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

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.

2024-02-05  Jakub Jelinek  <jakub@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.

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index a7cc5cee07baed4b1e6afe6b62264e10c0eebabf..e92f5731d9e2bd87fec582770fc10f11a127fbe4 100644 (file)
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -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);
        }
     }
 

diff --git a/gcc/testsuite/gcc.dg/bitint-84.c b/gcc/testsuite/gcc.dg/bitint-84.c
new file mode 100644 (file)
index 0000000..dffdf16
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/bitint-84.c
@@ -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__ ("");
+    }
+}