https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124901
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
> Did you check other places to schedule the pass?
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
When - fno-reorder-blocks -fno-reorder-blocks-and-partition options are added
to compile options, the newly added pass_late_combine does nothing. So, there
indeed are new opportunities only when BB reorders blocks (duplicates paths?).
[1] https://gcc.gnu.org/pipermail/gcc-patches/2026-April/713048.html
