Re: [PATCH] Improve alloca alignment

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

Jeff Law wrote:
On 07/26/2017 05:29 PM, Wilco Dijkstra wrote:

> > But then the check size_align % MAX_SUPPORTED_STACK_ALIGNMENT != 0
> > seems wrong too given that round_push uses a different alignment to align to. 
> I had started to dig into the history of this code, but just didn't have
> the time to do so fully before needing to leave for the day.  To some
> degree I was hoping you knew the rationale behind the test against
> MAX_SUPPORTED_STACK_ALIGNMENT and I wouldn't have to do a ton of digging :-)

I looked further into this - it appears this works correctly since it is only bypassed if
size_align is already maximally aligned. round_push aligns to the preferred alignment,
which may be lower or equal to MAX_SUPPORTED_STACK_ALIGNMENT (which is
at least STACK_BOUNDARY).

Here is the updated version:

ChangeLog:
2017-08-22  Wilco Dijkstra  <wdijkstr@arm.com>

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

diff --git a/gcc/explow.c b/gcc/explow.c
index 50074e281edd5270c76d29feac6b7a92f598d11d..d3148273030a010ece1f8ea1c14eef64bbf4e78a 100644
--- a/gcc/explow.c
+++ b/gcc/explow.c
@@ -1234,15 +1234,20 @@ 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,