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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/64308] Missed optimization: 64-bit divide used when 32-bit divide would work
Date: Mon, 15 Dec 2014 10:34:00 -0000	[thread overview]
Message-ID: <bug-64308-4-opayYYLRgX@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-64308-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64308

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
You would need symbolic ranges, b and ret are in [0,m-1]. And then you are
using that very specific x86 instruction that divides 64 bits by 32 bits but
only works if the result fits in 32 bits. It works here because (m-1)*(m-1)/m<m
is small enough, but that's very hard for the compiler to prove, and I don't
know of another architecture with a similar instruction.

(Related: PR58897, PR53100)


  reply	other threads:[~2014-12-15 10:34 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-14 18:39 [Bug tree-optimization/64308] New: " tavianator at gmail dot com
2014-12-15 10:34 ` glisse at gcc dot gnu.org [this message]
2014-12-20  1:29 ` [Bug tree-optimization/64308] " olegendo at gcc dot gnu.org
2022-01-28  0:52 ` pinskia at gcc dot gnu.org

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