[llvm-dev] LLC does not do proper copy propagation (or copy coalescing)

[Alex Susu via llvm-dev]

   Hello.
     Could you please tell me how can I optimize with the back end (llc) the following 
piece of assembly code generated by llc:

       // NOTE: my processor accepts loops in the form of REPEAT(num_times)..END_REPEAT
       R0 = ...
       REPEAT(256)
         R5 = R0; // basically unnecessary reg. copy
         REPEAT(256)
           R10 = LS[R4];
           R2 = LS[R5];
           R4 = R4 + R1;
           R5 = R5 + R1; // should be R0 = R0 + R1
           R10 = R2 * R10;
           R3 = R3 + R10;
         END_REPEAT;
         REDUCE R3;
         R0 = R5; // basically unnecessary reg. copy
       END_REPEAT;

     The above code has the deficiencies created basically by PHI elimination and not 
applying a proper register copy propagation on machine instructions before Register 
Allocation.

     I see 3 options to address my problem:
       - implement a case that handles this in PHI elimination (PHIElimination.cpp);
       - create a new pass that does copy propagation (based on DFA) on machine 
instructions before Register Allocation;
       - optimize copy coalescing such as the standard one or the one activated by 
-pbqp-coalescing in lib/CodeGen/RegAllocPBQP.cpp (there is an email also about PBQP 
coalescing at http://lists.llvm.org/pipermail/llvm-dev/2016-June/100523.html).

   Best regards,
     Alex