combine: Convert subreg-of-lshiftrt to zero_extract properly (PR78390)

r242414, for PR77881, introduces some bugs (PR78390, PR78438, PR78477).
It all has the same root cause: that patch makes combine convert every
lowpart subreg of a logical shift right to a zero_extract.  This cannot
work at all if it is not a constant shift, and it has to be a bit more
careful exactly which bits it extracts.


	PR target/77881
	PR bootstrap/78390
	PR target/78438
	PR bootstrap/78477
	* combine.c (make_compound_operation_int): Do not convert a subreg of
	a non-constant logical shift right to a zero_extract.  Handle the case
	where some zero bits have been shifted into the range covered by that
	subreg.

From-SVN: r242757

gcc/ChangeLog

+2016-11-23  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR target/77881
+	PR bootstrap/78390
+	PR target/78438
+	PR bootstrap/78477
+	* combine.c (make_compound_operation_int): Do not convert a subreg of
+	a non-constant logical shift right to a zero_extract.  Handle the case
+	where some zero bits have been shifted into the range covered by that
+	subreg.
+
 2016-11-23  Richard Sandiford  <richard.sandiford@arm.com>
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>

gcc/combine.c

@@ -8089,12 +8089,17 @@ make_compound_operation_int (machine_mode mode, rtx *x_ptr,
 	/* If the SUBREG is masking of a logical right shift,
 	   make an extraction.  */
 	if (GET_CODE (inner) == LSHIFTRT
+	    && CONST_INT_P (XEXP (inner, 1))
 	    && GET_MODE_SIZE (mode) < GET_MODE_SIZE (GET_MODE (inner))
 	    && subreg_lowpart_p (x))
 	  {
 	    new_rtx = make_compound_operation (XEXP (inner, 0), next_code);
+	    int width = GET_MODE_PRECISION (GET_MODE (inner))
+			- INTVAL (XEXP (inner, 1));
+	    if (width > mode_width)
+	      width = mode_width;
 	    new_rtx = make_extraction (mode, new_rtx, 0, XEXP (inner, 1),
-				       mode_width, 1, 0, in_code == COMPARE);
+				       width, 1, 0, in_code == COMPARE);
 	    break;
 	  }