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Under the hood, MBPI is doing a linear scan of every successor every

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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.

llvm-svn: 144530
This commit is contained in:
Chandler Carruth 2011-11-14 09:12:57 +00:00
parent 5c125698be
commit f89087744e
2 changed files with 18 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
include/llvm/CodeGen/MachineBranchProbabilityInfo.h
View File

@ -34,11 +34,6 @@ class MachineBranchProbabilityInfo : public ImmutablePass {
// weight to just "inherit" the non-zero weight of an adjacent successor.
static const uint32_t DEFAULT_WEIGHT = 16;
// Get sum of the block successors' weights, potentially scaling them to fit
// within 32-bits. If scaling is required, sets Scale based on the necessary
// adjustment. Any edge weights used with the sum should be divided by Scale.
uint32_t getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const;
public:
static char ID;
@ -55,6 +50,11 @@ public:
// DEFAULT_WEIGHT.
uint32_t getEdgeWeight(MachineBasicBlock *Src, MachineBasicBlock *Dst) const;
// Get sum of the block successors' weights, potentially scaling them to fit
// within 32-bits. If scaling is required, sets Scale based on the necessary
// adjustment. Any edge weights used with the sum should be divided by Scale.
uint32_t getSumForBlock(MachineBasicBlock *MBB, uint32_t &Scale) const;
// A 'Hot' edge is an edge which probability is >= 80%.
bool isEdgeHot(MachineBasicBlock *Src, MachineBasicBlock *Dst) const;

17
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -334,7 +334,15 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
const BranchProbability HotProb(4, 5); // 80%
MachineBasicBlock *BestSucc = 0;
BranchProbability BestProb = BranchProbability::getZero();
// FIXME: Due to the performance of the probability and weight routines in
// the MBPI analysis, we manually compute probabilities using the edge
// weights. This is suboptimal as it means that the somewhat subtle
// definition of edge weight semantics is encoded here as well. We should
// improve the MBPI interface to effeciently support query patterns such as
// this.
uint32_t BestWeight = 0;
uint32_t WeightScale = 0;
uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
SE = BB->succ_end();
@ -347,7 +355,8 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
continue;
}
BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
@ -360,10 +369,10 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
<< " (prob)"
<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc && BestProb >= SuccProb)
if (BestSucc && BestWeight >= SuccWeight)
continue;
BestSucc = *SI;
BestProb = SuccProb;
BestWeight = SuccWeight;
}
return BestSucc;
}