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Date: Sun, 28 Feb 2021 20:37:33 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: Mattias Andrée <maandree@....se>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH v2 2/2] Use modulo instead of mul+sub in
__secs_to_tm
* Mattias Andrée <maandree@....se> [2021-02-28 20:22:10 +0100]:
> On x86 modulo is free when doing division, so this removes
there should be no division.
div by const is transformed to mul and shift at -O1 and
that's what we should be using instead of manual hacks.
https://godbolt.org/z/Wsxq5h
> a multiplication and at the cost of replacing a conditional
> move with a conditional jump, but it still appears to be
> faster.
> (Similar architectures: nds32le)
>
> ARM doesn't have modulo, instead an multiply-and-subtract
> operation is done after the division, so the diffence
> here is either none at all, or a move and a multiply-and-add
> being replaced with a multiply-and-subtract.
> (Similar architectures: or1k)
>
> RISC-V on the other hand has a separate modulo
> instruction and will perform a separate modulo instead of
> an assignment, a multiplication, and an addition with
> this change. GCC does change how the modulo operation is
> realised depending on the optimisation level. I don't know
> how this affects the performance, however a simple test on
> x86 suggests that doing a modulo operations is actually
> faster than assign–multiply–add.
did you benchmark with CFLAGS=-O2 or -Os ?
> ---
> src/time/__secs_to_tm.c | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> index 62219df5..348e51ec 100644
> --- a/src/time/__secs_to_tm.c
> +++ b/src/time/__secs_to_tm.c
> @@ -39,16 +39,28 @@ int __secs_to_tm(long long t, struct tm *tm)
> qc_cycles--;
> }
>
> +#if 1
> + c_cycles = remdays / DAYS_PER_100Y;
> + remdays %= DAYS_PER_100Y;
> + if (c_cycles == 4) {
> + remdays += DAYS_PER_100Y;
> + c_cycles--;
> + }
> +#else
> c_cycles = remdays / DAYS_PER_100Y;
> if (c_cycles == 4) c_cycles--;
> remdays -= c_cycles * DAYS_PER_100Y;
> +#endif
>
> q_cycles = remdays / DAYS_PER_4Y;
> - remdays -= q_cycles * DAYS_PER_4Y;
> + remdays %= DAYS_PER_4Y;
>
> remyears = remdays / 365;
> - if (remyears == 4) remyears--;
> - remdays -= remyears * 365;
> + remdays %= 365;
> + if (remyears == 4) {
> + remdays += 365;
> + remyears--;
> + }
>
> leap = !remyears && (q_cycles || !c_cycles);
> yday = remdays + 31 + 28 + leap;
> --
> 2.30.1
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