[lno] tree loop optimization causes a problem in RTL alias analysis

[Zdenek Dvorak]

Hello,

> We have encountered a problem when the -ftree-loop-optimize flag is
> specified. The problem causes bad schedules due to unnecessary memory
> dependences. We compiled the below code example, and compared memory
> attributes in the RTL with and without -ftree-loop-optimize. It turned
> out that when -ftree-loop-optimize was enabled the memory attributes
> (MEM_EXPR) were lost.
> 
> int a[128];
> int b[128];
> 
> void f ()
> {
>   int i;
>   for (i = 0; i < 128; i++)
>     a[i] = b[i];
> }
> 
> We used powerpc-apple-darwin7.2.0 with the following compiler flags:
> "-O3 -ftree-loop-optimize -funroll-loops"
> "-O3 -fno-tree-loop-optimize -funroll-loops"
> 
> The following code from alias.c uses that information, and cannot
> proceed if that information is not present:

the problem (I think) is that the code above is strength-reduced to

int *p, *q;

  p = a;
  q = b;
  for (i = 0; i < 128; i++)
    *p++ = *q++;

Which does not contain the aliasing information accessible on rtl level.
By a coincidence, I am just testing a patch that should fix the problem.

Zdenek