Re: On the x86_64, does one have to zero a vector register before filling it completely ?

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Nov 28, 2009 at 3:21 AM, Toon Moene <toon@moene.org> wrote:
> L.S.,
>
> Due to the discussion on register allocation, I went back to a hobby of
> mine: Studying the assembly output of the compiler.
>
> For this Fortran subroutine (note: unless otherwise told to the Fortran
> front end, reals are 32 bit floating point numbers):
>
> ? ? ?subroutine sum(a, b, c, n)
> ? ? ?integer i, n
> ? ? ?real a(n), b(n), c(n)
> ? ? ?do i = 1, n
> ? ? ? ? c(i) = a(i) + b(i)
> ? ? ?enddo
> ? ? ?end
>
> with -O3 -S (GCC: (GNU) 4.5.0 20091123), I get this (vectorized) loop:
>
> ? ? ? ?xorps ? %xmm2, %xmm2
> ? ? ? ?....
> .L6:
> ? ? ? ?movaps ?%xmm2, %xmm0
> ? ? ? ?movaps ?%xmm2, %xmm1
> ? ? ? ?movlps ?(%r9,%rax), %xmm0
> ? ? ? ?movlps ?(%r8,%rax), %xmm1
> ? ? ? ?movhps ?8(%r9,%rax), %xmm0
> ? ? ? ?movhps ?8(%r8,%rax), %xmm1
> ? ? ? ?incl ? ?%ecx
> ? ? ? ?addps ? %xmm1, %xmm0
> ? ? ? ?movaps ?%xmm0, 0(%rbp,%rax)
> ? ? ? ?addq ? ?$16, %rax
> ? ? ? ?cmpl ? ?%ebx, %ecx
> ? ? ? ?jb ? ? ?.L6
>
> I'm not a master of x86_64 assembly, but this strongly looks like %xmm{0,1}
> have to be zero'd (%xmm2 is set to zero by xor'ing it with itself), before
> they are completely filled with the mov{l,h}ps instructions ?
>

I think it is used to avoid partial SSE register stall.


-- 
H.J.