Simplify pad_below implementation

This patch simplifies the alignment calculations in pad_below.
The first arm of the "if" was:

- taking GET_MODE_BITSIZE (always equal to GET_MODE_SIZE * BITS_PER_UNIT),
- rounding up to the next multiple of PARM_BOUNDARY
- converting bits to bytes and
- subtracting the GET_MODE_SIZE

so was in effect calculating the number of bytes needed to round
GET_MODE_SIZE up to (PARM_BOUNDARY / BITS_PER_UNIT).  That can be
done more directly as -size & (align - 1), which is easier to
convert to variable-sized modes.

2017-08-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* function.c (pad_below): Simplify padding calculation.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r251233

gcc/ChangeLog

@@ -2,6 +2,12 @@
 	    Alan Hayward  <alan.hayward@arm.com>
 	    David Sherwood  <david.sherwood@arm.com>
 
+	* function.c (pad_below): Simplify padding calculation.
+
+2017-08-21  Richard Sandiford  <richard.sandiford@linaro.org>
+	    Alan Hayward  <alan.hayward@arm.com>
+	    David Sherwood  <david.sherwood@arm.com>
+
 	* target.def (function_prologue): Remove frame size argument.
 	(function_epilogue): Likewise.
 	* doc/tm.texi: Regenerate.

gcc/function.c

@@ -4322,21 +4322,16 @@ pad_to_arg_alignment (struct args_size *offset_ptr, int boundary,
 static void
 pad_below (struct args_size *offset_ptr, machine_mode passed_mode, tree sizetree)
 {
+  unsigned int align = PARM_BOUNDARY / BITS_PER_UNIT;
   if (passed_mode != BLKmode)
-    {
-      if (GET_MODE_BITSIZE (passed_mode) % PARM_BOUNDARY)
-	offset_ptr->constant
-	  += (((GET_MODE_BITSIZE (passed_mode) + PARM_BOUNDARY - 1)
-	       / PARM_BOUNDARY * PARM_BOUNDARY / BITS_PER_UNIT)
-	      - GET_MODE_SIZE (passed_mode));
-    }
+    offset_ptr->constant += -GET_MODE_SIZE (passed_mode) & (align - 1);
   else
     {
       if (TREE_CODE (sizetree) != INTEGER_CST
-	  || (TREE_INT_CST_LOW (sizetree) * BITS_PER_UNIT) % PARM_BOUNDARY)
+	  || (TREE_INT_CST_LOW (sizetree) & (align - 1)) != 0)
 	{
 	  /* Round the size up to multiple of PARM_BOUNDARY bits.  */
-	  tree s2 = round_up (sizetree, PARM_BOUNDARY / BITS_PER_UNIT);
+	  tree s2 = round_up (sizetree, align);
 	  /* Add it in.  */
 	  ADD_PARM_SIZE (*offset_ptr, s2);
 	  SUB_PARM_SIZE (*offset_ptr, sizetree);