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

[Philip Reames via llvm-commits]

On 10/6/21 1:46 PM, Nikita Popov wrote:
> On Wed, Oct 6, 2021 at 10:32 PM Philip Reames 
> <listmail at philipreames.com <mailto: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.
Fair enough, thanks.  And again, sorry for the sharpness of tone on the 
original response.  That was uncalled for on my part.
>
> 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.

Hm, with D111191, I can see your point.  Amusingly enough the helper 
function is back to the signature I'd originally proposed, largely with 
this in mind.  I'm not a huge fan of the need for the double check, but 
given it has a demonstrated benefit, it seems called for.

Anyways, thanks!

>
> 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
>     <https://github.com/llvm/llvm-project/commit/17c20a6dfb7c89ac4eb13990308b731263345918>
>     > DIFF:
>     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 <mailto:llvm-commits at lists.llvm.org>
>     > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>     <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/bc468901/attachment.html>