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

[Nikita Popov via llvm-commits]

On Wed, Oct 6, 2021 at 10:32 PM Philip Reames <listmail at philipreames.com>
wrote:

> 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
>

Yes, this does recover the minor regression that calling dominates() here
introduced.

Also, I consider this the simpler code. I didn't like how D111191 couples
implementation details, by having to hardcode the nodes supported by
getDefiningScopeBound() in the caller as well. Now you can always call it
and use a non-null result.

Regards,
Nikita


> 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();
> >   }
> >
> >
> >
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20211006/f708c13a/attachment.html>