On 2/20/25 5:47 AM, Richard Biener wrote:
When SCCP does final value replacement we end up with unfolded IL like
__result_274 = _150 + 1;
...
__new_finish_106 = __result_274 + 3; <-- from SCCP
_115 = _150 + 4;
if (__new_finish_106 != _115)
this does only get rectified by the next full folding which happens
in forwprop4 which is after the strlen pass emitting the unwanted
diagnostic. The following mitigates this case in a similar way as
r15-7472 did for PR118817 - by ensuring we have the IL folded.
This is done by simply folding all immediate uses of the former
PHI def that SCCP replaces. All other more general approaches have
too much fallout at this point.
Bootstrapped and tested on x86_64-unknown-linux-gnu, OK?
Thanks,
Richard.
PR tree-optimization/118521
* tree-scalar-evolution.cc (final_value_replacement_loop):
Fold uses of the replaced PHI def.
* g++.dg/torture/pr118521.C: New testcase.
Guess I should have included "have scev clean up its own mess" as an
option in the BZ :-)
I dismissed pass reordering without even investigating. I'm always
skeptical of that as the solution.
I'd pondered how to make this work in a DOM context and even tried
something akin to what you're doing (though just in the debugger and I
think I used the single use edge callback which obviously didn't work).
Point being I think that's probably the best solution we're going to
see. LGTM.
I hesitate to even bring it up (but we need to). Do we want to consider
the state/future of these warnings for gcc-16? Is this stuff
salvageable? I think there's utility in these warnings, but they're
clearly a pain point across multiple contexts.
jeff
---
gcc/testsuite/g++.dg/torture/pr118521.C | 14 ++++++++++++++
gcc/tree-scalar-evolution.cc | 18 ++++++++++++++++++
2 files changed, 32 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/torture/pr118521.C
diff --git a/gcc/testsuite/g++.dg/torture/pr118521.C
b/gcc/testsuite/g++.dg/torture/pr118521.C
new file mode 100644
index 00000000000..1a66aca0e8d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/pr118521.C
@@ -0,0 +1,14 @@
+// { dg-do compile }
+// { dg-additional-options "-Wall" }
+
+#include <vector> // dg-bogus new_allocator.h:191 warning: writing 1 byte into
a region of size 0
+
+void bar(std::vector<char>);
+
+void foo()
+{
+ std::vector<char> v{1, 2};
+ v.insert(v.end(), 2, 0);
+ v.push_back(1);
+ bar(v);
+}
diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index 0ba85917d41..4ca08751a0d 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -284,6 +284,7 @@ along with GCC; see the file COPYING3. If not see
#include "tree-into-ssa.h"
#include "builtins.h"
#include "case-cfn-macros.h"
+#include "tree-eh.h"
static tree analyze_scalar_evolution_1 (class loop *, tree);
static tree analyze_scalar_evolution_for_address_of (class loop *loop,
@@ -3947,6 +3948,23 @@ final_value_replacement_loop (class loop *loop)
print_gimple_stmt (dump_file, SSA_NAME_DEF_STMT (rslt), 0);
fprintf (dump_file, "\n");
}
+
+ /* Re-fold immediate uses of the replaced def, but avoid
+ CFG manipulations from this function. For now only do
+ a single-level re-folding, not re-folding uses of
+ folded uses. */
+ if (! SSA_NAME_OCCURS_IN_ABNORMAL_PHI (rslt))
+ {
+ gimple *use_stmt;
+ imm_use_iterator imm_iter;
+ FOR_EACH_IMM_USE_STMT (use_stmt, imm_iter, rslt)
+ {
+ gimple_stmt_iterator gsi = gsi_for_stmt (use_stmt);
+ if (!stmt_can_throw_internal (cfun, use_stmt)
+ && fold_stmt (&gsi, follow_all_ssa_edges))
+ update_stmt (gsi_stmt (gsi));
+ }
+ }
}
return any;
