POINTER_PLUS_EXPR Vs. PLUS_EXPR

[a b]

I hit a problem about the 2 operands of a addr-plus instruction. My instruction is special because it is not commutative and requries address be the 2nd operand and the offset in the 3rd one. But my port generates PLUS_EXPR instead of POINTER_PLUS_EXPR and finally mistakenly switches the order of the address and offset.
The relevant code is in fold-const.c:
10281   switch (code)
10282     {
10283     case POINTER_PLUS_EXPR:
        ....
10292       /* INT +p INT -> (PTR)(INT + INT).  Stripping types allows for this. */
10293       if (INTEGRAL_TYPE_P (TREE_TYPE (arg1))
10294            && INTEGRAL_TYPE_P (TREE_TYPE (arg0)))
10295         return fold_convert_loc (loc, type,
10296                                  fold_build2_loc (loc, PLUS_EXPR, uintptrtype,
10297                                               fold_convert_loc (loc, uintptrtype,
10298                                                                 arg1),
10299                                               fold_convert_loc (loc, uintptrtype,
10300                                                                 arg0)));

the code seems to force gcc to generate a PLUS_EXPR instead of POINTER_PLUS_EXPR and I am not clear what is the context for the piece of code. 
The code was checked in by Andrew in 2007 (http://gcc.gnu.org/ml/fortran/2007-06/msg00163.html)
Could anybody kindly explain me what was going on with that piece of code?
or more specifically:
1. what does INT +p INT mean? shouldn't POINTER_PLUS_EXPR always better than PLUS_EXPR? why change to PLUS_EXPR?
2. In general, when we should use POINTER_PLUS_EXPR and PLUS_EXPR. I understand the good of POINTER_PLUS_EXPR in my case. but it seems that it is not strictly enforced that address+offset should always represented by POINTER_PLUS_EXPR . Then can anybody tell me what the rule that a POINTER_PLUS_EXPR should be or happens to be used?

Another irlevant question is where I can find the regression test result of the gcc releases such as 4.5/4.7? I try to find out which dejagnu tests have known-failures. 

thanks