Minor optimization of variable bit testing

gcc/
	* match.pd: New pattern to simplify (1 << n) & M ==/!= 0 for M
	being a power of 2.

gcc/testsuite
	* gcc.dg/tree-ssa/bittest.c: New test

2 files changed, 38 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 71cf6f9..cdab5e5 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3441,6 +3441,17 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	     (bit_and (convert (shift:shift_type (convert @3) @1)) { newmaskt; })
 	     (bit_and @4 { newmaskt; })))))))))))))
 
+/* ((1 << n) & M) != 0  -> n == log2 (M) */
+(for cmp (ne eq)
+       icmp (eq ne)
+ (simplify
+  (cmp
+   (bit_and
+    (nop_convert? (lshift integer_onep @0)) integer_pow2p@1) integer_zerop)
+  (if (INTEGRAL_TYPE_P (TREE_TYPE (@0)))
+   (icmp @0 { wide_int_to_tree (TREE_TYPE (@0),
+				wi::exact_log2 (wi::to_wide (@1))); }))))
+
 /* Fold (X {&,^,|} C2) << C1 into (X << C1) {&,^,|} (C2 << C1)
    (X {&,^,|} C2) >> C1 into (X >> C1) & (C2 >> C1).  */
 (for shift (lshift rshift)
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/bittest.c b/gcc/testsuite/gcc.dg/tree-ssa/bittest.c
new file mode 100644
index 0000000..7d712ca
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/bittest.c
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+
+void bar (void);
+
+void
+foo(unsigned int abc123)
+{
+  unsigned int xyzpdq = (1 << abc123);
+  if ((xyzpdq & 0x800) != 0)
+    bar();
+}
+
+void
+baz(unsigned int abc123)
+{
+  unsigned int xyzpdq = (1 << abc123);
+  if ((xyzpdq & 0x800) == 0)
+    bar();
+}
+
+/* What we want to verify is that the bit test against xyzpdq is
+   replaced with a test against abc123 which avoids the shifting
+   and bit ops.  */
+/* { dg-final { scan-tree-dump-not "xyzpdq" "optimized"} } */
+/* { dg-final { scan-tree-dump-times "if .abc123" 2 "optimized"} } */