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SubjectRe: [patch V2 16/17] lib/vdso: Allow architectures to override the ns shift operation
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On 07/02/2020 12:39, Thomas Gleixner wrote:
> From: Christophe Leroy <christophe.leroy@c-s.fr>
>
> On powerpc/32, GCC (8.1) generates pretty bad code for the ns >>= vd->shift
> operation taking into account that the shift is always <= 32 and the upper
> part of the result is likely to be zero. GCC makes reversed assumptions
> considering the shift to be likely >= 32 and the upper part to be like not
> zero.
>
> unsigned long long shift(unsigned long long x, unsigned char s)
> {
> return x >> s;
> }
>
> results in:
>
> 00000018 <shift>:
> 18: 35 25 ff e0 addic. r9,r5,-32
> 1c: 41 80 00 10 blt 2c <shift+0x14>
> 20: 7c 64 4c 30 srw r4,r3,r9
> 24: 38 60 00 00 li r3,0
> 28: 4e 80 00 20 blr
> 2c: 54 69 08 3c rlwinm r9,r3,1,0,30
> 30: 21 45 00 1f subfic r10,r5,31
> 34: 7c 84 2c 30 srw r4,r4,r5
> 38: 7d 29 50 30 slw r9,r9,r10
> 3c: 7c 63 2c 30 srw r3,r3,r5
> 40: 7d 24 23 78 or r4,r9,r4
> 44: 4e 80 00 20 blr
>
> Even when forcing the shift to be smaller than 32 with an &= 31, it still
> considers the shift as likely >= 32.
>
> Move the default shift implementation into an inline which can be redefined
> in architecture code via a macro.
>
> [ tglx: Made the shift argument u32 and removed the __arch prefix ]
>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Link: https://lore.kernel.org/r/b3d449de856982ed060a71e6ace8eeca4654e685.1580399657.git.christophe.leroy@c-s.fr
>


Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

[...]
--
Regards,
Vincenzo
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