On Mon, Nov 30, 2020 at 11:05 AM Nicolas Pitre <[email protected]> wrote: > > The ARM version of __div64_32() encapsulates a call to __do_div64 with > non-standard argument passing. In particular, __n is a 64-bit input > argument assigned to r0-r1 and __rem is an output argument sharing half > of that 40-r1 register pair.
Should `40` be `r0`? > > With __n being an input argument, the compiler is in its right to > presume that r0-r1 would still hold the value of __n past the inline > assembly statement. Normally, the compiler would have assigned non > overlapping registers to __n and __rem if the value for __n is needed > again. > > However, here we enforce our own register assignment and gcc fails to > notice the conflict. In practice this doesn't cause any problem as __n > is considered dead after the asm statement and *n is overwritten. > However this is not always guaranteed and clang rightfully complains. > > Let's fix it properly by making __n into an input-output variable. This > makes it clear that those registers representing __n have been modified. > Then we can extract __rem as the high part of __n with plain C code. > > This asm constraint "abuse" was likely relied upon back when gcc didn't > handle 64-bit values optimally Turns out that gcc is now able to ^ Missing punctuation (period after `optimally`). > optimize things and produces the same code with this patch applied. > > Signed-off-by: Nicolas Pitre <[email protected]> > Reviewed-by: Ard Biesheuvel <[email protected]> > Tested-by: Ard Biesheuvel <[email protected]> Reported-by: Antony Yu <[email protected]> > --- > > This is related to the thread titled "[RESEND,PATCH] ARM: fix > __div64_32() error when compiling with clang". My limited compile test > with clang appears to make it happy. If no more comments I'll push this > to RMK's patch system. > > diff --git a/arch/arm/include/asm/div64.h b/arch/arm/include/asm/div64.h > index 898e9c78a7..595e538f5b 100644 > --- a/arch/arm/include/asm/div64.h > +++ b/arch/arm/include/asm/div64.h > @@ -21,29 +21,20 @@ > * assembly implementation with completely non standard calling convention > * for arguments and results (beware). > */ > - > -#ifdef __ARMEB__ > -#define __xh "r0" > -#define __xl "r1" > -#else > -#define __xl "r0" > -#define __xh "r1" > -#endif > - > static inline uint32_t __div64_32(uint64_t *n, uint32_t base) > { > register unsigned int __base asm("r4") = base; > register unsigned long long __n asm("r0") = *n; > register unsigned long long __res asm("r2"); > - register unsigned int __rem asm(__xh); > - asm( __asmeq("%0", __xh) > + unsigned int __rem; > + asm( __asmeq("%0", "r0") > __asmeq("%1", "r2") > - __asmeq("%2", "r0") > - __asmeq("%3", "r4") > + __asmeq("%2", "r4") > "bl __do_div64" > - : "=r" (__rem), "=r" (__res) > - : "r" (__n), "r" (__base) > + : "+r" (__n), "=r" (__res) > + : "r" (__base) > : "ip", "lr", "cc"); > + __rem = __n >> 32; > *n = __res; > return __rem; The above 3 statement could be: ``` *n = __res; return __n >> 32; ``` > } -- Thanks, ~Nick Desaulniers
