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Message-Id: <20210228192712.1666688-2-maandree@kth.se>
Date: Sun, 28 Feb 2021 20:27:12 +0100
From: Mattias Andrée <maandree@....se>
To: musl@...ts.openwall.com
Cc: Mattias Andrée <maandree@....se>
Subject: [PATCH v3 2/2] Use modulo instead of mul+sub in __secs_to_tm
On x86 modulo is free when doing division, so this removes
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.
---
src/time/__secs_to_tm.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
index 62219df5..59b1fc8d 100644
--- a/src/time/__secs_to_tm.c
+++ b/src/time/__secs_to_tm.c
@@ -40,15 +40,21 @@ int __secs_to_tm(long long t, struct tm *tm)
}
c_cycles = remdays / DAYS_PER_100Y;
- if (c_cycles == 4) c_cycles--;
- remdays -= c_cycles * DAYS_PER_100Y;
+ remdays %= DAYS_PER_100Y;
+ if (c_cycles == 4) {
+ remdays += DAYS_PER_100Y;
+ c_cycles--;
+ }
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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