[gcc r15-7652] tree-optimization/118521 - bogus diagnostic from unreachable code

[Richard Biener via Gcc-cvs]

https://gcc.gnu.org/g:a2755339c6c9832467c573d956e91565943ecdc1

commit r15-7652-ga2755339c6c9832467c573d956e91565943ecdc1
Author: Richard Biener <rguent...@suse.de>
Date:   Thu Feb 20 13:37:05 2025 +0100

    tree-optimization/118521 - bogus diagnostic from unreachable code
    
    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.
    
            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.

Diff:
---
 gcc/testsuite/g++.dg/torture/pr118521.C | 14 ++++++++++++++
 gcc/tree-scalar-evolution.cc            | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/gcc/testsuite/g++.dg/torture/pr118521.C 
b/gcc/testsuite/g++.dg/torture/pr118521.C
new file mode 100644
index 000000000000..1a66aca0e8d2
--- /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 0ba85917d410..4ca08751a0d3 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;