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.
