Re: RFC: Another pass_late_combine after pass_reorder_blocks?

On Thu, Apr 16, 2026 at 1:57 PM Richard Biener <[email protected]> wrote:

> > I was interested to exercise opportunities, exposed by bbro pass (as
> > mentioned in [1]), so the natural place to put the new pass is after
> > bbro pass:
> >
> > On x86_32, IRA zeroes %ecx, which is later copied to %eax in the
> > terminal basic block:
> >
> >    12: NOTE_INSN_BASIC_BLOCK 3
> >     7: cx:SI=0
> >       REG_EQUAL 0
> >    45: pc=L36
> >    ...
> >    36: L36:
> >    39: NOTE_INSN_BASIC_BLOCK 7
> >    37: ax:SI=cx:SI
> >    38: use ax:SI
> >
> > This sequence is reordered in bbro pass to:
> >
> >    28: L28:
> >    12: NOTE_INSN_BASIC_BLOCK 7
> >    69: {cx:SI=0;clobber flags:CC;}
> >       REG_UNUSED flags:CC
> >    71: ax:SI=cx:SI
> >       REG_DEAD cx:SI
> >    72: use ax:SI
> >    73: NOTE_INSN_EPILOGUE_BEG
> >    74: bx:SI=[sp:SI++]
> >       REG_CFA_ADJUST_CFA sp:SI=sp:SI+0x4
> >       REG_CFA_RESTORE bx:SI
> >    75: si:SI=[sp:SI++]
> >       REG_CFA_ADJUST_CFA sp:SI=sp:SI+0x4
> >       REG_CFA_RESTORE si:SI
> >    76: simple_return
>
> Ah, so maybe we can have a late combine entry that can be invoked
> iff BB reorder does any path duplication only?  On GIMPLE we
> increasingly invoke pass workers directly from other passes
> in such case, VN even has a mode to operate on small portions
> of the CFG.

Please also note that with the i386 testcase, even before bbro pass
there is in _.c.350r.rtl_dce:

   25: L25:
   18: NOTE_INSN_BASIC_BLOCK 5
   19: cx:SI=ax:SI
   20: flags:CCZ=cmp([ax:SI],si:SI)
   21: pc={(flags:CCZ==0)?L36:pc}
      REG_DEAD flags:CCZ
      REG_BR_PROB 59055801
   22: NOTE_INSN_BASIC_BLOCK 6
   23: {dx:SI=dx:SI+0x1;clobber flags:CC;}
      REG_UNUSED flags:CC
   24: {ax:SI=ax:SI+0x4;clobber flags:CC;}
      REG_UNUSED flags:CC
   26: flags:CCZ=cmp(bx:SI,dx:SI)
   27: pc={(flags:CCZ!=0)?L25:pc}
      REG_DEAD flags:CCZ
      REG_BR_PROB 1014686025
   47: NOTE_INSN_BASIC_BLOCK 7
   48: pc=L28
   49: barrier
   36: L36:
   39: NOTE_INSN_BASIC_BLOCK 8
   37: ax:SI=cx:SI
      REG_DEAD cx:SI
   38: use ax:SI

(insn 19) and (insn 37) are also detected by pass_late_combine as
redundant. So, the pass should run even if BB reorder doesn't
duplicate paths to optimize some opportunities, exposed by previous
passes.

Uros.