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, 8 insertions, 0 deletions

diff --git a/gcc/explow.h b/gcc/explow.h
index 52113db..e12f90c 100644
--- a/gcc/explow.h
+++ b/gcc/explow.h
@@ -87,6 +87,14 @@ extern void record_new_stack_level (void);
 /* Allocate some space on the stack dynamically and return its address.  */
 extern rtx allocate_dynamic_stack_space (rtx, unsigned, unsigned, bool);
 
+/* Calculate the necessary size of a constant dynamic stack allocation from the
+   size of the variable area.  */
+extern void get_dynamic_stack_size (rtx *, unsigned, unsigned, HOST_WIDE_INT *);
+
+/* Returns the address of the dynamic stack space without allocating it.  */
+extern rtx get_dynamic_stack_base (HOST_WIDE_INT offset,
+				   unsigned required_align);
+
 /* Emit one stack probe at ADDRESS, an address within the stack.  */
 extern void emit_stack_probe (rtx);