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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/99930] Failure to optimize floating point
 -abs(x) in nontrivial code at -O2/3
Date: Tue, 06 Apr 2021 17:31:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99930

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
What happens here is
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dblob;f=3Dgcc/combine.c;h=3D3294575=
357bfcb19e589868da34364498a860dcf;hb=3DHEAD#l1884

"*<code><mode>2_1" for absneg:MODEF has a bare "use".  And then we trigger

  If the USE in INSN was for a pseudo register, the matching
  insn pattern will likely match any register; combining this
  with any other USE would only be safe if we knew that the
  used registers have identical values, or if there was
  something to tell them apart, e.g. different modes.  For
  now, we forgo such complicated tests and simply disallow
  combining of USES of pseudo registers with any other USE.

because both the abs and the neg have a bare use.

The patterns should be rewritten to not have such bare uses.  Alternatively
we can add some pretty-much-never-triggered code do combine to handle this
case.  Patches welcome.=