://bugs.llvm.org/show_bug.cgi?id=38789
 */
#define DMA_BIT_MASK(n)    (((n) == 0) ? 0ULL : ((2ULL<<((n)-1)))-1)

         Arnd