| Message ID | 20190307085246.1477426-1-arnd@arndb.de |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v2] dma-mapping: work around clang bug | expand |
Hi Arnd, On 2019-03-07 8:52 am, Arnd Bergmann wrote: > Clang has a rather annoying behavior of checking for integer > arithmetic problems in code paths that are discarded by gcc > before that perfoms the same checks. > > For DMA_BIT_MASK(64), this leads to a warning despite the > result of the macro being completely sensible: > > arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow] > .coherent_dma_mask = DMA_BIT_MASK(64), > > The best workaround I could come up with is to shift the > value twice, which makes the macro way less readable but > always has the same result. > > Link: https://bugs.llvm.org/show_bug.cgi?id=38789 > Signed-off-by: Arnd Bergmann <arnd@arndb.de> > --- > v2: fix off-by-one error > --- > include/linux/dma-mapping.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h > index 75e60be91e5f..9e438fe6b130 100644 > --- a/include/linux/dma-mapping.h > +++ b/include/linux/dma-mapping.h > @@ -138,7 +138,8 @@ struct dma_map_ops { > extern const struct dma_map_ops dma_virt_ops; > extern const struct dma_map_ops dma_dummy_ops; > > -#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. 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. Robin. > > #define DMA_MASK_NONE 0x0ULL > >
On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@arm.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
On Thu, Mar 7, 2019 at 10:28 AM Arnd Bergmann <arnd@arndb.de> wrote: > On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@arm.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) Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be> Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
On 2019-03-07 9:28 am, Arnd Bergmann wrote: > On Thu, Mar 7, 2019 at 10:17 AM Robin Murphy <robin.murphy@arm.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) Neat - it was too early in the morning for me to think of a succinct way to comment it, but that's great. I suspect there may be a redundant set of parentheses around the shift still, but other than that, Reviewed-by: Robin Murphy <robin.murphy@arm.com> Cheers, Robin.
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h index 75e60be91e5f..9e438fe6b130 100644 --- a/include/linux/dma-mapping.h +++ b/include/linux/dma-mapping.h @@ -138,7 +138,8 @@ struct dma_map_ops { extern const struct dma_map_ops dma_virt_ops; extern const struct dma_map_ops dma_dummy_ops; -#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) #define DMA_MASK_NONE 0x0ULL
Clang has a rather annoying behavior of checking for integer arithmetic problems in code paths that are discarded by gcc before that perfoms the same checks. For DMA_BIT_MASK(64), this leads to a warning despite the result of the macro being completely sensible: arch/arm/plat-iop/adma.c:146:24: error: shift count >= width of type [-Werror,-Wshift-count-overflow] .coherent_dma_mask = DMA_BIT_MASK(64), The best workaround I could come up with is to shift the value twice, which makes the macro way less readable but always has the same result. Link: https://bugs.llvm.org/show_bug.cgi?id=38789 Signed-off-by: Arnd Bergmann <arnd@arndb.de> --- v2: fix off-by-one error --- include/linux/dma-mapping.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) -- 2.20.0