[Question] How to deliver loop-related pragma information from TREE to RTL?

[Yangfei (Felix)]

Hello everyone,

    I'm thinking of the right way of adding some loop related pragmas to GCC. 
    An example:

    #pragma loop unroll = 2
    for (i = 0; i < n; i ++)
      {
        Whatever...
      }

    Here I want the unroll factor of the loop to be 2 when doing RTL loop 
unrolling. 
    But I find that it's kind of difficult to achieve this in GCC. 
    The reason is that the loop structures are initialized and finalized at run 
time. Also the RTL expand phase is kind of complex and it does not care about 
loops. 
    So how to deliver the pragma information to RTL loop unrolling pass? What 
the the elegant way of doing this? 
    Can anyone give some suggestions on this please? 
    Many thanks.