[MemorySSA] Update comment in PassBuilder

[ychen]

Summary

Is teaching the LoopFullUnrollPass to preserve MemorySSA very hard or
just impossible?

[asbirlea]

It's not impossible. It's hard and likely not worth it.
Preserving MemorySSA for this pass means creating MemoryAccesses for all blocks cloned when unrolling. Creating an access and then updating so many times is likely more expensive than recomputing MemorySSA from scratch.

I have a patch lined up where MemorySSA would be set to true when unroll added to LPM2, but preliminary testing didn't show this case to provide much value.

[ychen]

In D82618#2116101, @asbirlea wrote:

It's not impossible. It's hard and likely not worth it.
Preserving MemorySSA for this pass means creating MemoryAccesses for all blocks cloned when unrolling. Creating an access and then updating so many times is likely more expensive than recomputing MemorySSA from scratch.

I have a patch lined up where MemorySSA would be set to true when unroll added to LPM2, but preliminary testing didn't show this case to provide much value.

Thanks for the information @asbirlea . It is very helpful.

llvm/lib/Passes/PassBuilder.cpp

FunctionPassManager PassBuilder::buildO1FunctionSimplificationPipeline(
   // We provide the opt remark emitter pass for LICM to use. We only need to do
   // this once as it is immutable.
   FPM.addPass(
       RequireAnalysisPass<OptimizationRemarkEmitterAnalysis, Function>());
   FPM.addPass(createFunctionToLoopPassAdaptor(
       std::move(LPM1), EnableMSSALoopDependency, DebugLogging));
   FPM.addPass(SimplifyCFGPass());
   FPM.addPass(InstCombinePass());
-  // The loop passes in LPM2 (IndVarSimplifyPass, LoopIdiomRecognizePass,
-  // LoopDeletionPass and LoopFullUnrollPass) do not preserve MemorySSA.
+  // The loop passes in LPM2 (LoopFullUnrollPass) do not preserve MemorySSA.
   // *All* loop passes must preserve it, in order to be able to use it.
   FPM.addPass(createFunctionToLoopPassAdaptor(
       std::move(LPM2), /*UseMemorySSA=*/false, DebugLogging));
 
   // Delete small array after loop unroll.
   FPM.addPass(SROA());
 
   // Specially optimize memory movement as it doesn't look like dataflow in SSA.