From: "H. Nikolaus Schaller" <hns@goldelico.com>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>
Cc: Arnd Bergmann <arnd@arndb.de>,
Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
Huacai Chen <chenhuacai@kernel.org>,
Huacai Chen <chenhuacai@loongson.cn>,
Jiaxun Yang <jiaxun.yang@flygoat.com>,
linux-arch@vger.kernel.org, linux-mips@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] MIPS: Avoid DIVU in `__div64_32' is result would be zero
Date: Wed, 21 Apr 2021 18:05:37 +0200 [thread overview]
Message-ID: <284CBC37-0F4F-4077-A172-7E47C90B8B43@goldelico.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2104200331110.44318@angie.orcam.me.uk>
> Am 20.04.2021 um 04:50 schrieb Maciej W. Rozycki <macro@orcam.me.uk>:
>
> We already check the high part of the divident against zero to avoid the
nit-picking: s/divident/dividend/
(seems to come from from Latin "dividendum" = the number that is to be divided).
> costly DIVU instruction in that case, needed to reduce the high part of
> the divident, so we may well check against the divisor instead and set
> the high part of the quotient to zero right away. We need to treat the
> high part the divident in that case though as the remainder that would
> be calculated by the DIVU instruction we avoided.
>
> This has passed correctness verification with test_div64 and reduced the
> module's average execution time down to 1.0445s and 0.2619s from 1.0668s
> and 0.2629s respectively for an R3400 CPU @40MHz and a 5Kc CPU @160MHz.
Impressive.
>
> Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
> ---
> I have made an experimental change on top of this to put `__div64_32' out
> of line, and that increases the averages respectively up to 1.0785s and
> 0.2705s. Not a terrible loss, especially compared to generic times quoted
> with 3/4, but still, so I think it would best be made where optimising for
> size, as noted in the cover letter.
> ---
> arch/mips/include/asm/div64.h | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> Index: linux-3maxp-div64/arch/mips/include/asm/div64.h
> ===================================================================
> --- linux-3maxp-div64.orig/arch/mips/include/asm/div64.h
> +++ linux-3maxp-div64/arch/mips/include/asm/div64.h
> @@ -68,9 +68,11 @@
> \
> __high = __div >> 32; \
> __low = __div; \
> - __upper = __high; \
> \
> - if (__high) { \
> + if (__high < __radix) { \
> + __upper = __high; \
> + __high = 0; \
> + } else { \
> __asm__("divu $0, %z1, %z2" \
> : "=x" (__modquot) \
> : "Jr" (__high), "Jr" (__radix)); \
next prev parent reply other threads:[~2021-04-21 16:05 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-20 2:50 [PATCH 0/4] Reinstate and improve MIPS `do_div' implementation Maciej W. Rozycki
2021-04-20 2:50 ` [PATCH 1/4] lib/math: Add a `do_div' test module Maciej W. Rozycki
2021-04-20 2:50 ` [PATCH 2/4] div64: Correct inline documentation for `do_div' Maciej W. Rozycki
2021-04-20 2:50 ` [PATCH 3/4] MIPS: Reinstate platform `__div64_32' handler Maciej W. Rozycki
2021-04-22 18:36 ` Guenter Roeck
2021-04-22 20:43 ` Maciej W. Rozycki
2021-04-20 2:50 ` [PATCH 4/4] MIPS: Avoid DIVU in `__div64_32' is result would be zero Maciej W. Rozycki
2021-04-21 16:05 ` H. Nikolaus Schaller [this message]
2021-04-21 16:16 ` Maciej W. Rozycki
2021-04-22 7:56 ` Thomas Bogendoerfer
2021-04-22 9:12 ` Maciej W. Rozycki
2021-04-22 11:08 ` Thomas Bogendoerfer
2021-04-22 20:47 ` Maciej W. Rozycki
2021-04-27 12:16 ` Maciej W. Rozycki
2021-04-22 11:17 ` Andreas Schwab
2021-04-21 12:01 ` [PATCH 0/4] Reinstate and improve MIPS `do_div' implementation Thomas Bogendoerfer
2021-04-21 13:12 ` Maciej W. Rozycki
2021-04-21 16:00 ` H. Nikolaus Schaller
2021-04-21 19:04 ` Maciej W. Rozycki
2021-04-22 5:53 ` H. Nikolaus Schaller
2021-04-22 13:39 ` Jiaxun Yang
2021-04-22 15:58 ` Maciej W. Rozycki
2021-04-22 16:00 ` H. Nikolaus Schaller
2021-04-22 16:55 ` Maciej W. Rozycki
2021-04-22 17:06 ` H. Nikolaus Schaller
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