where can I find documentation on RTL?

[Jonathan Wilson]

Specificly things like gen_reg_rtx, gen_rtx_REG, emit_call_insn, 
start_sequence, get_insns etc?
I looked in the GCC internals manual and I didnt find what I wanted.

Basicly, I am inside legitimize_tls_address and I cant figure out just what 
I should be doing to turn the passed in rtx x into the return value.

What will the input rtx to legitimize_tls_address look like?