Messages in this thread Patch in this message | | | Date | Mon, 30 Nov 2020 14:05:27 -0500 (EST) | From | Nicolas Pitre <> | Subject | [PATCH] __div64_32(): straighten up inline asm constraints |
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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.
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 optimize things and produces the same code with this patch applied.
Signed-off-by: Nicolas Pitre <nico@fluxnic.net> Reviewed-by: Ard Biesheuvel <ardb@kernel.org> Tested-by: Ard Biesheuvel <ardb@kernel.org> ---
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; }
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