Loop Unroll a constant number of times?

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#1

Is it possible to unroll a loop (forcibly if necessary) with llvm (possibly the -loop-unroll pass) a constant number of times.

I believe that I read that the -unroll-count=x option was removed, correct? So is there some other way to do this or is this just not possible in llvm?

#2

Ryan,
I see the following command line options in LoopUnrollPass.cpp:

static cl::opt<unsigned>
UnrollThreshold("unroll-threshold", cl::init(150), cl::Hidden,
  cl::desc("The cut-off point for automatic loop unrolling"));

static cl::opt<unsigned>
UnrollCount("unroll-count", cl::init(0), cl::Hidden,
  cl::desc("Use this unroll count for all loops, for testing purposes"));

static cl::opt<bool>
UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
  cl::desc("Allows loops to be partially unrolled until "
           "-unroll-threshold loop size is reached."));

static cl::opt<bool>
UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
  cl::desc("Unroll loops with run-time trip counts"));

I assume the first two are most applicable to what you're trying to do. I've never used these options, so it would be a good idea to look at the source to see what these are really doing.

Also, you'll probably need to put -mllvm before the argument to pass it to the backend (clang -mllvm -unroll-threshold=10).

You can also use the -debug-only=loop-unroll option to get debug information. It too requires the -mllvm option.

Hope this helps,
  Chad

#3

See clang -mllvm -help-hidden. If that doesn't work, llc -help-hidden.

Probably want something like -mllvm -unroll-runtime -mllvm -unroll-count=N.

As with any experimental options, look at the source, and -mllvm -debug-only=loop-unroll output to understand what it's doing.

Andy

#4

Andrew,

Thanks. Unfortunately, the runtime won’t really work for what I’m doing but it seems that I can statically unroll just fine for the most part.

Thanks.