diff --git a/gcc/simplify-rtx.cc b/gcc/simplify-rtx.cc
index fa20665..e62bf56 100644
--- a/gcc/simplify-rtx.cc
+++ b/gcc/simplify-rtx.cc
@@ -1366,9 +1366,8 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	break;
 
       /* If operand is something known to be positive, ignore the ABS.  */
-      if (GET_CODE (op) == FFS || GET_CODE (op) == ABS
-	  || val_signbit_known_clear_p (GET_MODE (op),
-					nonzero_bits (op, GET_MODE (op))))
+      if (val_signbit_known_clear_p (GET_MODE (op),
+				     nonzero_bits (op, GET_MODE (op))))
 	return op;
 
       /* If operand is known to be only -1 or 0, convert ABS to NEG.  */
@@ -1615,6 +1614,24 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 	    }
 	}
 
+      /* We can canonicalize SIGN_EXTEND (op) as ZERO_EXTEND (op) when
+         we know the sign bit of OP must be clear.  */
+      if (val_signbit_known_clear_p (GET_MODE (op),
+				     nonzero_bits (op, GET_MODE (op))))
+	return simplify_gen_unary (ZERO_EXTEND, mode, op, GET_MODE (op));
+
+      /* (sign_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
+      if (GET_CODE (op) == SUBREG
+	  && subreg_lowpart_p (op)
+	  && GET_MODE (SUBREG_REG (op)) == mode
+	  && is_a <scalar_int_mode> (mode, &int_mode)
+	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
+	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
+	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION (int_mode)
+	  && (nonzero_bits (SUBREG_REG (op), mode)
+	      & ~(GET_MODE_MASK (op_mode)>>1)) == 0)
+	return SUBREG_REG (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
@@ -1765,6 +1782,18 @@ simplify_context::simplify_unary_operation_1 (rtx_code code, machine_mode mode,
 				     op0_mode);
 	}
 
+      /* (zero_extend:DI (subreg:SI (ctz:DI ...))) is (ctz:DI ...).  */
+      if (GET_CODE (op) == SUBREG
+	  && subreg_lowpart_p (op)
+	  && GET_MODE (SUBREG_REG (op)) == mode
+	  && is_a <scalar_int_mode> (mode, &int_mode)
+	  && is_a <scalar_int_mode> (GET_MODE (op), &op_mode)
+	  && GET_MODE_PRECISION (int_mode) <= HOST_BITS_PER_WIDE_INT
+	  && GET_MODE_PRECISION (op_mode) < GET_MODE_PRECISION (int_mode)
+	  && (nonzero_bits (SUBREG_REG (op), mode)
+	      & ~GET_MODE_MASK (op_mode)) == 0)
+	return SUBREG_REG (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