[Bug rtl-optimization/100342] [10/11 Regression] wrong code with -O2 -fno-dse -fno-forward-propagate -mno-sse2

[ubizjak at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100342

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

Starting with:

_.cmpelim

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


The pass converts:

_.pro_and_epilogue

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


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

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