Allocate constant size dynamic stack space in the prologue

The attached patch fixes a warning during Linux kernel compilation
on S/390 due to -mwarn-dynamicstack and runtime alignment of stack
variables with constant size causing cfun->calls_alloca to be set
(even if alloca is not used at all).  The patched code places
constant size runtime aligned variables in the "virtual stack
vars" area instead of creating a "virtual stack dynamic" area.

This behaviour is activated by defining

  #define ALLOCATE_DYNAMIC_STACK_SPACE_IN_PROLOGUE 1

in the backend; otherwise the old logic is used.

The kernel uses runtime alignment for the page structure (aligned
to 16 bytes), and apart from triggereing the alloca warning
(-mwarn-dynamicstack), the current Gcc also generates inefficient
code like

  aghi %r15,-160  # prologue: create stack frame
    lgr %r11,%r15   # prologue: generate frame pointer
      aghi %r15,-32   # space for dynamic stack

which could be simplified to

  aghi %r15,-192

(if later optimization passes are able to get rid of the frame
pointer).  Is there a specific reason why the patched behaviour
shouldn't be used for all platforms?

--

As the placement of runtime aligned stack variables with constant
size is done completely in the middleend, I don't see a way to fix
this in the backend.

gcc/ChangeLog:

2016-07-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	* cfgexpand.c (expand_stack_vars): Implement synamic stack space
	allocation in the prologue.
	* explow.c (get_dynamic_stack_base): New function to return an address
	expression for the dynamic stack base.
	(get_dynamic_stack_size): New function to do the required dynamic stack
	space size calculations.
	(allocate_dynamic_stack_space): Use new functions.
	(align_dynamic_address): Move some code from
	allocate_dynamic_stack_space to new function.
	* explow.h (get_dynamic_stack_base, get_dynamic_stack_size): Export.

gcc/testsuite/ChangeLog:

2016-07-18  Dominik Vogt  <vogt@linux.vnet.ibm.com>

	* gcc.target/s390/warn-dynamicstack-1.c: New test.
	* gcc.dg/stack-usage-2.c (foo3): Adapt expected warning.
	stack-layout-dynamic-1.c: New test.

From-SVN: r238432

1 files changed, 17 insertions, 5 deletions

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 984ac4a..9a2837b 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -1053,6 +1053,7 @@ expand_stack_vars (bool (*pred) (size_t), struct stack_vars_data *data)
   HOST_WIDE_INT large_size = 0, large_alloc = 0;
   rtx large_base = NULL;
   unsigned large_align = 0;
+  bool large_allocation_done = false;
   tree decl;
 
   /* Determine if there are any variables requiring "large" alignment.
@@ -1096,11 +1097,6 @@ expand_stack_vars (bool (*pred) (size_t), struct stack_vars_data *data)
 	  large_size &= -(HOST_WIDE_INT)alignb;
 	  large_size += stack_vars[i].size;
 	}
-
-      /* If there were any, allocate space.  */
-      if (large_size > 0)
-	large_base = allocate_dynamic_stack_space (GEN_INT (large_size), 0,
-						   large_align, true);
     }
 
   for (si = 0; si < n; ++si)
@@ -1186,6 +1182,22 @@ expand_stack_vars (bool (*pred) (size_t), struct stack_vars_data *data)
 	  /* Large alignment is only processed in the last pass.  */
 	  if (pred)
 	    continue;
+
+	  /* If there were any variables requiring "large" alignment, allocate
+	     space.  */
+	  if (large_size > 0 && ! large_allocation_done)
+	    {
+	      HOST_WIDE_INT loffset;
+	      rtx large_allocsize;
+
+	      large_allocsize = GEN_INT (large_size);
+	      get_dynamic_stack_size (&large_allocsize, 0, large_align, NULL);
+	      loffset = alloc_stack_frame_space
+		(INTVAL (large_allocsize),
+		 PREFERRED_STACK_BOUNDARY / BITS_PER_UNIT);
+	      large_base = get_dynamic_stack_base (loffset, large_align);
+	      large_allocation_done = true;
+	    }
 	  gcc_assert (large_base != NULL);
 
 	  large_alloc += alignb - 1;