[gcc(refs/vendors/riscv/heads/gcc-15-with-riscv-opts)] tree-optimization/122502 - avoid folding during imm use walk with active ranger

[Jeff Law]

https://gcc.gnu.org/g:e7133b951a4d61566f0b0207f8b06e59bffea7e4

commit e7133b951a4d61566f0b0207f8b06e59bffea7e4
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Oct 31 08:57:39 2025 +0100

    tree-optimization/122502 - avoid folding during imm use walk with active ranger
    
    The following works around an unfortunate interaction with ranger
    and immediate use walking.  An actual solution needs more thoughts.
    
            PR tree-optimization/122502
            * tree-scalar-evolution.cc (final_value_replacement_loop):
            Avoid folding from within FOR_EACH_IMM_USE_STMT due to active
            ranger.
    
            * gcc.dg/torture/pr122502.c: New testcase.
    
    (cherry picked from commit c070cfb7b4f7850ace889725b4f788f4ae769840)

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr122502.c | 21 +++++++++++++++++++++
 gcc/tree-scalar-evolution.cc            | 10 ++++++++--
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/torture/pr122502.c b/gcc/testsuite/gcc.dg/torture/pr122502.c
new file mode 100644
index 000000000000..5e2cb2e8163f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr122502.c
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+
+short int *ts;
+
+void
+c2 (unsigned long long int s4, int ns)
+{
+  short int *b2 = (short int *)&ns;
+
+  while (ns != 0)
+    {
+      int xn;
+
+      for (xn = 0; xn < 3; ++xn)
+        for (*b2 = 0; *b2 < 2; ++*b2)
+          s4 += xn;
+      if (s4 != 0)
+        b2 = ts;
+      ++ns;
+    }
+}
diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index 7d9aea88f04d..ce0e2f4109f0 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -3968,11 +3968,17 @@ final_value_replacement_loop (class loop *loop)
 	{
 	  gimple *use_stmt;
 	  imm_use_iterator imm_iter;
+	  auto_vec<gimple *, 4> to_fold;
 	  FOR_EACH_IMM_USE_STMT (use_stmt, imm_iter, rslt)
+	    if (!stmt_can_throw_internal (cfun, use_stmt))
+	      to_fold.safe_push (use_stmt);
+	  /* Delay folding until after the immediate use walk is completed
+	     as we have an active ranger and that might walk immediate
+	     uses of rslt again.  See PR122502.  */
+	  for (gimple *use_stmt : to_fold)
 	    {
 	      gimple_stmt_iterator gsi = gsi_for_stmt (use_stmt);
-	      if (!stmt_can_throw_internal (cfun, use_stmt)
-		  && fold_stmt (&gsi, follow_all_ssa_edges))
+	      if (fold_stmt (&gsi, follow_all_ssa_edges))
 		update_stmt (gsi_stmt (gsi));
 	    }
 	}