combine: Fix up simplify_shift_const_1 for nested ROTATEs [PR97386]

The following testcases are miscompiled (the first one since my improvements
to rotate discovery on GIMPLE, the other one for many years) because
combiner optimizes nested ROTATEs with narrowing SUBREG in between (i.e.
the outer rotate is performed in shorter precision than the inner one) to
just one ROTATE of the rotated constant.  While that (under certain
conditions) can work for shifts, it can't work for rotates where we can only
do that with rotates of the same precision.

2020-10-13  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/97386
	* combine.c (simplify_shift_const_1): Don't optimize nested ROTATEs if
	they have different modes.

	* gcc.c-torture/execute/pr97386-1.c: New test.
	* gcc.c-torture/execute/pr97386-2.c: New test.

(cherry picked from commit f76949cee9560d04d5417481dbcda5ca089c9ebc)

diff --git a/gcc/combine.c b/gcc/combine.c
index f69413a34d02ad206019450a4bca58f20f6e0ccb..35505cc5311427356274126d5aa96fe6ff019e4d 100644 (file)
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -11002,8 +11002,11 @@ simplify_shift_const_1 (enum rtx_code code, machine_mode result_mode,
                break;
              /* For ((int) (cstLL >> count)) >> cst2 just give up.  Queuing
                 up outer sign extension (often left and right shift) is
-                hardly more efficient than the original.  See PR70429.  */
-             if (code == ASHIFTRT && int_mode != int_result_mode)
+                hardly more efficient than the original.  See PR70429.
+                Similarly punt for rotates with different modes.
+                See PR97386.  */
+             if ((code == ASHIFTRT || code == ROTATE)
+                 && int_mode != int_result_mode)
                break;
 
              rtx count_rtx = gen_int_shift_amount (int_result_mode, count);

diff --git a/gcc/testsuite/gcc.c-torture/execute/pr97386-1.c b/gcc/testsuite/gcc.c-torture/execute/pr97386-1.c
new file mode 100644 (file)
index 0000000..c50e038
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr97386-1.c
@@ -0,0 +1,16 @@
+/* PR rtl-optimization/97386 */
+
+__attribute__((noipa)) unsigned char
+foo (unsigned int c)
+{
+  return __builtin_bswap16 ((unsigned long long) (0xccccLLU << c | 0xccccLLU >> ((-c) & 63)));
+}
+
+int
+main ()
+{
+  unsigned char x = foo (0);
+  if (__CHAR_BIT__ == 8 && __SIZEOF_SHORT__ == 2 && x != 0xcc)
+    __builtin_abort ();
+  return 0;
+}

diff --git a/gcc/testsuite/gcc.c-torture/execute/pr97386-2.c b/gcc/testsuite/gcc.c-torture/execute/pr97386-2.c
new file mode 100644 (file)
index 0000000..e61829d
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr97386-2.c
@@ -0,0 +1,20 @@
+/* PR rtl-optimization/97386 */
+
+__attribute__((noipa)) unsigned
+foo (int x)
+{
+  unsigned long long a = (0x800000000000ccccULL << x) | (0x800000000000ccccULL >> (64 - x));
+  unsigned int b = a;
+  return (b << 24) | (b >> 8);
+}
+
+int
+main ()
+{
+  if (__CHAR_BIT__ == 8
+      && __SIZEOF_INT__ == 4
+      &&  __SIZEOF_LONG_LONG__ == 8
+      && foo (1) != 0x99000199U)
+    __builtin_abort ();
+  return 0;
+}