[gcc r17-3252] dce: Don't remove lhs for calls for no_delete case [PR126815]

https://gcc.gnu.org/g:1374c269134364e5241051c45363e1b0c381f550

commit r17-3252-g1374c269134364e5241051c45363e1b0c381f550
Author: Andrea Pinski <[email protected]>
Date:   Wed Aug 12 17:58:22 2026 -0700

    dce: Don't remove lhs for calls for no_delete case [PR126815]
    
    r17-3204-ge02ce3b8c4574c added a no_delete to simple_dce_from_worklist.
    With the no_delete option, for calls where the lhs would be removed,
    simple_dce_from_worklist would insert an assignment after the call.
    But with exceptions the call is required to be last stmt in the basic block.
    Note inserting before the call would not work either because of requirements
    of returns twice functions need to be the first stmt of the basic block (
    see gimple_verify_flow_info).
    So the fix is instead just not removing the lhs for no_delete case.
    
    Pushed as obvious after a bootstrap/test on x86_64-linux-gnu.
    
            PR tree-optimization/126815
    
    gcc/ChangeLog:
    
            * tree-ssa-dce.cc (simple_dce_from_worklist): Just don't
            remove the lhs for no_delete case rather than adding a
            new stmt.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/torture/pr126815-1.C: New test.
    
    Signed-off-by: Andrea Pinski <[email protected]>

Diff:
---
 gcc/testsuite/g++.dg/torture/pr126815-1.C | 20 ++++++++++++++++++++
 gcc/tree-ssa-dce.cc                       | 13 +++----------
 2 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/gcc/testsuite/g++.dg/torture/pr126815-1.C b/gcc/testsuite/g++.dg/torture/pr126815-1.C
new file mode 100644
index 000000000000..227fdb4505f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/torture/pr126815-1.C
@@ -0,0 +1,20 @@
+// PR tree-optimization/126815
+// { dg-do compile }
+
+
+struct Guard { ~Guard (); };      // EH cleanup: makes f() end its basic block
+int f (int);
+void sink (int);
+
+void h (int a, int b, int c)
+{
+  Guard g;
+  int t;
+  if (c)
+    t = f (a);                    // throwing call feeds the PHI
+  else
+    t = b;                        // non-constant: prevents jump threading
+  if (t == 42)                    // t's only use
+    __builtin_unreachable ();
+  sink (a);
+}
diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
index bf818ad2c919..9fdd102442a6 100644
--- a/gcc/tree-ssa-dce.cc
+++ b/gcc/tree-ssa-dce.cc
@@ -2224,19 +2224,12 @@ simple_dce_from_worklist (bitmap worklist, bitmap need_eh_cleanup,
       if (gimple_has_side_effects (t))
 	{
 	  gcall *call = dyn_cast <gcall *> (t);
-	  if (call)
+	  // For no delete don't remove the lhs.
+	  if (call && no_delete)
 	    {
 	      gimple_call_set_lhs (call, NULL_TREE);
 	      update_stmt (call);
-	      if (no_delete)
-		{
-		  tree zero = build_zero_cst (TREE_TYPE (def));
-		  gassign *new_stmt = gimple_build_assign (def, zero);
-		  gimple_stmt_iterator gsi = gsi_for_stmt (t);
-		  gsi_insert_after (&gsi, new_stmt, GSI_SAME_STMT);
-		}
-	      else
-		release_ssa_name (def);
+	      release_ssa_name (def);
 	    }
 	  continue;
 	}