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 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