From: Segher Boessenkool <segher@kernel.crashing.org>
To: Jiufu Guo <guojiufu@linux.ibm.com>
Cc: gcc-patches@gcc.gnu.org, rguenther@suse.de,
jeffreyalaw@gmail.com, richard.sandiford@arm.com,
bergner@linux.ibm.com, linkw@gcc.gnu.org
Subject: Re: [PATCH V2] Optimize '(X - N * M) / N' to 'X / N - M' if valid
Date: Fri, 9 Jun 2023 15:26:14 -0500 [thread overview]
Message-ID: <20230609202614.GR19790@gate.crashing.org> (raw)
In-Reply-To: <20230607082111.2773414-1-guojiufu@linux.ibm.com>
Hi!
On Wed, Jun 07, 2023 at 04:21:11PM +0800, Jiufu Guo wrote:
> This patch tries to optimize "(X - N * M) / N" to "X / N - M".
> For C code, "/" towards zero (trunc_div), and "X - N * M" maybe
> wrap/overflow/underflow. So, it is valid that "X - N * M" does
> not cross zero and does not wrap/overflow/underflow.
Is it ever valid semi-generally when N does not divide X?
Say X=5, N=2, M=3. Then the original expression evaluates to 0, but the
new one to -1. Whenever one of the divisions rounds up and the other
rounds down you have this problem.
Segher
next prev parent reply other threads:[~2023-06-09 20:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-07 8:21 Jiufu Guo
2023-06-09 8:44 ` Richard Biener
2023-06-09 13:38 ` Jiufu Guo
2023-06-09 20:26 ` Segher Boessenkool [this message]
2023-06-12 2:40 ` Jiufu Guo
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