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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/100342] [10/11 Regression] wrong code with -O2
 -fno-dse -fno-forward-propagate -mno-sse2
Date: Tue, 04 May 2021 15:14:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100342

--- Comment #5 from Uro=C5=A1 Bizjak <ubizjak at gmail dot com> ---
The problem can be seen in _.pro_and_epilogue pass:

Starting with:

_.cmpelim

 2741: r14:DI=3D[sp:DI+0x38]
  ...
  368: di:DI=3Dr14:DI
  ...
  613: si:QI=3Dr14:QI
  ...
 2737: bp:DI=3Dr14:DI
  ...
  658: strict_low_part(ax:QI)=3Dbp:QI
 2275: dx:DI=3Dsi:DI
  ...
 2287: dx:DI=3Dbp:DI
  710: dx:DI=3Dbswap(dx:DI)


The pass converts:

_.pro_and_epilogue

rescanning insn with uid =3D 658.
insn 658: replaced reg 6 with 4
verify found no changes in insn with uid =3D 658.
verify found no changes in insn with uid =3D 2275.
deleting insn with uid =3D 2287.


 2741: r14:DI=3D[sp:DI+0x38]
  ...
  368: di:DI=3Dr14:DI
  ...
  613: si:QI=3Dr14:QI
  ...
 2737: bp:DI=3Dr14:DI
  ...
  658: strict_low_part(ax:QI)=3Dsi:QI
 2275: dx:DI=3Dsi:DI
  ...
 2287: ?
  710: dx:DI=3Dbswap(dx:DI)

Please note how the optimization removes insn 2287, assuming that rsi holds=
 the
whole 64bit value ... Which is not true!=