[gcc r10-8902] combine: Fix up simplify_shift_const_1 for nested ROTATEs [PR97386]

[Jakub Jelinek]

https://gcc.gnu.org/g:1a98b22b0468214ae8463d075dacaeea1d46df15

commit r10-8902-g1a98b22b0468214ae8463d075dacaeea1d46df15
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Oct 13 19:13:26 2020 +0200

    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:
---
 gcc/combine.c                                   |  7 +++++--
 gcc/testsuite/gcc.c-torture/execute/pr97386-1.c | 16 ++++++++++++++++
 gcc/testsuite/gcc.c-torture/execute/pr97386-2.c | 20 ++++++++++++++++++++
 3 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/gcc/combine.c b/gcc/combine.c
index f69413a34d0..35505cc5311 100644
--- 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
index 00000000000..c50e0380a65
--- /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
index 00000000000..e61829d71ab
--- /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;
+}