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

[Toon Moene]

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 ?

Am I missing something ?

[ BTW, the induction variable %ecx could have been eliminated,
   because %rax also counts upwards (but 16 at a time instead of 1) ]

Thanks for any insight,

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
At home: http://moene.org/~toon/
Progress of GNU Fortran: http://gcc.gnu.org/gcc-4.5/changes.html