[aarch64-port-dev ] RFC: Change stack bang sequence

[Andrew Haley]

The stack bang size changed to 25 4k pages due to 8172144, Implement
"JEP 270: Reserved Stack Areas for Critical Sections" and 8173339, Fix
minimum stack size computations.

Unfortunately this means that we now generate three instructions for a
stack bang if we are using 4k pages.

This patch changes

  0x0000007fa8f748d4: mov	x9, #0xffffffffffffc000    	// #-16384
  0x0000007fa8f748d8: movk	x9, #0xfffe, lsl #16
  0x0000007fa8f748dc: str	xzr, [sp,x9]

to

  0x0000007f88bae8d4: sub	x9, sp, #0x14, lsl #12
  0x0000007f88bae8d8: str	xzr, [x9]

There's no measurable speed difference on a system I have tested, so
do we care?  I suppose we do, given that it makes the code slightly
smaller.  Opinions welcome.

diff -r ddabaeb7d186 src/cpu/aarch64/vm/macroAssembler_aarch64.hpp
--- a/src/cpu/aarch64/vm/macroAssembler_aarch64.hpp	Wed Jun 28 10:58:19 2017 +0200
+++ b/src/cpu/aarch64/vm/macroAssembler_aarch64.hpp	Fri Jun 30 15:29:47 2017 +0100
@@ -949,8 +949,8 @@
   void bang_stack_with_offset(int offset) {
     // stack grows down, caller passes positive offset
     assert(offset > 0, "must bang with negative offset");
-    mov(rscratch2, -offset);
-    str(zr, Address(sp, rscratch2));
+    sub(rscratch2, sp, offset);
+    str(zr, Address(rscratch2));
   }

   // Writes to stack successive pages until offset reached to check for

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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