Re: Caused by unknown alignment, was: Re: On the x86_64, does one have to zero a vector register before filling it completely ?

[Toon Moene <toon at moene dot org>]

Toon Moene wrote:

> This is where IPA could help.  I created the following main program:
>
>       real a(10), b(10), c(10)
>       a = 0.
>       b = 1.
>       print '(3(1x,z16))', loc(a), loc(b), loc(c)
>       call sum(a, b, c, 10)
>       print *, c(5)
>       end

> So the alignment of a, b and c is known and is correct for vectorization 
> - still the loop in the subroutine looks like this (objdump -S a.out):

Inlining the "sum.f" subroutine by hand:

      integer i
      real a(10), b(10), c(10)
      a = 0.
      b = 1.
      print '(3(1x,z16))', loc(a), loc(b), loc(c)
      do i = 1, 10
         c(i) = a(i) + b(i)
      enddo
      print *, c(5)
      end

*does* lead to better code:

        movaps  1056(%rsp), %xmm0
        movq    %rbp, %rdi
        addps   1008(%rsp), %xmm0
        movq    $.LC2, 488(%rsp)
        movaps  %xmm0, 960(%rsp)
        movl    $9, 496(%rsp)
        movaps  1072(%rsp), %xmm0
        movl    $128, 480(%rsp)
        addps   1024(%rsp), %xmm0
        movl    $6, 484(%rsp)
        movaps  %xmm0, 976(%rsp)
        movss   1088(%rsp), %xmm0
        addss   1040(%rsp), %xmm0
        movss   %xmm0, 992(%rsp)
        movss   1092(%rsp), %xmm0
        addss   1044(%rsp), %xmm0
        movss   %xmm0, 996(%rsp)

i.e., a completely unrolled and (SLP) vectorized code.

So the potential is there - what we just need is an Alignment 
Propagation Pass (analogous to the Constant and the Range Propagation pass).

--
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