On Mon, 21 Mar 2005 13:45:19 +0100 (CET), Richard Guenther
<[EMAIL PROTECTED]> wrote:
> Hi!
>
> On mainline we now use loop versioning and peeling for alignment
> for the following loop (-march=pentium4):
>
> void foo3(float * __restrict__ a, float * __restrict__ b,
> float * __restrict__ c)
> {
> int i;
> for (i=0; i<4; ++i)
> a[i] = b[i] + c[i];
> }
>
> which results only in slower and larger code. I also cannot
> see why we zero the mm registers before loading and why we
> load them high/low separated:
>
> .L13:
> xorps %xmm1, %xmm1
> movlps (%edx,%esi), %xmm1
> movhps 8(%edx,%esi), %xmm1
> xorps %xmm0, %xmm0
> movlps (%edx,%ebx), %xmm0
> movhps 8(%edx,%ebx), %xmm0
> addps %xmm0, %xmm1
> movaps %xmm1, (%edx,%eax)
> addl $1, %ecx
> addl $16, %edx
> cmpl %ecx, -16(%ebp)
> ja .L13
>
> but the point is, there is nothing to win vectorizing the loop
> in the first place if we do not know alignment before.
Uh, and with -funroll-loops we seem to be lost completely, as we
produce peeling/loops for a eight times four rolling loop! Where is
the information about the loop counter gone??
It looks like vectorization interacts badly with the rest of the loop
optimizers.
Ugh.
Richard.
