TREE_USED and DECL_READ_P

GCC Edition:7.3.0 I find in source code/gcc/c/c-decl.c ,  Line 1265 to 1281 decides what situation leads to warnings for unused variables I am confused about line 1266: I think only the DECL_READ_P is enough for program to  decides the warning for unused variables, so I can elmiminate " !TREE_USED(

What is the difference between datatype "const_tree" and "tree" ?

In the function emit_side_effect_warnings, I find its inputs are "location_t loc" and "tree expr". And, there is a function warn_if_unused_value called by emit_side_effect_warnings: emit_side_effect_warnings(location_t loc, tree expr){    ...     else     warn_if_unused_value(expr,loc) } To

What is dump_file in gcc ?

I have ever seen "fprintf(dump_file,"something writes into dump_file") many times. I want to know what dump_file is and how can I check its content ?

I want to know the information about the function arguments

When I modify the gcc source code, sometimes I do not know what parameters does the function call. For example,  bool warn_if_unused_value(const tree exp,location_t locus){    function declaration ... } I want to know what "exp" and "locus" are by using fprintf such as fprintf(stderr,"%s\n%s\n",e

I want to dump something when I compile the program. How should I do ?

My work is to modify the gcc source code so as to customize the warning message, when the programmer writing the program violating some rules.  When the violation occurs, I want to reveal some message such as "guanjen375 warning: the rule ### is violated" on the window. How should I do ? Should I