[PATCH] [Runtime Unrolling] use a loop to simplify the runtime unrolling prologue.

[Kevin Qin]

Runtime unrolling will create a prologue to execute the extra iterations which is can't divided by the unroll factor. It generates an if-then-else sequence to jump into a factor -1 times unrolled loop body, like

    extraiters = tripcount % loopfactor
    if (extraiters == 0) jump Loop:
    if (extraiters == loopfactor) jump L1
    if (extraiters == loopfactor-1) jump L2
    ...
    L1:  LoopBody;
    L2:  LoopBody;
    ...
    if tripcount < loopfactor jump End
    Loop:
    ...
    End:

It means if the unroll factor is 4, the loop body will be 7 times unrolled, 3 are in loop prologue, and 4 are in the loop.
This patch is to use a loop to execute the extra iterations in prologue, like

        extraiters = tripcount % loopfactor
        if (extraiters == 0) jump Loop:
        else jump Prol
 Prol:  LoopBody;
        extraiters -= 1                 // Omitted if unroll factor is 2.
        if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
        if (tripcount < loopfactor) jump End
 Loop:
 ...
 End:

Then when unroll factor is 4, the loop body will be copied by only 5 times, 1 in the prologue loop, 4 in the original loop. And if the unroll factor is 2, new loop won't be created, just as the original solution.

On AArch64 target, if  runtime unrolling enabled, after applying this patch, the code size will drop by 10%.

Also, the sequence of if-then-else sequence is saved, which could bring very slightly performance benefit, which is less than 0.1% on X86 and AArch64 target.

So overall, this patch can bring a lot of code size improvement, and have no harm to performance.

Is it OK to commit?

Thanks,
Kevin

http://reviews.llvm.org/D5147

Files:
  lib/Transforms/Utils/LoopUnrollRuntime.cpp
  test/Transforms/LoopUnroll/PowerPC/a2-unrolling.ll
  test/Transforms/LoopUnroll/runtime-loop.ll
  test/Transforms/LoopUnroll/runtime-loop1.ll
  test/Transforms/LoopUnroll/runtime-loop2.ll
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