[PATCH 1/1] ARM : missing corrupted reg in __do_div_asm

[Dave Martin]

On Wed, Mar 30, 2016 at 03:27:01AM +0000, 陈刚(Gangchen) wrote:
> On 03/29/2016 06:56 PM, Dave Martin wrote:
> > On Tue, Mar 29, 2016 at 11:34:18AM +0100, Russell King - ARM Linux wrote:
> >> On Tue, Mar 29, 2016 at 11:26:05AM +0100, Dave Martin wrote:
> >>> On Tue, Mar 29, 2016 at 12:19:49PM +0200, Arnd Bergmann wrote:
> >>>> On Monday 28 March 2016 12:19:03 Chen Gang wrote:
> >>>>> __xl(R0 in little endian system, or R1 in big endian system) is corrupted
> >>>>> after calling __do_div64 and compiler is not informed about this in
> >>>>> macro __do_div_asm. If n is used again afterwards, __xl won't be
> >>>>> reloaded and n will contain incorrect value.
> >>>>>
> >>>>> Signed-off-by: Chen Gang <gangchen at rdamicro.com>
> >>>>> Signed-off-by: Chen Gang <chengang.beijing at gmail.com>
> >>>>> ---
> >>>> How did you find this? Did you run into this problem on a live system
> >>>> or see it through inspection?
> >>>>
> >>>>>   arch/arm/include/asm/div64.h | 6 ++++--
> >>>>>   1 file changed, 4 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h
> >>>>> index e1f0776..1a6e91a 100644
> >>>>> --- a/arch/arm/include/asm/div64.h
> >>>>> +++ b/arch/arm/include/asm/div64.h
> >>>>> @@ -35,12 +35,14 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
> >>>>>          register unsigned long long __n   asm("r0") = *n;
> >>>>>          register unsigned long long __res asm("r2");
> >>>>>          register unsigned int __rem       asm(__xh);
> >>>>> +       register unsigned int __clobber   asm(__xl);
> >>>>>          asm(    __asmeq("%0", __xh)
> >>>>>                  __asmeq("%1", "r2")
> >>>>> +               __asmeq("%3", "r0")
> >>>>> +               __asmeq("%4", "r4")
> >>>>>                  __asmeq("%2", "r0")
> >>>>> -               __asmeq("%3", "r4")
> >>>>>                  "bl     __do_div64"
> >>>>> -               : "=r" (__rem), "=r" (__res)
> >>>>> +               : "=r" (__rem), "=r" (__res), "=r" (__clobber)
> >>>>>                  : "r" (__n), "r" (__base)
> >>>>>                  : "ip", "lr", "cc");
> >>>>>          *n = __res;
> >>>> Doesn't the clobber normally go in the third line along with
> >>>> "ip" and "lr"?
> >>> Since __xl is not used for any real argument to the asm, I think
> >>> we can just add __xl to the clobber list directly, without needing
> >>> to introduce an extra register variable ... no?
> >> No, you can't.  The clobber list is not allowed to specify registers
> >> that may be used for input or output operands, and since __xl may be
> >> r0, and __n _is_ r0, you can't specify r0 in the clobber list.
> > Hmm, you're right -- in which case the change looks reasonable.
> >
> > I wonder whether the following would be cleaner than having these
> > aliased arguments:
> >
> > 	asm(	/* ... */
> > 		"bl	__do_div64"
> > 		: "+r" (__n), "=r" (__res)
> > 		: "r" (__base)
> > 		: "ip", "lr", "cc");
> > 	*n = __res;
> > 	return __n >> 32;
> >
> > (providing that GCC doesn't make a mess of the "easy" shift).
> I tried your proposal. It didn't make any difference: this is inline 
> function and gcc just ignores your trick.

What doesn't work for you when using this method?

Why does the fact that this is an inline function make a difference?

Cheers
---Dave