Mac OS X i486 ABI -- 16-byte stack alignment

[Landon Fuller]

I've been working on getting the JDK  running on Mac OS X Leopard /  
x86, based on the BSD's JDK16 porting work -- so far I've gotten  
'Hello World' running, while anything more complex quickly runs into  
stack alignment issues -- Mac OS X's x86 calling conventions require  
a 16-byte aligned stack pointer at function entry.

The first issue I've hit is alignment in the  
MacroAssembler::call_VM_base() code generation. It doesn't look like  
I can determine the current stack alignment within the call_VM()  
methods, and there's no gaurantee that the frame started 16-byte  
aligned, either. Given that, I was thinking that doing an alignment  
fixup in call_VM_base() might be a workable solution -- something like:

	/* Arguments */
	int argSize = arg_count * wordSize;

	pushl %esi		; save %esi, we'll use it to store the current sp
	movl %esp, %esi
	andl $-16, %esp	; force sp to a known (16-byte) alignment
	if (argSize & 15) {
		// Pad SP for alignment if the pushed args aren't going to leave us  
16-byte aligned
		subl $((argSize + 15) & -16) - argSize, %esp;
	}
	... push arguments ...
	call
	movl %esi, %esp	; restore pre-alignment sp
	popl %esi		; restore esi

However, this means that call_VM_base() needs to push the arguments  
on to the stack itself, rather than relying on the call_VM()  
implementations to do so. Thus, I was thinking about modifying  
call_VM_base() to accept an arg array:

   virtual void call_VM_base(           // returns the register  
containing the thread upon return
     Register oop_result,               // where an oop-result ends  
up if any; use noreg otherwise
     Register java_thread,              // the thread if computed  
before     ; use noreg otherwise
     Register last_java_sp,             // to set up last_Java_frame  
in stubs; use noreg otherwise
     address  entry_point,              // the entry point
     int      number_of_arguments,      // the number of arguments (w/ 
o thread) to pop after the call
     Register args[],                   // call arguments, left-to- 
right ordered <-- NEW ARGUMENT
     bool     check_exceptions          // whether to check for  
pending exceptions after return
   );

That way, call_VM can fixup the alignment, push the args, do the  
call, and restore esp.

Thoughts, criticism, and suggestions are very much appreciated, as  
are suggestions on where else I should be worrying about stack  
alignment. This is the first I've looked at the hotspot code, so  
hopefully my evaluation is not completely off in the weeds.

Thanks!
-landonf
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