Improve alloca alignment

This patch improves alloca alignment.  Currently alloca reserves
too much space as it aligns twice, and generates unnecessary stack
alignment code.

When the requested alignment is lower than the stack alignment, no
extra alignment is needed.  If the requested alignment is higher,
we need to increase the size by the difference of the requested 
alignment and the stack alignment.  As a result, the alloca alignment
is exactly as expected:

alloca (16):
	sub	sp, sp, #16
	mov	x1, sp

alloca (x):
	add	x0, x0, 15
	and	x0, x0, -16
	sub	sp, sp, x0
	mov	x0, sp

__builtin_alloca_with_align (x, 512):
	add	x0, x0, 63
	and	x0, x0, -16
	sub	sp, sp, x0
	add	x0, sp, 63
	and	x0, x0, -64

    gcc/
	* explow.c (get_dynamic_stack_size): Improve dynamic alignment.

From-SVN: r251713

gcc/ChangeLog

+2017-09-05  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	* explow.c (get_dynamic_stack_size): Improve dynamic alignment.
+
 2017-09-05  Richard Biener  <rguenther@suse.de>
 
 	PR tree-optimization/82084

gcc/explow.c

@@ -1240,15 +1240,20 @@ get_dynamic_stack_size (rtx *psize, unsigned size_align,
      example), so we must preventively align the value.  We leave space
      in SIZE for the hole that might result from the alignment operation.  */
 
-  extra = (required_align - BITS_PER_UNIT) / BITS_PER_UNIT;
-  size = plus_constant (Pmode, size, extra);
-  size = force_operand (size, NULL_RTX);
-
-  if (flag_stack_usage_info && pstack_usage_size)
-    *pstack_usage_size += extra;
+  /* Since the stack is presumed to be aligned before this allocation,
+     we only need to increase the size of the allocation if the required
+     alignment is more than the stack alignment.  */
+  if (required_align > STACK_BOUNDARY)
+    {
+      extra = (required_align - STACK_BOUNDARY) / BITS_PER_UNIT;
+      size = plus_constant (Pmode, size, extra);
+      size = force_operand (size, NULL_RTX);
+      if (size_align > STACK_BOUNDARY)
+	size_align = STACK_BOUNDARY;
 
-  if (extra && size_align > BITS_PER_UNIT)
-    size_align = BITS_PER_UNIT;
+      if (flag_stack_usage_info && pstack_usage_size)
+	*pstack_usage_size += extra;
+    }
 
   /* Round the size to a multiple of the required stack alignment.
      Since the stack is presumed to be rounded before this allocation,