SSA alias representation

[Diego Novillo]

On 2/19/08 2:27 PM, Fran Baena wrote:
> Hi everybody,
> 
> i am studing how gcc carries out Alias Representation and some questions appear.
> 
> For instance, given this code portion:
> 
>  if ( ... )
>    p  = &a;
>  else
>    if ( ... )
>      p = &b;
>    else
>      p = &c;
> 
>  a = 5;
>  b = 3;
>  d = *p4;
> 
> My questions are:
> 
> - both p like *p need a Name Memory Tag structure? It is enough with only one?

Pointer dereferences do not receive NMTs, only the pointers.  So a name 
tag will be associated with the SSA name resulting from the PHI node 
created at the end of that if() tree.

> - About versioning : every name can be versioned, cannot it?

Sorry, I don't understand the question.  SSA names have a version 
number, yes.  Is that what you're asking?

>    - when are virtual operands inserted? Do they have to be versioned
> at same time that real operands? For instance, # a6 = VDEF <a5>; a7 =
> 3;, this implies that alias computation is processed at same time that
> SSA Renaming?

No, the memory SSA web is built after the first stage of alias analysis. 
  First, the program is put in register SSA form, a few passes later the 
memory SSA UD web is built on top.


>    - Could the others names (TMT, NMT,  SFT, etc.) be versioned?
> (every name that is able to be part of a virtual operand could be
> versioned....)

Yes.  When a pointer does not have a specific set of symbols in its 
points-to set, its memory tag is used.


Diego.