[llvm] r247497 - Clean up: Refactoring the hardcoded value of 6 for FindAvailableLoadedValue()'s parameter MaxInstsToScan.

Larisse Voufo via llvm-commits llvm-commits at lists.llvm.org
Tue Sep 15 10:55:28 PDT 2015


Hello,

Please see r247497 below for context.

Could anyone please shed some light on the specific reasons why
MaxInstsToScan is set to '6'?
I need to increase the value, at least to '8' on the call of
FindAvailableLoadedValue() from InstCombiner::visitLoadInst();
and I would like to know that this is a non-destructive change to make.
I am unable to access emails on the mailing list that go as far back as
r60148 (Nov 26, 2008) to dig up the reasons.

Thanks,
-- Larisse.


On Fri, Sep 11, 2015 at 6:41 PM, Larisse Voufo via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: lvoufo
> Date: Fri Sep 11 20:41:55 2015
> New Revision: 247497
>
> URL: http://llvm.org/viewvc/llvm-project?rev=247497&view=rev
> Log:
> Clean up: Refactoring the hardcoded value of 6 for
> FindAvailableLoadedValue()'s parameter MaxInstsToScan.
>
> Modified:
>     llvm/trunk/include/llvm/Analysis/Loads.h
>     llvm/trunk/lib/Analysis/Lint.cpp
>     llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
>     llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
>
> Modified: llvm/trunk/include/llvm/Analysis/Loads.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/Loads.h?rev=247497&r1=247496&r2=247497&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/Analysis/Loads.h (original)
> +++ llvm/trunk/include/llvm/Analysis/Loads.h Fri Sep 11 20:41:55 2015
> @@ -29,15 +29,28 @@ class MDNode;
>  bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
>                                   unsigned Align);
>
> +/// DEF_MAX_INSTS_TO_SCAN - the default number of maximum instructions
> +/// to scan in the block, used by FindAvailableLoadedValue().
> +/// FindAvailableLoadedValue() was introduced in r60148, to improve jump
> +/// threading in part by eliminating partially redundant loads.
> +/// At that point, the value of MaxInstsToScan was already set to '6'
> +/// without documented explanation.
> +/// FIXME: Ask r60148 author for details, and complete this documentation.
> +/// NOTE: As of now, every use of FindAvailableLoadedValue() uses this
> default
> +/// value of '6'.
> +#ifndef DEF_MAX_INSTS_TO_SCAN
> +#define DEF_MAX_INSTS_TO_SCAN 6
> +#endif
> +
>  /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting
> at
>  /// the instruction before ScanFrom) checking to see if we have the value
> at
>  /// the memory address *Ptr locally available within a small number of
>  ///  instructions. If the value is available, return it.
>  ///
> -/// If not, return the iterator for the last validated instruction that
> the
> +/// If not, return the iterator for the last validated instruction that
> the
>  /// value would be live through.  If we scanned the entire block and
> didn't
>  /// find something that invalidates *Ptr or provides it, ScanFrom would be
> -/// left at begin() and this returns null.  ScanFrom could also be left
> +/// left at begin() and this returns null.  ScanFrom could also be left
>  ///
>  /// MaxInstsToScan specifies the maximum instructions to scan in the
> block.
>  /// If it is set to 0, it will scan the whole block. You can also
> optionally
> @@ -48,7 +61,7 @@ bool isSafeToLoadUnconditionally(Value *
>  /// is found, it is left unmodified.
>  Value *FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
>                                  BasicBlock::iterator &ScanFrom,
> -                                unsigned MaxInstsToScan = 6,
> +                                unsigned MaxInstsToScan =
> DEF_MAX_INSTS_TO_SCAN,
>                                  AliasAnalysis *AA = nullptr,
>                                  AAMDNodes *AATags = nullptr);
>
>
> Modified: llvm/trunk/lib/Analysis/Lint.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/Lint.cpp?rev=247497&r1=247496&r2=247497&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Analysis/Lint.cpp (original)
> +++ llvm/trunk/lib/Analysis/Lint.cpp Fri Sep 11 20:41:55 2015
> @@ -829,8 +829,9 @@ Value *Lint::findValueImpl(Value *V, boo
>      for (;;) {
>        if (!VisitedBlocks.insert(BB).second)
>          break;
> -      if (Value *U = FindAvailableLoadedValue(L->getPointerOperand(),
> -                                              BB, BBI, 6, AA))
> +      if (Value *U =
> +          FindAvailableLoadedValue(L->getPointerOperand(),
> +                                   BB, BBI, DEF_MAX_INSTS_TO_SCAN, AA))
>          return findValueImpl(U, OffsetOk, Visited);
>        if (BBI != BB->begin()) break;
>        BB = BB->getUniquePredecessor();
>
> Modified:
> llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp?rev=247497&r1=247496&r2=247497&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
> (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
> Fri Sep 11 20:41:55 2015
> @@ -750,8 +750,9 @@ Instruction *InstCombiner::visitLoadInst
>    // separated by a few arithmetic operations.
>    BasicBlock::iterator BBI = &LI;
>    AAMDNodes AATags;
> -  if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(),
> BBI,
> -                                                     6, AA, &AATags)) {
> +  if (Value *AvailableVal =
> +      FindAvailableLoadedValue(Op, LI.getParent(), BBI,
> +                               DEF_MAX_INSTS_TO_SCAN, AA, &AATags)) {
>      if (LoadInst *NLI = dyn_cast<LoadInst>(AvailableVal)) {
>        unsigned KnownIDs[] = {
>          LLVMContext::MD_tbaa,
> @@ -822,7 +823,7 @@ Instruction *InstCombiner::visitLoadInst
>        }
>
>        // load (select (cond, null, P)) -> load P
> -      if (isa<ConstantPointerNull>(SI->getOperand(1)) &&
> +      if (isa<ConstantPointerNull>(SI->getOperand(1)) &&
>            LI.getPointerAddressSpace() == 0) {
>          LI.setOperand(0, SI->getOperand(2));
>          return &LI;
>
> Modified: llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp?rev=247497&r1=247496&r2=247497&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/JumpThreading.cpp Fri Sep 11 20:41:55
> 2015
> @@ -769,7 +769,7 @@ bool JumpThreading::ProcessBlock(BasicBl
>      // If we're branching on a conditional, LVI might be able to determine
>      // it's value at the branch instruction.  We only handle comparisons
>      // against a constant at this time.
> -    // TODO: This should be extended to handle switches as well.
> +    // TODO: This should be extended to handle switches as well.
>      BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
>      Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
>      if (CondBr && CondConst && CondBr->isConditional()) {
> @@ -877,7 +877,7 @@ bool JumpThreading::SimplifyPartiallyRed
>    BasicBlock::iterator BBIt = LI;
>
>    if (Value *AvailableVal =
> -        FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
> +        FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt)) {
>      // If the value if the load is locally available within the block,
> just use
>      // it.  This frequently occurs for reg2mem'd allocas.
>      //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
> @@ -922,7 +922,8 @@ bool JumpThreading::SimplifyPartiallyRed
>      // Scan the predecessor to see if the value is available in the pred.
>      BBIt = PredBB->end();
>      AAMDNodes ThisAATags;
> -    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB,
> BBIt, 6,
> +    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB,
> BBIt,
> +                                                    DEF_MAX_INSTS_TO_SCAN,
>                                                      nullptr, &ThisAATags);
>      if (!PredAvailable) {
>        OneUnavailablePred = PredBB;
>
>
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