On Mon, 22 Jun 2026, Konstantinos Eleftheriou wrote: > > After tail-merge merges duplicate blocks, the predecessors that branched to > them now branch to a single successor. This series re-runs ifcombine on those > predecessors so that sequential comparisons feeding a common successor are > combined into one condition; on targets with conditional compares the backend > then emits ccmp, and elsewhere it helps wherever ifcombine's cost model > already > prefers the combined form. > > 1/3 exports the ifcombine helpers via a new header. > 2/3 relaxes the uninit predicate analysis so a combined-condition shape does > not produce bogus -Wmaybe-uninitialized warnings. > 3/3 collects the candidate predecessors after tail-merge and runs ifcombine > on them to combine the conditions. > > Bootstrapped and regression-tested on aarch64-linux-gnu and > x86_64-linux-gnu.
This series is OK. For fun I also asked gemini-cli to review it and it in the end said "The patch series is of **production-grade quality**. It solves PR102793 with an elegant and general-purpose design that avoids hacks, leverages existing compiler frameworks (`ifcombine` and `predicate-analysis`), and successfully prevents regressions on critical loop transformations." Ha! Thanks, Richard. > Changes in v3: > - Drop the targetm.have_ccmp () gate; the post-tail-merge ifcombine now runs > on > all targets, leaving profitability to ifcombine's own cost model. > - Replace the maybe_undef_at_dom_frontier_p work-around with a fix in the > uninit > predicate analysis (relax the definition predicate by the maybe-undef > operands' incoming-edge conditions). > - apply_clusters: hoist the immediate-dominator check ahead of > recognize_if_then_else and assert the gcond. > - Add gcc.dg/uninit-pred-13.c covering the uninit interaction. > - Make the pr102793 tests target-independent (force the combine with > --param logical-op-non-short-circuit=1, drop the ccmp dg-skip-if) and use > __UINT64_TYPE__. > - ifcombine: do not combine the conditions guarding a loop. Extend the > existing niter-safety check (which already leaves integer/pointer loop exit > conditions alone) to loop guards too, so the post-tail-merge combine no > longer makes a guarded loop's number-of-iterations unanalyzable; this > restores the IV elimination in gcc.dg/tree-ssa/ivopt_mult_1.c and > ivopt_mult_2.c. > - pr117123.C: force --param logical-op-non-short-circuit=0 so this > value-numbering test keeps its short-circuit form (3 ifs) on all targets, > rather than being combined to 2 where the target defaults to > non-short-circuit (e.g. x86). > > Konstantinos Eleftheriou (3): > ifcombine: Add tree-ssa-ifcombine.h and update function signatures > uninit: Relax PHI def predicate by maybe-undef edge conditions > tail-merge: Combine conditions of merged blocks for ccmp [PR102793] > > gcc/gimple-predicate-analysis.cc | 96 ++++++++++++++++++++-- > gcc/gimple-predicate-analysis.h | 2 + > gcc/testsuite/g++.dg/tree-ssa/pr117123.C | 2 +- > gcc/testsuite/gcc.dg/tree-ssa/pr102793-1.c | 49 +++++++++++ > gcc/testsuite/gcc.dg/tree-ssa/pr102793-2.c | 50 +++++++++++ > gcc/testsuite/gcc.dg/uninit-pred-13.c | 60 ++++++++++++++ > gcc/tree-ssa-ifcombine.cc | 41 +++++++-- > gcc/tree-ssa-ifcombine.h | 27 ++++++ > gcc/tree-ssa-tail-merge.cc | 60 +++++++++++++- > 9 files changed, 370 insertions(+), 17 deletions(-) > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr102793-1.c > create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr102793-2.c > create mode 100644 gcc/testsuite/gcc.dg/uninit-pred-13.c > create mode 100644 gcc/tree-ssa-ifcombine.h > > -- Richard Biener <[email protected]> SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Jochen Jaser, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
