combine: Punt on out of range rotate counts [PR93505]

What happens on this testcase is with the out of bounds rotate we get:
Trying 13 -> 16:
   13: r129:SI=r132:DI#0<-<0x20
      REG_DEAD r132:DI
   16: r123:DI=r129:SI<0
      REG_DEAD r129:SI
Successfully matched this instruction:
(set (reg/v:DI 123 [ <retval> ])
    (const_int 0 [0]))
during combine.  So, perhaps we could also change simplify-rtx.c to punt
if it is out of bounds rather than trying to optimize anything.
Or, but probably GCC11 material, if we decide that ROTATE/ROTATERT doesn't
have out of bounds counts or introduce targetm.rotate_truncation_mask,
we should truncate the argument instead of punting.
Punting is better for backports though.

2020-01-30  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/93505
	* combine.c (simplify_comparison) <case ROTATE>: Punt on out of range
	rotate counts.

	* gcc.c-torture/compile/pr93505.c: New test.

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.c-torture/compile/pr93505.c b/gcc/testsuite/gcc.c-torture/compile/pr93505.c
new file mode 100644
index 00000000000..0627962eae5
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr93505.c
@@ -0,0 +1,15 @@
+/* PR middle-end/93505 */
+
+unsigned a;
+
+unsigned
+foo (unsigned x)
+{
+  unsigned int y = 32 - __builtin_bswap64 (-a);
+  /* This would be UB (x << 32) at runtime.  Ensure we don't
+     invoke UB in the compiler because of that (visible with
+     bootstrap-ubsan).  */
+  x = x << y | x >> (-y & 31);
+  x >>= 31;
+  return x;
+}