The following commit has been merged into the timers/core branch of tip: Commit-ID: d34b9b611b4c23816dde3c8bde531d7e8a5c27f5 Gitweb: https://git.kernel.org/tip/d34b9b611b4c23816dde3c8bde531d7e8a5c27f5 Author: Christophe Leroy <christophe.leroy@xxxxxx> AuthorDate: Fri, 07 Feb 2020 13:39:03 +01:00 Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> CommitterDate: Mon, 17 Feb 2020 14:40:28 +01:00 lib/vdso: Allow architectures to override the ns shift operation 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@xxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Tested-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx> Link: https://lore.kernel.org/r/b3d449de856982ed060a71e6ace8eeca4654e685.1580399657.git.christophe.leroy@xxxxxx Link: https://lkml.kernel.org/r/20200207124403.857649978@xxxxxxxxxxxxx --- lib/vdso/gettimeofday.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c index 8eb6d1e..b95aef9 100644 --- a/lib/vdso/gettimeofday.c +++ b/lib/vdso/gettimeofday.c @@ -39,6 +39,13 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult) } #endif +#ifndef vdso_shift_ns +static __always_inline u64 vdso_shift_ns(u64 ns, u32 shift) +{ + return ns >> shift; +} +#endif + #ifndef __arch_vdso_hres_capable static inline bool __arch_vdso_hres_capable(void) { @@ -80,7 +87,7 @@ static int do_hres_timens(const struct vdso_data *vdns, clockid_t clk, ns = vdso_ts->nsec; last = vd->cycle_last; ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult); - ns >>= vd->shift; + ns = vdso_shift_ns(ns, vd->shift); sec = vdso_ts->sec; } while (unlikely(vdso_read_retry(vd, seq))); @@ -148,7 +155,7 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk, ns = vdso_ts->nsec; last = vd->cycle_last; ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult); - ns >>= vd->shift; + ns = vdso_shift_ns(ns, vd->shift); sec = vdso_ts->sec; } while (unlikely(vdso_read_retry(vd, seq)));
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