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Message-ID: <20210228205212.7ef9e8b4.maandree@kth.se>
Date: Sun, 28 Feb 2021 20:52:12 +0100
From: Mattias Andrée <maandree@....se>
To: musl@...ts.openwall.com
Subject: Re: [PATCH v2 2/2] Use modulo instead of mul+sub in
__secs_to_tm
On Sun, 28 Feb 2021 20:37:33 +0100
Szabolcs Nagy <nsz@...t70.net> wrote:
> * 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
For -Os, the currently used optimisation, it does division.
But for other optimisations, it makes no difference as the
compiler will do a multiply–subtract either way.
>
> > 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 ?
I guess it must have been -O0 or -Os, but what I did was
I made a trivial program and checked that assembly output,
to see which method was faster. The important part here
was that the compiler didn't change the division operation,
so adding optimisation might have bad the test pointless.
As I wrote, for RISC-V the compiler did exactly what was
written, no matter the optimisation level, that is, for
RISC-V I tried, -O0, -O1, -O2, -O3, and -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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