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Date: Sun, 28 Feb 2021 12:06:15 -0500
From: Rich Felker <dalias@...c.org>
To: Mattias Andrée <maandree@....se>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] Remove unnecessary if in __secs_to_tm
On Sun, Feb 28, 2021 at 04:09:12PM +0100, Mattias Andrée wrote:
> Since years divisible by 100 but not by 400 are not leap years,
> q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
> ---
> src/time/__secs_to_tm.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> index 093d9021..2d0c0b2c 100644
> --- a/src/time/__secs_to_tm.c
> +++ b/src/time/__secs_to_tm.c
> @@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
> remdays -= c_cycles * DAYS_PER_100Y;
>
> q_cycles = remdays / DAYS_PER_4Y;
> - if (q_cycles == 25) q_cycles--;
> - remdays -= q_cycles * DAYS_PER_4Y;
> + remdays %= DAYS_PER_4Y;
>
> remyears = remdays / 365;
> if (remyears == 4) remyears--;
I think you're right about the condition being impossible -- it looks
like the error in thinking was that, while 400Y and 4Y are strictly
larger than 4*100Y and 4*1Y respectively, 100Y is smaller than 25*4Y.
However, changing the -= to %= is not desirable. The point of the -=
has nothing to do with the edge case that can't happen; it's to avoid
a modulo operation. Since the divisor is a constant though maybe the
compiler can generate the same code for both, anyway..?
Rich
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