[gcc r14-8798] lower-bitint: Remove single label _BitInt switches [PR113737]

[gcc r14-8798] lower-bitint: Remove single label _BitInt switches [PR113737]

[Jakub Jelinek]

https://gcc.gnu.org/g:dede174fbb57bdd3e26f322b6096d53edf0089c4

commit r14-8798-gdede174fbb57bdd3e26f322b6096d53edf0089c4
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Mon Feb 5 10:57:39 2024 +0100

    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:
---
 gcc/gimple-lower-bitint.cc       |  9 ++++++++-
 gcc/testsuite/gcc.dg/bitint-84.c | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index a7cc5cee07ba..e92f5731d9e2 100644
--- 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
index 000000000000..dffdf160a845
--- /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__ ("");
+    }
+}