[PATCH][ARM][thumb1] Reduce lr save for leaf function with non-far jump

["Joey Ye" <joey dot ye at arm dot com>]

Current GCC thumb1 has an annoying problem that always assuming far branch.
So it forces to save lr, even when unnecessarily. The most extreme case
complained by partner is:

// compiled with "-mthumb -mcpu=cortex-m0 -Os".
void foo() { for (;;); }
=>
foo:
	push	{lr}  // Crazy!!!
.L2:
	b	.L2

The reason is that thumb1 far jump is only resolved in the very late pass
"shorten_branch". Prologue/epilogue pass doesn't actually know a branch is
far or not from its attribute. It has to conservatively save/restore lr
whenever there is a branch.

This patch tries to fix it with a simple heuristic, i.e., using function
size to decide if a far jump will likely be used. Function size information
is meaningful in prologue/epilogue pass. The heuristic uses following check
to decide if lr should be saved for far jump:

function_size * 3 >= 2048 // yes: save lr for possible far jump. No: don't
save lr for far jump

The scheme has an issue: if some corner case does break above condition,
there is no chance to fix-up but to ICE. But the heuristic condition is very
conservative. It is base on the worse normal condition that each instruction
is associated with a 4 byte literal ( (2+4)/2=3, blooming size by 3 times ).
I can't think of a real case to trigger the ICE. So I think it should work.

Other approaches than the heuristic scheme are too expensive to implement
for this small size/performance issue. I did explored some but none of them
persuaded myself.

Tests passed:
* build libgcc, libstdc++, newlib, libm
* make check-gcc with cpu=cortex-m0
* Small and extreme test cases

ChangeLog:

2012-12-20  Joey Ye  <joey.ye@arm.com>

	* config/arm/arm.c(thumb1_final_prescan_insn): 
	Assert lr save for real far jump.
	(thumb_far_jump_used_p): Count instruction size and set 
     far_jump_used.

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 327ef22..ad79451 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -21790,6 +21857,11 @@ thumb1_final_prescan_insn (rtx insn)
       else if (conds != CONDS_NOCOND)
 	cfun->machine->thumb1_cc_insn = NULL_RTX;
     }
+
+    /* Check if unexpected far jump is used.  */
+    if (cfun->machine->lr_save_eliminated
+        && get_attr_far_jump (insn) == FAR_JUMP_YES)
+      internal_error("Unexpected thumb1 far jump");
 }
 
 int
@@ -21815,6 +21887,8 @@ static int
 thumb_far_jump_used_p (void)
 {
   rtx insn;
+  bool far_jump = false;
+  unsigned int func_size = 0;
 
   /* This test is only important for leaf functions.  */
   /* assert (!leaf_function_p ()); */
@@ -21870,6 +21944,26 @@ thumb_far_jump_used_p (void)
 	  && get_attr_far_jump (insn) == FAR_JUMP_YES
 	  )
 	{
+	  far_jump = true;
+	}
+      func_size += get_attr_length (insn);
+    }
+
+  /* Attribute far_jump will always be true for thumb1 before
shorten_branch
+     pass. So checking far_jump attribute before shorten_branch isn't much
+     useful.
+     
+     Following heuristic tries to estimate more accruately if a far jump
may 
+     finally be used. The heuristic is very conservative as there is no
chance
+     to roll-back the decision of not to use far jump.
+
+     Thumb1 long branch offset is -2048 to 2046. The worst case is each
2-byte
+     insn is assiociated with a 4 byte constant pool. Using function size 
+     2048/3 as the threshold is conservative enough.  */
+  if (far_jump)
+    {
+      if ((func_size * 3) >= 2048)
+        {
 	  /* Record the fact that we have decided that
 	     the function does use far jumps.  */
 	  cfun->machine->far_jump_used = 1;