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:

> Tim Prince wrote:
> 
>  > If you want those, you must request them with -mtune=barcelona.
> 
> OK, so it is an alignment issue (with -mtune=barcelona):
> 
> .L6:
>         movups  0(%rbp,%rax), %xmm0
>         movups  (%rbx,%rax), %xmm1
>         incl    %ecx
>         addps   %xmm1, %xmm0
>         movaps  %xmm0, (%r8,%rax)
>         addq    $16, %rax
>         cmpl    %r10d, %ecx
>         jb      .L6

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

[ The first print statement assures me a, b and c are 16 byte aligned
   (hexadecimal printout of address ends in 0), the second one prevents
   optimization from throwing away everything. ]

and then compiling with:

$ /usr/snp/bin/gfortran -O3 -flto -fwhole-program main.f sum.f

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):

   4004d0:       0f 28 c2                movaps %xmm2,%xmm0
   4004d3:       0f 28 ca                movaps %xmm2,%xmm1
   4004d6:       0f 12 04 07             movlps (%rdi,%rax,1),%xmm0
   4004da:       0f 12 0c 06             movlps (%rsi,%rax,1),%xmm1
   4004de:       0f 16 44 07 08          movhps 0x8(%rdi,%rax,1),%xmm0
   4004e3:       0f 16 4c 06 08          movhps 0x8(%rsi,%rax,1),%xmm1
   4004e8:       ff c1                   inc    %ecx
   4004ea:       0f 58 c1                addps  %xmm1,%xmm0
   4004ed:       41 0f 29 04 02          movaps %xmm0,(%r10,%rax,1)
   4004f2:       48 83 c0 10             add    $0x10,%rax
   4004f6:       44 39 c9                cmp    %r9d,%ecx
   4004f9:       72 d5                   jb     4004d0 <main+0x1c0>

-- 
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