Re-apply: Drop excess size used for run time allocated stack variables.

The patch got reverted after hitting PR77359 which turned out to be a
rs6000 backend problem.  Reapplying after the PR got fixed.

gcc/ChangeLog:

2016-11-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	Re-apply after PR bootstrap/77359 is fixed:
	2016-08-23  Dominik Vogt  <vogt@linux.vnet.ibm.com>

        * explow.c (get_dynamic_stack_size): Take known alignment of stack
        pointer + STACK_DYNAMIC_OFFSET into account when calculating the
        size needed.

--This line, and those below, will be
ignored--

M    gcc/ChangeLog
M    gcc/explow.c

From-SVN: r242590

gcc/ChangeLog

 2016-11-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>
 
+	Re-apply after PR bootstrap/77359 is fixed:
+	2016-08-23  Dominik Vogt  <vogt@linux.vnet.ibm.com>
+
+        * explow.c (get_dynamic_stack_size): Take known alignment of stack
+        pointer + STACK_DYNAMIC_OFFSET into account when calculating the
+        size needed.
+
+2016-11-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>
+
 	PR bootstrap/77359
 	* config/rs6000/rs6000.c (rs6000_stack_info): Properly align local
 	variables in functions calling alloca.  Also update the ASCII

gcc/explow.c

@@ -1233,9 +1233,15 @@ 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;
+  unsigned known_align = REGNO_POINTER_ALIGN (VIRTUAL_STACK_DYNAMIC_REGNUM);
+  if (known_align == 0)
+    known_align = BITS_PER_UNIT;
+  if (required_align > known_align)
+    {
+      extra = (required_align - known_align) / 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;