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Date: Sun, 13 Mar 2016 12:50:19 -0700 From: Andrew Pinski <apinski@...ium.com> To: pinskia@...il.com, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org Cc: Andrew Pinski <apinski@...ium.com> Subject: [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv On many cores, udiv with a large value is slow, expand instead the division out to be what GCC would have generated for the divide by 1000. On ThunderX, the speeds up gettimeofday by 5%. Signed-off-by: Andrew Pinski <apinski@...ium.com> --- arch/arm64/kernel/vdso/gettimeofday.S | 20 ++++++++++++++++---- 1 files changed, 16 insertions(+), 4 deletions(-) diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S index efa79e8..e5caef9 100644 --- a/arch/arm64/kernel/vdso/gettimeofday.S +++ b/arch/arm64/kernel/vdso/gettimeofday.S @@ -64,10 +64,22 @@ ENTRY(__kernel_gettimeofday) bl __do_get_tspec seqcnt_check w9, 1b - /* Convert ns to us. */ - mov x13, #1000 - lsl x13, x13, x12 - udiv x11, x11, x13 + /* Undo the shift. */ + lsr x11, x11, x12 + + /* Convert ns to us (division by 1000 by using multiply high). + * This is how GCC converts the division by 1000 into. + * This is faster than divide on most cores. + */ + mov x13, 63439 + movk x13, 0xe353, lsl 16 + lsr x11, x11, 3 + movk x13, 0x9ba5, lsl 32 + movk x13, 0x20c4, lsl 48 + /* x13 = 0x20c49ba5e353f7cf */ + umulh x11, x11, x13 + lsr x11, x11, 4 + stp x10, x11, [x0, #TVAL_TV_SEC] 2: /* If tz is NULL, return 0. */ -- 1.7.2.5
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