[llvm-dev] Legacy Loop Pass Manager question

[Bhatu via llvm-dev]

Ping!

On Thu, Aug 9, 2018 at 2:46 PM, Bhatu <cs12b1010 at iith.ac.in> wrote:

> Hi,
>
> If we add multiple loop passes to the pass manager in
> PassManagerBuilder.cpp consecutively without any func/module pass in
> between, I used to think they would belong to the same loop pass manager.
> But it does not seem to be the case.
>
> For example for this code snippet
>   PM.add(createIndVarSimplifyPass());        // Canonicalize indvars
>   MPM.add(createLoopIdiomPass());             // Recognize idioms like
> memset.
>   addExtensionsToPM(EP_LateLoopOptimizations, MPM);
>   MPM.add(createLoopDeletionPass());          // Delete dead loops
> ..
>   if (!DisableUnrollLoops)
>     MPM.add(createSimpleLoopUnrollPass(OptLevel));    // Unroll small
> loops
>
> I see -debug-pass=Structure output:
>         Scalar Evolution Analysis
>         Loop Pass Manager
>           Induction Variable Simplification
>           Recognize loop idioms
>           Delete dead loops
>           Unroll loops
>         MergedLoadStoreMotion
>
> Which is in line to what I thought. However for this code snippet:
>     if (EnableUnrollAndJam) {
>       // Unroll and Jam. We do this before unroll but need to be in a
> separate
>       // loop pass manager in order for the outer loop to be processed by
>       // unroll and jam before the inner loop is unrolled.
>       MPM.add(createLoopUnrollAndJamPass(OptLevel));
>     }
>
>     MPM.add(createLoopUnrollPass(OptLevel));    // Unroll small loops
>
> I see:
>       Loop-Closed SSA Form Pass
>       Loop Pass Manager
>         Unroll and Jam loops
>       Loop Pass Manager
>         Unroll loops
>       Lazy Branch Probability Analysis
>
> Here two different loop pass managers were created. What is the difference
> in between these two cases? How was the loop pass manager split up in the
> second case?
>
>
> --
> Regards
> Bhatu
>



-- 
Regards
Bhatu
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