llvm dev - Jan 2014 - [LLVMdev] Loop unrolling a function

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
here<http://llvm.org/docs/Passes.html#loop-unroll-unroll-loops>)
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?

Thanks,
Skye
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Hi Skye,

If I remember correctly, our loop unrolling pass requires loop rotation in order
to be very effective.  Can you try adding that before you run the unroller to
see if it helps?

--Owen

On Jan 14, 2014, at 09:32 PM, Skye Wanderman-Milne <skye at cloudera.com>
wrote:

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 here) 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?

Thanks,
Skye
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The loop rotation pass does modify the function, which I'm guessing means
that a FunctionPassManager can be used to run LoopPasses (this is not
obvious to me after looking through the FunctionPassManager code).
Unfortunately none of the other passes I'm using (ScalarEvolution, LCSSA,
IndVarSimplify, and LoopUnroll) appear to have an effect.

I verified that the function can be loop unrolled by opt, so I'm currently
using opt to narrow down which passes are required to get this result. Once
I figure that out I'll see if I can get a FunctionPassManager to run those
passes over just the function. I am open to other suggestions though :)

Skye


On Tue, Jan 14, 2014 at 10:22 PM, Owen Anderson <resistor at mac.com>
wrote:
> Hi Skye,
>
> If I remember correctly, our loop unrolling pass requires loop rotation in
> order to be very effective.  Can you try adding that before you run the
> unroller to see if it helps?
>
> --Owen
>
> On Jan 14, 2014, at 09:32 PM, Skye Wanderman-Milne <skye at
cloudera.com>
> wrote:
>
> 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
> here <http://llvm.org/docs/Passes.html#loop-unroll-unroll-loops>) 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?
>
> Thanks,
> Skye
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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Hi Skye,

just stumbled over this thread. I started writing a tool for getting loop
information, e.g. count variable with initialisation, increment and so on. If
you are interested, I could share the code with you.

Cheers,
Sebastian

On 15.01.2014, at 06:32, Skye Wanderman-Milne <skye at cloudera.com>
wrote:
> 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
here) 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?
> 
> Thanks,
> Skye
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev