https://gcc.gnu.org/g:0d34e73b71ce199f52de227c4101256484feaa78
commit r16-3331-g0d34e73b71ce199f52de227c4101256484feaa78
Author: Richard Sandiford <richard.sandif...@arm.com>
Date: Thu Aug 21 16:16:02 2025 +0100
rtl-ssa: Add missing live-out uses [PR121619]
This PR is another bug in the rtl-ssa code to manage live-out uses.
It seems that this didn't get much coverage until recently.
In the testcase, late-combine first removed a register-to-register
move by substituting into all uses, some of which were in other EBBs.
This was done after checking make_uses_available, which (as expected)
says that single dominating definitions are available everywhere
that the definition dominates. But the update failed to add
appropriate live-out uses, so a later parallelisation attempt
tried to move the new destination into a later block.
gcc/
PR rtl-optimization/121619
* rtl-ssa/functions.h (function_info::commit_make_use_available):
Declare.
* rtl-ssa/blocks.cc (function_info::commit_make_use_available):
New function.
* rtl-ssa/changes.cc (function_info::apply_changes_to_insn): Use it.
gcc/testsuite/
PR rtl-optimization/121619
* gcc.dg/pr121619.c: New test.
Diff:
---
gcc/rtl-ssa/blocks.cc | 35 +++++++++++++++++++++++++++++++++++
gcc/rtl-ssa/changes.cc | 6 +++++-
gcc/rtl-ssa/functions.h | 1 +
gcc/testsuite/gcc.dg/pr121619.c | 33 +++++++++++++++++++++++++++++++++
4 files changed, 74 insertions(+), 1 deletion(-)
diff --git a/gcc/rtl-ssa/blocks.cc b/gcc/rtl-ssa/blocks.cc
index a57b9e15f13c..568f96585402 100644
--- a/gcc/rtl-ssa/blocks.cc
+++ b/gcc/rtl-ssa/blocks.cc
@@ -359,6 +359,41 @@ function_info::live_out_value (bb_info *bb, set_info *set)
return set;
}
+// Make USE's definition available at USE, if it isn't already. Assume that
+// the caller has properly used make_use_available to check that this is
+// possible.
+void
+function_info::commit_make_use_available (use_info *use)
+{
+ // We only need to handle single dominating definitions here.
+ // Other cases are handled by degenerate phis, with create_degenerate_phi
+ // creating any necessary live-out uses.
+ set_info *def = use->def ();
+ if (def
+ && use->is_reg ()
+ && is_single_dominating_def (def)
+ && use->ebb () != def->ebb ())
+ {
+ // If USE's EBB has DEF's EBB as its single predecessor, it's enough
+ // to add a live-out use to the former's predecessor block. Otherwise,
+ // conservatively add a live-out use at the end of DEF's block, so that
+ // DEF cannot move further down. Doing a minimal yet accurate update
+ // would be an O(n.log(n)) operation in the worst case.
+ auto ebb_cfg_bb = def->ebb ()->first_bb ()->cfg_bb ();
+ if (single_pred_p (ebb_cfg_bb))
+ {
+ bb_info *pred_bb = this->bb (single_pred (ebb_cfg_bb));
+ if (pred_bb->ebb () == def->ebb ())
+ {
+ add_live_out_use (pred_bb, def);
+ return;
+ }
+ }
+ add_live_out_use (def->bb (), def);
+ return;
+ }
+}
+
// Add PHI to EBB and enter it into the function's hash table.
void
function_info::append_phi (ebb_info *ebb, phi_info *phi)
diff --git a/gcc/rtl-ssa/changes.cc b/gcc/rtl-ssa/changes.cc
index 00e6c3185644..f2fc9826c4f1 100644
--- a/gcc/rtl-ssa/changes.cc
+++ b/gcc/rtl-ssa/changes.cc
@@ -713,7 +713,11 @@ function_info::apply_changes_to_insn (insn_change &change,
// Add all uses, now that their position is final.
for (use_info *use : change.new_uses)
- add_use (use);
+ {
+ if (use->def ())
+ commit_make_use_available (use);
+ add_use (use);
+ }
// Copy the uses and definitions.
unsigned int num_defs = change.new_defs.size ();
diff --git a/gcc/rtl-ssa/functions.h b/gcc/rtl-ssa/functions.h
index 27cbc18178a7..ba805072ba4f 100644
--- a/gcc/rtl-ssa/functions.h
+++ b/gcc/rtl-ssa/functions.h
@@ -309,6 +309,7 @@ private:
void add_live_out_use (bb_info *, set_info *);
set_info *live_out_value (bb_info *, set_info *);
+ void commit_make_use_available (use_info *);
void append_phi (ebb_info *, phi_info *);
void remove_phi (phi_info *);
diff --git a/gcc/testsuite/gcc.dg/pr121619.c b/gcc/testsuite/gcc.dg/pr121619.c
new file mode 100644
index 000000000000..a63896d244af
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr121619.c
@@ -0,0 +1,33 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -fno-gcse -fno-tree-ter -fno-guess-branch-probability
-fno-forward-propagate" } */
+
+int printf(const char *, ...);
+long a, c, d;
+char b;
+int main() {
+f : {
+ short g = 100;
+ int h = 1;
+ while (1) {
+ char i = 0;
+ if (a)
+ i = h = -b;
+ short j = g;
+ c = h ^ g;
+ g = ~(-h / c + 1);
+ if (b > 6) {
+ a = g && -1;
+ goto f;
+ }
+ if (j < 100)
+ printf("%ld\n", d);
+ if (g - 1)
+ break;
+ b = i;
+ }
+ int k = 2L % g;
+ if (k)
+ goto f;
+ }
+ return 0;
+}
