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340d596509
time it is queried to compute the probability of a single successor. This makes computing the probability of every successor of a block in sequence... really really slow. ;] This switches to a linear walk of the successors rather than a quadratic one. One of several quadratic behaviors slowing this pass down. I'm not really thrilled with moving the sum code into the public interface of MBPI, but I don't (at the moment) have ideas for a better interface. My direction I'm thinking in for a better interface is to have MBPI actually retain much more state and make *all* of these queries cheap. That's a lot of work, and would require invasive changes. Until then, this seems like the least bad (ie, least quadratic) solution. Suggestions welcome. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144530 91177308-0d34-0410-b5e6-96231b3b80d8
85 lines
3.2 KiB
C++
85 lines
3.2 KiB
C++
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//==- MachineBranchProbabilityInfo.h - Machine Branch Probability Analysis -==//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass is used to evaluate branch probabilties on machine basic blocks.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
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#define LLVM_CODEGEN_MACHINEBRANCHPROBABILITYINFO_H
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#include "llvm/Pass.h"
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#include "llvm/Support/BranchProbability.h"
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#include <climits>
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namespace llvm {
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class raw_ostream;
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class MachineBasicBlock;
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class MachineBranchProbabilityInfo : public ImmutablePass {
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// Default weight value. Used when we don't have information about the edge.
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// TODO: DEFAULT_WEIGHT makes sense during static predication, when none of
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// the successors have a weight yet. But it doesn't make sense when providing
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// weight to an edge that may have siblings with non-zero weights. This can
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// be handled various ways, but it's probably fine for an edge with unknown
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// weight to just "inherit" the non-zero weight of an adjacent successor.
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static const uint32_t DEFAULT_WEIGHT = 16;
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public:
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static char ID;
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MachineBranchProbabilityInfo() : ImmutablePass(ID) {
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PassRegistry &Registry = *PassRegistry::getPassRegistry();
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initializeMachineBranchProbabilityInfoPass(Registry);
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}
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesAll();
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}
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// Return edge weight. If we don't have any informations about it - return
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// DEFAULT_WEIGHT.
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uint32_t getEdgeWeight(MachineBasicBlock *Src, MachineBasicBlock *Dst) const;
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// Get sum of the block successors' weights, potentially scaling them to fit
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// within 32-bits. If scaling is required, sets Scale based on the necessary
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// adjustment. Any edge weights used with the sum should be divided by Scale.
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uint32_t getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const;
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// A 'Hot' edge is an edge which probability is >= 80%.
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bool isEdgeHot(MachineBasicBlock *Src, MachineBasicBlock *Dst) const;
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// Return a hot successor for the block BB or null if there isn't one.
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// NB: This routine's complexity is linear on the number of successors.
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MachineBasicBlock *getHotSucc(MachineBasicBlock *MBB) const;
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// Return a probability as a fraction between 0 (0% probability) and
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// 1 (100% probability), however the value is never equal to 0, and can be 1
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// only iff SRC block has only one successor.
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// NB: This routine's complexity is linear on the number of successors of
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// Src. Querying sequentially for each successor's probability is a quadratic
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// query pattern.
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BranchProbability getEdgeProbability(MachineBasicBlock *Src,
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MachineBasicBlock *Dst) const;
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// Print value between 0 (0% probability) and 1 (100% probability),
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// however the value is never equal to 0, and can be 1 only iff SRC block
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// has only one successor.
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raw_ostream &printEdgeProbability(raw_ostream &OS, MachineBasicBlock *Src,
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MachineBasicBlock *Dst) const;
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};
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}
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#endif
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