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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/78103] Failure to optimize with __builtin_clzl
Date: Tue, 27 Jul 2021 13:53:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D78103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gn=
u.org

--- Comment #19 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 51211
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51211&action=3Dedit
gcc12-pr78103.patch

Updated untested patch that uses define_insn_and_split in 2 cases instead of
the combine splitters so that it can work around the lack of LOG_LINKS.
The patch doesn't do anything with -mlzcnt, even when bsr or bsr + lea woul=
d be
shorter than mov + lzcnt + sub, but I'm afraid we can't do anything about t=
hat,
we've lost information whether it is UB on zero or not with -mlzcnt and for
zero input all this is quite unsafe.=