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.
