[gcc r16-9412] tree-cfgcleanup: Don't remove forwarder blocks (with phis) with phis that have abnormal uses [PR1253

https://gcc.gnu.org/g:6933427282a2f1a22d0e967e16f41b419f83c11e

commit r16-9412-g6933427282a2f1a22d0e967e16f41b419f83c11e
Author: Andrew Pinski <[email protected]>
Date:   Wed May 20 13:20:15 2026 -0700

    tree-cfgcleanup: Don't remove forwarder blocks (with phis) with phis that have abnormal uses [PR125396]
    
    This was a latent bug in the checks for removing of a forwarder block which has a phi.
    
    Take:
          <bb 15> [local count: 182536112]:
          # arr1$0_32 = PHI <arr1$0_36(ab)(11)>
    
          <bb 14> [local count: 206998870]:
          # arr1$0_43(ab) = PHI <arr1$0_25(ab)(13), 9(15)>
          # sj12_44(ab) = PHI <sj12_31(ab)(13), arr1$0_32(15)>
          f8 ();
    
    Here bb 15 predecessor is a normal edge.
    So when merging the forwarder bb 15 into bb14 we end up with:
          <bb 12> [local count: 206998870]:
          # arr1$0_42(ab) = PHI <arr1$0_24(ab)(11), 9(9)>
          # sj12_43(ab) = PHI <sj12_30(ab)(11), arr1$0_35(ab)(9)>
          f8 ();
    and now there is an overlap of live range of arr1$0_35 and arr1$0_42.
    So we need to reject the case where we have phis and the phi arguments that
    use abnormal uses.
    
    Changes since v1:
    * v2: Look at phi arguments of the forwarder block rather than the dest bb
          having an abnormal edge out.
    * v3: Fix bb_phis_references_abnormal_uses to use the gimple_phi_num_args to
          search over the phi arguments. Also fix the commit message which was wrong.
    
    Bootstrapped and tested on x86_64-linux-gnu.
    
            PR tree-optimization/125396
    
    gcc/ChangeLog:
    
            * tree-cfgcleanup.cc (bb_phis_references_abnormal_uses): New function.
            (maybe_remove_forwarder_block): Check to make sure the
            forwarder block does not have a phi that references ssa name that has
            abnormal uses.
    
    gcc/testsuite/ChangeLog:
    
            * gcc.dg/torture/pr125396-1.c: New test.
    
    Signed-off-by: Andrew Pinski <[email protected]>
    (cherry picked from commit ec79b0779b6eeb9ef3b8a45f46a596cbb75a7844)

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr125396-1.c | 38 +++++++++++++++++++++++++++++++
 gcc/tree-cfgcleanup.cc                    | 27 ++++++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/torture/pr125396-1.c b/gcc/testsuite/gcc.dg/torture/pr125396-1.c
new file mode 100644
index 000000000000..b0557d2f03f0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr125396-1.c
@@ -0,0 +1,38 @@
+/* { dg-do compile } */
+/* PR tree-optimization/125396 */
+
+
+char g7;
+int g13, g26;
+int g() __attribute__((returns_twice));
+void f8()
+{
+    int arr1[2];
+    int c2, c4;
+    char v6;
+    int sj12;
+    int sj14;
+lbl_br1:
+    if (g13) goto lbl_br3;
+    if (c4)
+    {
+        c4 = 0;
+        goto lbl_br6;
+    }
+    switch (v6)
+    case 1:
+        c4 = 1;
+    sj14 = g();
+    g13 = sj12;
+lbl_br3:
+    arr1[0] = 0;
+    sj12 = g();
+    c2 = g7;
+    g26 = 5;
+    if (c2) __builtin_unreachable();
+lbl_br6:
+    sj12 = arr1[0];
+    arr1[0] = 9;
+    if (g26) f8();
+    goto lbl_br1;
+}
diff --git a/gcc/tree-cfgcleanup.cc b/gcc/tree-cfgcleanup.cc
index 6f0ae0b08de7..7fe2b7ad5cbb 100644
--- a/gcc/tree-cfgcleanup.cc
+++ b/gcc/tree-cfgcleanup.cc
@@ -464,6 +464,28 @@ move_debug_stmts_from_forwarder (basic_block src,
     }
 }
 
+/* Returns true if a phi of the basic block BB has an
+   use of a ssa name that is used in an abnormal edge.  */
+
+static bool
+bb_phis_references_abnormal_uses (basic_block bb)
+{
+  auto phis = phi_nodes (bb);
+  gimple_stmt_iterator i;
+  for (i = gsi_start (phis); !gsi_end_p (i); gsi_next (&i))
+    {
+      gphi *phi = as_a<gphi*>(*i);
+      for (unsigned indx = 0; indx < gimple_phi_num_args (phi); indx++)
+	{
+	  tree value = gimple_phi_arg_def (phi, indx);
+	  if (TREE_CODE (value) == SSA_NAME
+	      && SSA_NAME_OCCURS_IN_ABNORMAL_PHI (value))
+	    return true;
+	}
+    }
+  return false;
+}
+
 /* Return true if basic block BB does nothing except pass control
    flow to another block and that we can safely insert a label at
    the start of the successor block and was removed.
@@ -550,6 +572,11 @@ maybe_remove_forwarder_block (basic_block bb, bool can_split = false)
   if (has_phi && gimple_seq_empty_p (phi_nodes (dest)))
     return false;
 
+  /* When we have phi, make sure the phi does not have any uses of
+     names used in abnormal phis.  */
+  if (has_phi && bb_phis_references_abnormal_uses (bb))
+    return false;
+
   /* Now walk through the statements backward.  We can ignore labels,
      anything else means this is not a forwarder block.  */
   for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi_prev (&gsi))