Inserting named labels through a GCC plugin

Question

I'm writing a gcc plugin to find out target basic blocks to branch instructions, and need to match instructions and basic blocks with compiled binaries, so I decided to explicitly annotate all the branch
instructions and basic blocks with named labels, then from the compiler plugin I assign named labels to branch instructions and basic blocks, and from symbol table I can get offset to these instructions
and basic blocks. Is this the right way to do it, or I shouldn't do this task in compiling phase?

When I was trying with this method, I wrote a plugin with some code like this:

unsigned int
first_rtl_exec(void)
{
 rtx insn, bb_start, bb_end;
 basic_block bb;
 FOR_EACH_BB(bb)
 {
 bb_start = gen_label_rtx();
 LABEL_NAME(bb_start) = "BB Start";
 LABEL_NUSES(bb_start) = 1;
 bb_end = gen_label_rtx();
 LABEL_NAME(bb_end) = "BB End";
 LABEL_NUSES(bb_end) = 1;

 emit_label_before(bb_start, BB_HEAD(bb));
 emit_label_after(bb_end, BB_END(bb));
 }

 return 0;
}

I insert this pass before final pass. But when compiling C program with this plugin, I got error:

foo.c:7:1: internal compiler error: in final, at final.c:1958
 }
 ^

It seems this is not the correct way to insert labels. Am I using emit_label_* in a wrong way, at wrong time, or I'm totally in the wrong direction?