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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102014] [missed optimization] __uint128_t % uint64_t emits a call to __umodti3 instead of div instruction
Date: Sun, 22 Aug 2021 17:18:33 +0000	[thread overview]
Message-ID: <bug-102014-4-WjwKJeTxC8@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-102014-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102014

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
No, that is not a safe optimization.  The x86 128-bit by 64-bit DIV instruction
will #DE if the quotient is larger than ~(uint64_t) 0, you won't get a modulo
in that case even when the modulo is guaranteed to be representable.
Consider e.g. a == b == 0x3fffffffffffffffULL and n == 3.

  reply	other threads:[~2021-08-22 17:18 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-22 16:56 [Bug c/102014] New: " kamkaz at windowslive dot com
2021-08-22 17:18 ` jakub at gcc dot gnu.org [this message]
2021-08-22 18:41 ` [Bug c/102014] " pinskia at gcc dot gnu.org

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