[LLVMdev] Loop unrolling a function

Skye Wanderman-Milne skye at cloudera.com
Tue Jan 14 21:32:36 PST 2014

Hi all,

I'm attempting to perform loop unrolling on a single function using the C++
API. Maybe I missed something in the docs, but I cannot figure out a way to
do this. The function I'm working with is very simple, containing a single
for loop and not much else. I compiled the function to IR using clang with
no optimizations enabled.

My initial plan was to first run the IndVarSimplify pass (as suggested
and then the LoopUnroll pass on the function. I tried using a
FunctionPassManager to do this, even though the docs state FPMs are used
with FunctionPasses and not LoopPasses, which didn't complain but had no
effect on the function. Is adding LoopPasses to a FPM supposed to be a
no-op? Is this supposed to work and I'm not running the right combination
of passes to get loop unrolling?

I also tried creating a LoopInfo object over my function with a FPM so I
could call UnrollLoop() manually, but the LoopInfo didn't return any loops,
perhaps because I'm missing some preliminary passes.

The last option I considered was copying the function into it's own module,
using a PassManager, and then copying the function back into the original
module. I didn't try this option as creating a valid stand-alone module
with the single function will potentially be difficult.

Does anyone have a suggestion for how to do this?

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