Tweak Thumb register allocation order.

[Paul Brook]

The arm backend is currently setup to allocate r12(ip) before call clobbered 
low register (r4-r7).  However in thumb mode using a high register is often 
more expensive than saving an additional low register.  The patch below 
defines ORDER_REGS_FOR_LOCAL_ALLOC to correct this.

Tested on arm-none-eabi.
Applied to SVN trunk.

Paul

2008-08-04  Paul Brook  <paul@codesourcery.com>

	gcc/
	* cofig/arm/arm.c (thumb_core_reg_alloc_order): New.
	(arm_order_regs_for_local_alloc): New function.
	* config/arm/arm-protos.h (arm_order_regs_for_local_alloc): Add
	prototype.
	* config/arm/arm.h (ORDER_REGS_FOR_LOCAL_ALLOC): Define.

Index: gcc/config/arm/arm.c
===================================================================
--- gcc/config/arm/arm.c	(revision 138613)
+++ gcc/config/arm/arm.c	(working copy)
@@ -19046,4 +19046,28 @@ arm_mangle_type (const_tree type)
   return NULL;
 }
 
+/* Order of allocation of core registers for Thumb: this allocation is
+   written over the corresponding initial entries of the array
+   initialized with REG_ALLOC_ORDER.  We allocate all low registers
+   first.  Saving and restoring a low register is usually cheaper than
+   using a call-clobbered high register.  */
+
+static const int thumb_core_reg_alloc_order[] =
+{
+   3,  2,  1,  0,  4,  5,  6,  7,
+  14, 12,  8,  9, 10, 11, 13, 15
+};
+
+/* Adjust register allocation order when compiling for Thumb.  */
+
+void
+arm_order_regs_for_local_alloc (void)
+{
+  const int arm_reg_alloc_order[] = REG_ALLOC_ORDER;
+  memcpy(reg_alloc_order, arm_reg_alloc_order, sizeof (reg_alloc_order));
+  if (TARGET_THUMB)
+    memcpy (reg_alloc_order, thumb_core_reg_alloc_order,
+            sizeof (thumb_core_reg_alloc_order));
+}
+
 #include "gt-arm.h"
Index: gcc/config/arm/arm.h
===================================================================
--- gcc/config/arm/arm.h	(revision 138613)
+++ gcc/config/arm/arm.h	(working copy)
@@ -1080,6 +1080,9 @@ extern int arm_structure_size_boundary;
    127						\
 }
 
+/* Use different register alloc ordering for Thumb.  */
+#define ORDER_REGS_FOR_LOCAL_ALLOC arm_order_regs_for_local_alloc ()
+
 /* Interrupt functions can only use registers that have already been
    saved by the prologue, even if they would normally be
    call-clobbered.  */
Index: gcc/config/arm/arm-protos.h
===================================================================
--- gcc/config/arm/arm-protos.h	(revision 138613)
+++ gcc/config/arm/arm-protos.h	(working copy)
@@ -208,4 +208,6 @@ extern void arm_lang_object_attributes_i
 
 extern const char *arm_mangle_type (const_tree);
 
+extern void arm_order_regs_for_local_alloc (void);
+
 #endif /* ! GCC_ARM_PROTOS_H */