Code Flow for RTL generation

[kernel coder]

hi,
   I'm trying to figure out the code flow of gcc.But i'm not been
able to locate the location at which an expression is compared with
pattern given in XXXX.md file.

I think XXXX.md file is resposible fo generation of emit-insn.c
file.But again at which point the functions in  emit-insn.c file are
called for generation of RTL file.

Please tell me where in code the pattern matching is done for RTL generation.

thanks,
shazad