if-to-switch: Don't skip the first condition bb when find_conditions in if-to-switch [PR105740]

The if condition is at last of first bb, so side effect statement in first BB
doesn't matter, then the first if condition could also be folded to switch
table.

gcc/ChangeLog:

	PR target/105740
	* gimple-if-to-switch.cc (find_conditions): Don't skip the first
	condition bb.

gcc/testsuite/ChangeLog:

	PR target/105740
	* gcc.dg/tree-ssa/if-to-switch-11.c: New test.

Signed-off-by: Xionghu Luo <xionghuluo@tencent.com>

2 files changed, 31 insertions, 1 deletions

diff --git a/gcc/gimple-if-to-switch.cc b/gcc/gimple-if-to-switch.cc
index 5dcfe5b..ca276d7 100644
--- a/gcc/gimple-if-to-switch.cc
+++ b/gcc/gimple-if-to-switch.cc
@@ -389,7 +389,9 @@ find_conditions (basic_block bb,
   if (cond == NULL)
     return;
 
-  if (!no_side_effect_bb (bb))
+  /* An empty conditions_in_bbs indicates we are processing the first
+     basic-block then no need check side effect.  */
+  if (!conditions_in_bbs->is_empty () && !no_side_effect_bb (bb))
     return;
 
   tree lhs = gimple_cond_lhs (cond);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-11.c b/gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-11.c
new file mode 100644
index 0000000..3dffee04
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-11.c
@@ -0,0 +1,28 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-iftoswitch-optimized" } */
+
+struct f {
+  int len;
+  int arr[4];
+};
+
+int
+test (struct f const *const f)
+{
+  if (f->arr[3] == 1) {
+    return 12;
+  } else if (f->arr[3] == 2) {
+    return 27;
+  } else if (f->arr[3] == 3) {
+    return 38;
+  } else if (f->arr[3] == 4) {
+    return 18;
+  } else if (f->arr[3] == 5) {
+    return 58;
+  } else if (f->arr[3] == 6) {
+    return 68;
+  }
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump "Canonical GIMPLE case clusters: 1 2 3 4 5 6" "iftoswitch" } } */