how to find variable related to a virtual ssa name

[Richard Biener]

On Tue, May 12, 2020 at 2:44 PM 易会战 via Gcc <gcc@gcc.gnu.org> wrote:
>
> hi, I am working on gcc ssa name. For each function, we can traverse all defined ssa name by macro FOR_EACH_SSA_NAME. If a ssa name is default definition for a symbol (check SSA_NAME_IS_DEFAULT_DEF) , I can get the symbol by SSA_NAME_VAR. But for a virtual DEFAULT DEF, I cannot get it, SSA_NAME_VAR return a identifier named .MEM. I cannot find which variable related to the default definition. Why and how I should find the related variable?
>
>
> By the way , I give my current work,  I wish find a MEM_REF refer to global/heap memory or local stack. I try my best to get a correct memory type. Since MEM_REF have a base address, which is often a ssa name. Athough it is not virtual ssa name. But I find just check ssa name data flow is not enough to get the info.
> For example, a malloc function allocate some heap memory and record the address in a global ptr. On gimple ssa IR, the malloc function return a address assigned to a ssa name , then ssa name assign the value to the global ptr. When i check ssa name defined by the global ptr, I donot know if the ptr point to global memory or local memory.
> Please see the gimple code:
> _2 = malloc()
> ptr = _2
> _3 = ptr
> MEM_REF[BASE _3]
> I wish get _3  is a address pointing to global memory. But just from _3=ptr, cannot judge it. 
> I wish memory SSA can help solve the problem.

memory SSA will not solve this problem.  You can instead query
points-to information
on _3 for example by calling ptr_deref_may_alias_global_p (_3) which internally
looks at SSA_NAME_PTR_INFO which contains the solution of the
points-to computation.

Richard.

>
> Or gcc gives the info at other pass? wish get some advice. Thanks a lot.