Messages in this thread | | | From | Ard Biesheuvel <> | Date | Mon, 30 Nov 2020 11:12:33 +0100 | Subject | Re: [RESEND,PATCH] ARM: fix __div64_32() error when compiling with clang |
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(+ Nico)
On Mon, 30 Nov 2020 at 11:11, Ard Biesheuvel <ardb@kernel.org> wrote: > > On Mon, 23 Nov 2020 at 08:39, Antony Yu <swpenim@gmail.com> wrote: > > > > __do_div64 clobbers the input register r0 in little endian system. > > According to the inline assembly document, if an input operand is > > modified, it should be tied to a output operand. This patch can > > prevent compilers from reusing r0 register after asm statements. > > > > Signed-off-by: Antony Yu <swpenim@gmail.com> > > --- > > arch/arm/include/asm/div64.h | 5 +++-- > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h > > index 898e9c78a7e7..809efc51e90f 100644 > > --- a/arch/arm/include/asm/div64.h > > +++ b/arch/arm/include/asm/div64.h > > @@ -39,9 +39,10 @@ static inline uint32_t __div64_32(uint64_t *n, uint32_t base) > > asm( __asmeq("%0", __xh) > > __asmeq("%1", "r2") > > __asmeq("%2", "r0") > > - __asmeq("%3", "r4") > > + __asmeq("%3", "r0") > > + __asmeq("%4", "r4") > > "bl __do_div64" > > - : "=r" (__rem), "=r" (__res) > > + : "=r" (__rem), "=r" (__res), "=r" (__n) > > : "r" (__n), "r" (__base) > > : "ip", "lr", "cc"); > > *n = __res; > > -- > > 2.23.0 > > > > Agree that using r0 as an input operand only is incorrect, and not > only on Clang. The compiler might assume that r0 will retain its value > across the asm() block, which is obviously not the case. > > However, your patch will likely break big-endian, since in that case, > __xh == r0, and so it will appear twice. > > Perhaps it would be better to change the type of __rem to unsigned > long long as well?
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