On Jul 15, 2007, at 11:55 AM, sig wrote:
> Everywhere when some method uses foo struct, generator places
> following line in function:
> register struct foo * foo = &fum;

I believe we only generate that if the foo structure was used in the  
routine more than once.
On powerpc this was a clue that the structure pointer should be in a  
register which gain us some performance
in earlier versions of GC. In later GCC compilers it seems they  
ignore the register hint now.  I once tried to use
the GCC global register hint, which worked quite well, but was  
fraught with issues if all the plugins were not
recompiled and if foo was not setup before anyone invoked a interp.c  
routine as part of VM setup.


>
> and then uses everywhere  foo->bar.
> So, the difference in compiled code when using foo struct or not is  
> minimal:
>
>   mov reg, [bar]   <- using globals
>   mov reg, [foo + bar_offset]  <- with foo
>
> Of course, this depends how well GCC optimizes code, but in optimal
> case - difference between loading value using direct pointer or using
> base+offset is a just few cycles. And i don't think that this may
> cause a major speed degradation.

A cycle here, a cycle there, add up to real cycles.
This is the first byte code in intel assembler properly optimized.

L10161:
	addl	$1, %esi
	movzbl	(%esi), %ebx
	addl	$4, %edi
	movl	_foo, %eax
	movl	84(%eax), %eax
	movl	4(%eax), %eax
	movl	%eax, (%edi)
	movl	512(%esp,%ebx,4), %eax
L10421:
	jmp	*%eax


less than optimal compiles can result in 12 instructions,  9 versus  
12 instructions does equal a difference in real physical time.


--
======================================================================== 
===
John M. McIntosh <johnmci at smalltalkconsulting.com>
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
======================================================================== 
===