Re: [PATCH] cciss: Fix warnings during compilation under 32bitenvironment

Andrew Morton
2007-04-20T21:12:31Z

On Fri, 20 Apr 2007 16:20:59 -0400
James Bottomley <James.Bottomley@SteelEye.com> wrote:

> On Fri, 2007-04-20 at 12:30 -0700, Andrew Morton wrote:
> > On Fri, 20 Apr 2007 14:50:06 -0400
> > James Bottomley <James.Bottomley@SteelEye.com> wrote:
> >
> > > > CONFIG_LBD=y gives us an additional 3kb of instructions on i386
> > > > allnoconfig. Other architectures might do less well. It's not a huge
> > > > difference, but that's the way in which creeping bloatiness happens.
> > >
> > > OK, sure, but if we really care about this saving, then unconditionally
> > > casting to u64 is therefore wrong as well ... this is starting to open
> > > quite a large can of worms ...
> > >
> > > For the record, if we have to do this, I fancy sector_upper_32() ... we
> > > should already have some similar accessor for dma_addr_t as well.
> >
> > hm. How about this?
> >
> > --- a/include/linux/kernel.h~upper-32-bits
> > +++ a/include/linux/kernel.h
> > @@ -40,6 +40,17 @@ extern const char linux_proc_banner[];
> > #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> > #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
> >
> > +/**
> > + * upper_32_bits - return bits 32-63 of a number
> > + * @n: the number we're accessing
> > + *
> > + * A basic shift-right of a 64- or 32-bit quantity. Use this to suppress
> > + * the "right shift count >= width of type" warning when that quantity is
> > + * 32-bits.
> > + */
> > +#define upper_32_bits(n) (((u64)(n)) >> 32)
>
> Won't this have the unwanted side effect of promoting everything in a
> calculation to long long on 32 bit platforms, even if n was only 32
> bits?

bummer.

> > +
> > +
> > #define KERN_EMERG "<0>" /* system is unusable */
> > #define KERN_ALERT "<1>" /* action must be taken immediately */
> > #define KERN_CRIT "<2>" /* critical conditions */
> > _
> >
> > It seems to generate the desired code. I avoided Alan's ((n >> 31) >> 1)
> > trick because it'll generate peculiar results with signed 64-bit
> > quantities.
>
> I've seen the trick done similarly with ((n >> 16) >> 16) which
> shouldn't have the issue.

That works if we know the caller is treating the return value as 32 bits,
but we don't know that.

If we have

#define upper_32_bits(x) ((x >> 16) >> 16)

then

upper_32_bits(0x8888777766665555)

will return 0x88887777 if it's treated as 32-bits, but it'll return
0xffffffff88887777 if the caller is using 64-bits.

I spose

#define upper_32_bits(x) ((u32)((x >> 16) >> 16))

will do the trick.