[llvm] 17c20a6 - [SCEV] Avoid unnecessary domination checks (NFC)

[Philip Reames via llvm-commits]

Er, do you actually have evidence of this being a problem?  I would be 
very surprised if calling DT was even *remotely* expensive here, and I 
would strongly prefer the simpler code.

Philip

On 10/6/21 1:14 PM, Nikita Popov via llvm-commits wrote:
> Author: Nikita Popov
> Date: 2021-10-06T22:14:04+02:00
> New Revision: 17c20a6dfb7c89ac4eb13990308b731263345918
>
> URL: https://github.com/llvm/llvm-project/commit/17c20a6dfb7c89ac4eb13990308b731263345918
> DIFF: https://github.com/llvm/llvm-project/commit/17c20a6dfb7c89ac4eb13990308b731263345918.diff
>
> LOG: [SCEV] Avoid unnecessary domination checks (NFC)
>
> When determining the defining scope, avoid repeatedly querying
> dominationg against the function entry instruction. This ends up
> begin a very common case that we can handle more efficiently.
>
> Added:
>      
>
> Modified:
>      llvm/include/llvm/Analysis/ScalarEvolution.h
>      llvm/lib/Analysis/ScalarEvolution.cpp
>
> Removed:
>      
>
>
> ################################################################################
> diff  --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
> index df1b7c2ad157..a38b1a299163 100644
> --- a/llvm/include/llvm/Analysis/ScalarEvolution.h
> +++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
> @@ -1921,9 +1921,10 @@ class ScalarEvolution {
>     SCEV::NoWrapFlags getNoWrapFlagsFromUB(const Value *V);
>   
>     /// Return a scope which provides an upper bound on the defining scope of
> -  /// 'S'.  Specifically, return the first instruction in said bounding scope.
> +  /// 'S'. Specifically, return the first instruction in said bounding scope.
> +  /// Return nullptr if the scope is trivial (function entry).
>     /// (See scope definition rules associated with flag discussion above)
> -  const Instruction *getDefiningScopeBound(const SCEV *S);
> +  const Instruction *getNonTrivialDefiningScopeBound(const SCEV *S);
>   
>     /// Return a scope which provides an upper bound on the defining scope for
>     /// a SCEV with the operands in Ops.
>
> diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
> index b767adac6a59..14f5dd51c406 100644
> --- a/llvm/lib/Analysis/ScalarEvolution.cpp
> +++ b/llvm/lib/Analysis/ScalarEvolution.cpp
> @@ -6585,25 +6585,24 @@ SCEV::NoWrapFlags ScalarEvolution::getNoWrapFlagsFromUB(const Value *V) {
>     return isSCEVExprNeverPoison(BinOp) ? Flags : SCEV::FlagAnyWrap;
>   }
>   
> -const Instruction *ScalarEvolution::getDefiningScopeBound(const SCEV *S) {
> +const Instruction *
> +ScalarEvolution::getNonTrivialDefiningScopeBound(const SCEV *S) {
>     if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(S))
>       return &*AddRec->getLoop()->getHeader()->begin();
>     if (auto *U = dyn_cast<SCEVUnknown>(S))
>       if (auto *I = dyn_cast<Instruction>(U->getValue()))
>         return I;
> -  // All SCEVs are bound by the entry to F
> -  return &*F.getEntryBlock().begin();
> +  return nullptr;
>   }
>   
>   const Instruction *
>   ScalarEvolution::getDefiningScopeBound(ArrayRef<const SCEV *> Ops) {
> -  const Instruction *Bound = &*F.getEntryBlock().begin();
> -  for (auto *S : Ops) {
> -    auto *DefI = getDefiningScopeBound(S);
> -    if (DT.dominates(Bound, DefI))
> -      Bound = DefI;
> -  }
> -  return Bound;
> +  const Instruction *Bound = nullptr;
> +  for (auto *S : Ops)
> +    if (auto *DefI = getNonTrivialDefiningScopeBound(S))
> +      if (!Bound || DT.dominates(Bound, DefI))
> +        Bound = DefI;
> +  return Bound ? Bound : &*F.getEntryBlock().begin();
>   }
>   
>   
>
>
>          
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