Understand pointer deferences in GIMPLE

[Shuai Wang via Gcc]

Hello,

I am trying to traverse the GIMPlE statements and identify all pointer
differences (i.e., memory load and store). For instance, something like:

  **_4* = 0;
   ...
  _108 = (signed char *) _107;
  _109 = **_108*;

After some quick searches in the GCC codebase, I am thinking to use the
following statements to identify variables like _4 and _108:

    tree op0 = gimple_op(stmt, 0);    // get the left variable

    if (TREE_CODE (op0) == SSA_NAME) {
      struct ptr_info_def *pi = SSA_NAME_PTR_INFO (op0);
      if (pi) {
        std::cerr << "find a store\n";
        return STORE;
      }
    }

However, to my surprise, variables like _4 just cannot be matched. Actually
_4 and _108 will be both treated as "NOT" SSA_NAME, and therefore cannot
satisfy the first if condition anyway.

So here is my question:

1. How come variables like _4 and _108 are NOT ssa forms?
2. then, what would be the proper way of identifying pointer dereferences,
something like *_4 = 0; and _109 = *_108 + 1?

Best,
Shuai