[llvm] r204434 - [Constant Hoisting] Lazily compute the idom and cache the result.

[Eric Christopher]

On Thu, Mar 20, 2014 at 11:04 PM, Juergen Ributzka <juergen at apple.com> wrote:
> Author: ributzka
> Date: Fri Mar 21 01:04:39 2014
> New Revision: 204434
>
> URL: http://llvm.org/viewvc/llvm-project?rev=204434&view=rev
> Log:
> [Constant Hoisting] Lazily compute the idom and cache the result.
>

Sounds nice, does it help in compile time?

-eric

> Related to <rdar://problem/16381500>
>
> Modified:
>     llvm/trunk/lib/Transforms/Scalar/ConstantHoisting.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/ConstantHoisting.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/ConstantHoisting.cpp?rev=204434&r1=204433&r2=204434&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/ConstantHoisting.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/ConstantHoisting.cpp Fri Mar 21 01:04:39 2014
> @@ -87,9 +87,10 @@ struct ConstantCandidate {
>  struct RebasedConstantInfo {
>    ConstantUseListType Uses;
>    Constant *Offset;
> +  mutable BasicBlock *IDom;
>
>    RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
> -    : Uses(Uses), Offset(Offset) { }
> +    : Uses(Uses), Offset(Offset), IDom(nullptr) { }
>  };
>
>  /// \brief A base constant and all its rebased constants.
> @@ -152,6 +153,17 @@ private:
>      Entry = nullptr;
>    }
>
> +  /// \brief Find the common dominator of all uses and cache the result for
> +  /// future lookup.
> +  BasicBlock *getIDom(const RebasedConstantInfo &RCI) const {
> +    if (RCI.IDom)
> +      return RCI.IDom;
> +    RCI.IDom = findIDomOfAllUses(RCI.Uses);
> +    assert(RCI.IDom && "Invalid IDom.");
> +    return RCI.IDom;
> +  }
> +
> +  BasicBlock *findIDomOfAllUses(const ConstantUseListType &Uses) const;
>    Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
>    Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
>    void collectConstantCandidates(Instruction *Inst, unsigned Idx,
> @@ -202,6 +214,32 @@ bool ConstantHoisting::runOnFunction(Fun
>    return MadeChange;
>  }
>
> +/// \brief Find nearest common dominator of all uses.
> +/// FIXME: Replace this with NearestCommonDominator once it is in common code.
> +BasicBlock *
> +ConstantHoisting::findIDomOfAllUses(const ConstantUseListType &Uses) const {
> +  // Collect all basic blocks.
> +  SmallPtrSet<BasicBlock *, 8> BBs;
> +  for (auto const &U : Uses)
> +    BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
> +
> +  if (BBs.count(Entry))
> +    return Entry;
> +
> +  while (BBs.size() >= 2) {
> +    BasicBlock *BB, *BB1, *BB2;
> +    BB1 = *BBs.begin();
> +    BB2 = *std::next(BBs.begin());
> +    BB = DT->findNearestCommonDominator(BB1, BB2);
> +    if (BB == Entry)
> +      return Entry;
> +    BBs.erase(BB1);
> +    BBs.erase(BB2);
> +    BBs.insert(BB);
> +  }
> +  assert((BBs.size() == 1) && "Expected only one element.");
> +  return *BBs.begin();
> +}
>
>  /// \brief Find the constant materialization insertion point.
>  Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
> @@ -224,11 +262,12 @@ Instruction *ConstantHoisting::findMatIn
>  Instruction *ConstantHoisting::
>  findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
>    assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
> -  // Collect all basic blocks.
> +  // Collect all IDoms.
>    SmallPtrSet<BasicBlock *, 8> BBs;
>    for (auto const &RCI : ConstInfo.RebasedConstants)
> -    for (auto const &U : RCI.Uses)
> -      BBs.insert(U.Inst->getParent());
> +    BBs.insert(getIDom(RCI));
> +
> +  assert(!BBs.empty() && "No dominators!?");
>
>    if (BBs.count(Entry))
>      return &Entry->front();
>
>
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