On 6/16/2026 1:44 AM, Konstantinos Eleftheriou wrote:
This patch converts the fold-mem-offsets pass from DF to RTL-SSA.
Along with this conversion, the way the pass collects information
was completely reworked. Instead of visiting each instruction multiple
times, this is now done only once.
Most significant changes are:
* The pass operates mainly on insn_info objects from RTL-SSA.
* Single iteration over all nondebug INSNs for identification
of fold-mem-roots. Then walk of the fold-mem-roots' DEF-chain
to collect foldable constants.
* The class fold_mem_info holds vectors for the DEF-chain of
the to-be-folded INSNs (fold_agnostic_insns, which don't need
to be adjusted, and fold_insns, which need their constant to
be set to zero).
* Introduction of a single-USE mode, which only collects DEFs,
that have a single USE and therefore are safe to transform
(the fold-mem-root will be the final USE). This mode is fast
and will always run (unless disabled via -fno-fold-mem-offsets).
* Introduction of a multi-USE mode, which allows DEFs to have
multiple USEs, but all USEs must be part of any fold-mem-root's
DEF-chain. The analysis of all USEs is expensive and therefore,
this mode is disabled for highly connected CFGs, unless
-fexpensive-optimizations (enabled by default at -O2) forces it on.
Note that multi-USE mode will miss some opportunities that the
single-USE mode finds (e.g. multi-USE mode fails for
fold-mem-offsets-3.c).
The following testing was done:
* Bootstrapped and regtested on aarch64-linux, x86-64-linux and
arm-linux.
* SPEC CPU tested on aarch64.
A compile time analysis with `/bin/time -v ./install/usr/local/bin/gcc -O2
all.i`
(all.i from PR117922) shows:
* -fno-fold-mem-offsets: 1681 s (user time) / 23626232 kBytes (max resident
set size)
* -ffold-mem-offsets: 1849 s (user time) / 23625708 kBytes (max resident
set size)
Multi-USE mode (on by default at -O2 via -fexpensive-optimizations) does
not have an impact on the duration or the memory footprint.
gcc/ChangeLog:
PR rtl-optimization/117922
* fold-mem-offsets.cc (INCLUDE_ALGORITHM): Added definition.
(INCLUDE_FUNCTIONAL): Likewise.
(class pass_fold_mem_offsets): Moved to bottom of file.
(class change_info): New.
(def_shadowed_by_cond_exec_p): New.
(get_single_def_in_bb): Converted to RTL-SSA.
(get_fold_mem_offset_root): Converted to RTL-SSA.
(get_uses): Removed.
(fold_offsets): Converted to RTL-SSA.
(fold_offsets_value): New.
(fold_offsets_1): Converted to RTL-SSA.
(has_foldable_uses_p): Converted to RTL-SSA.
(get_fold_mem_root): Removed.
(insn_uses_not_in_bitmap): New.
(drop_unsafe_candidates): New.
(do_commit_offset): Converted to RTL-SSA.
(do_analysis): Removed.
(do_commit_insn): Converted to RTL-SSA.
(do_fold_info_calculation): Removed.
(sort_changes): New.
(struct regno_changes): New.
(sort_pairs): New.
(do_check_validity): Removed.
(get_last_def): New.
(move_uses_to_prev_def): New.
(compute_validity_closure): Removed.
(change_in_vec_p): New.
(cancel_changes_for_group): New.
(find_keys_to_remove): New.
(free_changes_info): New.
(update_insns): New.
(fold_mem_offsets_1): New.
(pass_fold_mem_offsets::execute): Moved to bottom of file.
(fold_mem_offsets): New.
gcc/testsuite/ChangeLog:
* g++.target/aarch64/fold-mem-offsets.C: New test.
* gcc.target/aarch64/fold-mem-offsets.c: New test.
Co-authored-by: Christoph Müllner <[email protected]>
OK. Obviously with a change this large there is meaningful chance
something will go wrong. I'll keep my eye on the tester for regressions.
jeff
