x86_64 varargs setup jump table

[Richard Henderson <rth at redhat dot com>]

On 07/17/2010 07:25 AM, Bernd Schmidt wrote:
>  	leaq	0(,%rax,4), %rcx
>  	movl	$.L2, %eax
>  	subq	%rcx, %rax
>  	jmp	*%rax

I've often thought this was over-engineering in the x86_64 abi.
This jump table is trading memory bandwidth for unpredictability
in the branch target.

I've often wondered if we'd get better performance if we changed
to a simple comparison against zero.  I.e.

	test	%al,%al
	jz	1f
	// 8 xmm stores
1:

H.J., do you think you'd be able to measure performance on this?



r~