[PATCH] simplify-rtx: Simplify sign_extend of lshiftrt to zero_extend (PR68330)

[Segher Boessenkool]

Since r230164, in PR68330 combine ends up with a sign_extend of an
lshiftrt by some constant, and it does not know to morph that into a
zero_extract (the extend will always extend with zeroes).  I think
it is best to let simplify-rtx always replace such a sign_extend by
a zero_extend, after which everything works as expected.

Bootstrapped and tested on powerpc64-linux.  Is this okay for trunk?


Segher


2015-11-15  Segher Boessenkool  <segher@kernel.crashing.org>

	PR rtl-optimization/68330
	* simplify-rtx.c (simplify_unary_operation_1): Simplify SIGN_EXTEND
	of LSHIFTRT by a non-zero constant integer.

---
 gcc/simplify-rtx.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index c4fc42a..413d61b 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -1462,6 +1462,13 @@ simplify_unary_operation_1 (enum rtx_code code, machine_mode mode, rtx op)
 	    }
 	}
 
+      /* (sign_extend:M (lshiftrt:N <X> (const_int I))) is better as
+         (zero_extend:M (lshiftrt:N <X> (const_int I))) if I is not 0.  */
+      if (GET_CODE (op) == LSHIFTRT
+	  && CONST_INT_P (XEXP (op, 1))
+	  && XEXP (op, 1) != const0_rtx)
+	return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op));
+
 #if defined(POINTERS_EXTEND_UNSIGNED)
       /* As we do not know which address space the pointer is referring to,
 	 we can do this only if the target does not support different pointer
-- 
1.9.3