[PATCH] lower-bitint: Remove single label _BitInt switches [PR113737]

[Jakub Jelinek]

Hi!

The following testcase ICEs, because group_case_labels_stmt optimizes
  switch (a.0_7) <default: <L6> [50.00%], case 0: <L7> [50.00%], case 2: <L7> 
[50.00%]>
where L7 block starts with __builtin_unreachable (); to
  switch (a.0_7) <default: <L6> [50.00%]>
and single label GIMPLE_SWITCH is something the switch expansion refuses to
lower:
  if (gimple_switch_num_labels (m_switch) == 1
      || range_check_type (index_type) == NULL_TREE)
    return false;
(range_check_type never returns NULL for BITINT_TYPE), but the gimple
lowering pass relies on all large/huge _BitInt switches to be lowered
by that pass.

The following patch just removes those after making the single successor
edge EDGE_FALLTHRU.  I've done it even if !optimize just in case in case
we'd end up with single case label from earlier passes.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-02-05  Jakub Jelinek  <ja...@redhat.com>

        PR tree-optimization/113737
        * gimple-lower-bitint.cc (gimple_lower_bitint): If GIMPLE_SWITCH
        has just a single label, remove it and make single successor edge
        EDGE_FALLTHRU.

        * gcc.dg/bitint-84.c: New test.

--- gcc/gimple-lower-bitint.cc.jj       2024-02-02 11:30:05.801776658 +0100
+++ gcc/gimple-lower-bitint.cc  2024-02-03 12:49:52.997777574 +0100
@@ -5832,7 +5832,14 @@ gimple_lower_bitint (void)
 
          if (optimize)
            group_case_labels_stmt (swtch);
-         switch_statements.safe_push (swtch);
+         if (gimple_switch_num_labels (swtch) == 1)
+           {
+             single_succ_edge (bb)->flags |= EDGE_FALLTHRU;
+             gimple_stmt_iterator gsi = gsi_for_stmt (swtch);
+             gsi_remove (&gsi, true);
+           }
+         else
+           switch_statements.safe_push (swtch);
        }
     }
 
--- gcc/testsuite/gcc.dg/bitint-84.c.jj 2024-02-03 12:56:08.153622744 +0100
+++ gcc/testsuite/gcc.dg/bitint-84.c    2024-02-03 12:57:05.425835789 +0100
@@ -0,0 +1,32 @@
+/* PR tree-optimization/113737 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-O2 -std=c23" } */
+
+#if __BITINT_MAXWIDTH__ >= 129
+_BitInt(129) a;
+#else
+_BitInt(63) a;
+#endif
+
+int b[1], c;
+
+int
+foo (void)
+{
+  switch (a)
+  case 0:
+  case 2:
+    return 1;
+  return 0;
+}
+
+void
+bar (int i)
+{
+  for (;; ++i)
+    {
+      c = b[i];
+      if (!foo ())
+       __asm__ ("");
+    }
+}

        Jakub