[tree-ssa] Out of SSA status and issues

[Michael Matz]

Hi,

On 13 May 2003, Diego Novillo wrote:

> Why?
>
>      1. foo()
>      2. {
>      3.   int i, *p;
>      4.
>      5.   p = malloc();
>      6.   i = *p;
>      7.   return i + 9;
>      8. }
>
> I see nothing wrong in replacing 'i + 9' with '*p + 9'.

I do.  *p could have been changed in between 6 and 7.  The useful thing of
SSA is, that there is exactly one definition of an entity, and that is the
reason that you can copy-propagate extremely easily (because you know,
that the source of the copy can't be possibly changed after that copy
insn, because the def must have reached it, and that was the only def).

To be able to propagate *p you have to explicitely proove by some means
that it still is unchanged at the places of the uses of the copied-into
entity.  This would be exactly detrimental to the usefullness of copy
propagation.

I.e. *p is not a register but an expression, and a copy instruction copies
registers to registers.

> Not that tree-ssa will do anything with this code, the default
> type-based aliasing is too conservative, but PTA may disambiguate this.

Of course you _can_ optimize such cases, but it's not a subset of copy
propagation but, hmm, un-PRE or something like that.


Ciao,
Michael.