On Thu, Apr 16, 2026 at 2:35 PM Uros Bizjak <[email protected]> wrote: > > On Thu, Apr 16, 2026 at 2:29 PM Uros Bizjak <[email protected]> wrote: > > > > 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. > > Hm, no. 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?).
I have filled enhancement PR124901 [1] and summarized all discussion there. [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124901 Uros.
