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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/115144] [15 Regression] 2% performance
 regression for some codes with r15-518-g99b1daae18c095
Date: Wed, 22 May 2024 07:13:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115144

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot =
gnu.org
   Last reconfirmed|                            |2024-05-22
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Hans-Peter Nilsson from comment #7)
> (In reply to Richard Biener from comment #6)
[...]
> >  I do want to play with
> > sinking to the start of the else {, but without doing any lifetime anal=
ysis
> > I fear that's going to be worse in the average as the current location
> > at least ensures we're close to the first use of the DEF we sink.
>=20
> Thank you in advance and for the look this far!  I haven't looked closer =
at
> what happens with later passes in main, but looking at the generated
> assembly code, the "sinking" of a division has the eventual effect of
> increasing register pressure; see the previously attached dumps.

Indeed, we have originally

  _38 =3D _36 / _37;
  _39 =3D _36 % _37;
  r2_78 =3D (signed char) _39;

where both _36 and _37 die (but _39 and _38 are live for a lot longer).  We
sink the _38 def across

  <bb 28> [local count: 173045540]:
  # iftmp.10_49 =3D PHI <iftmp.10_79(27), _31(51)>
  if (_41 >=3D iftmp.10_49)
    goto <bb 42>; [0.00%]
  else
    goto <bb 29>; [100.00%]

  <bb 29> [local count: 173045540]:
  r1.13_43 =3D (unsigned char) _38;

which the original profile check avoided.  I'll note the above is a more
sensible case where to avoid such sinking but I'll also note that sinking
does not look at register pressure (or basically whether a sinking
increases or decreases register pressure) at all and generally GIMPLE
passes are not supposed to do this (it's also not an easy feat).=