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ARM: 9034/1: __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 r0-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.

Reported-by: Antony Yu <swpenim@gmail.com>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Nicolas Pitre 2020-11-30 22:42:36 +01:00 committed by Russell King
parent 9fa2e7af3d
commit e64ab473dd
1 changed files with 6 additions and 15 deletions
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21
arch/arm/include/asm/div64.h
View file

@ -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;
}