[PATCH] D108750: [PowerPC] common chains to reuse offsets to reduce register pressure

[Jinsong Ji via Phabricator via llvm-commits]

jsji added inline comments.


================
Comment at: llvm/lib/Target/PowerPC/PPCLoopInstrFormPrep.cpp:500
+    const SCEV *Diff = SE->getMinusSCEV(LSCEV, B.BaseSCEV);
+    if (isValidChainCommoningDiff(Diff)) {
+      B.Elements.push_back(BucketElement(Diff, MemI));
----------------
jsji wrote:
> Can we also abstract this check so that we can also reuse `addOneCandidate` to do this.
> 
> Something like:
> 
> ```
>  for (auto &B : Buckets) {
>     const SCEV *Diff = SE->getMinusSCEV(LSCEV, B.BaseSCEV);
>     if ( const auto *Diff = isValidDiff(Diff, LSCEV, B.BaseSCEV)) {
>       B.Elements.push_back(BucketElement(Diff, MemI));
>       FoundBucket = true;
>       break;
>     }
>   }
> ```
> 
> then  we use `isValidConstantDiff` for others, but `isValidChainCommoningDiff` for this.
> 
> ```
> isValidConstantDiff(Diff, LSCEV, B.BaseSCEV) ={
> return dyn_cast<SCEVConstant>(Diff)
> }
> 
> isValidChainCommoningDiff(Diff, LSCEV, B.BaseSCEV) ={
> 
>   assert(LSCEV && "Invalid SCEV for Ptr value.");
> 
>   // Don't mess up previous dform prepare.
>   if (isa<SCEVConstant>(LSCEV))
>     return null_ptr;
> 
>   // A single integer type offset.
>   if (isa<SCEVUnknown>(LSCEV) && LSCEV->getType()->isIntegerTy())
>     return Diff;
> 
>   const SCEVNAryExpr *ASCEV = dyn_cast<SCEVNAryExpr>(LSCEV);
>   if (!ASCEV)
>     return null_ptr;
> 
>   for (const SCEV *Op : ASCEV->operands())
>     if (!Op->getType()->isIntegerTy())
>       return null_ptr;
> 
>   return Diff;
> 
> 
> }
> 
> 
> ```
```
isValidChainCommoningDiff(Diff, LSCEV, B.BaseSCEV) ={


  assert(LSCEV && "Invalid SCEV for Ptr value.");

  if (cast<SCEVAddRecExpr>(B.BaseSCEV)->getStepRecurrence(*SE) !=
        cast<SCEVAddRecExpr>(LSCEV)->getStepRecurrence(*SE))
      return null_ptr;

  // Don't mess up previous dform prepare.
  if (isa<SCEVConstant>(LSCEV))
    return null_ptr;

  // A single integer type offset.
  if (isa<SCEVUnknown>(LSCEV) && LSCEV->getType()->isIntegerTy())
    return Diff;

  const SCEVNAryExpr *ASCEV = dyn_cast<SCEVNAryExpr>(LSCEV);
  if (!ASCEV)
    return null_ptr;

  for (const SCEV *Op : ASCEV->operands())
    if (!Op->getType()->isIntegerTy())
      return null_ptr;

  return Diff;


}
```


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D108750/new/

https://reviews.llvm.org/D108750