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[BranchProbabilityInfo] Handle irreducible loops.

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Summary:
Compute the strongly connected components of the CFG and fall back to
use these for blocks that are in loops that are not detected by
LoopInfo when computing loop back-edge and exit branch probabilities.

Reviewers: dexonsmith, davidxl

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D39385

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317094 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Geoff Berry
2017-11-01 15:16:50 +00:00
parent 697969187c
commit 4bf7c62ec4
3 changed files with 128 additions and 11 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/Analysis/BranchProbabilityInfo.cpp
+80 -10
View File
@@ -13,6 +13,7 @@
#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/LoopInfo.h"
@@ -424,25 +425,73 @@ bool BranchProbabilityInfo::calcPointerHeuristics(const BasicBlock *BB) {
return true;
}
static int getSCCNum(const BasicBlock *BB,
const BranchProbabilityInfo::SccInfo &SccI) {
auto SccIt = SccI.SccNums.find(BB);
if (SccIt == SccI.SccNums.end())
return -1;
return SccIt->second;
}
// Consider any block that is an entry point to the SCC as a header.
static bool isSCCHeader(const BasicBlock *BB, int SccNum,
BranchProbabilityInfo::SccInfo &SccI) {
assert(getSCCNum(BB, SccI) == SccNum);
// Lazily compute the set of headers for a given SCC and cache the results
// in the SccHeaderMap.
if (SccI.SccHeaders.size() <= static_cast<unsigned>(SccNum))
SccI.SccHeaders.resize(SccNum + 1);
auto &HeaderMap = SccI.SccHeaders[SccNum];
bool Inserted;
BranchProbabilityInfo::SccHeaderMap::iterator HeaderMapIt;
std::tie(HeaderMapIt, Inserted) = HeaderMap.insert(std::make_pair(BB, false));
if (Inserted) {
bool IsHeader = llvm::any_of(make_range(pred_begin(BB), pred_end(BB)),
[&](const BasicBlock *Pred) {
return getSCCNum(Pred, SccI) != SccNum;
});
HeaderMapIt->second = IsHeader;
return IsHeader;
} else
return HeaderMapIt->second;
}
// Calculate Edge Weights using "Loop Branch Heuristics". Predict backedges
// as taken, exiting edges as not-taken.
bool BranchProbabilityInfo::calcLoopBranchHeuristics(const BasicBlock *BB,
const LoopInfo &LI) {
const LoopInfo &LI,
SccInfo &SccI) {
int SccNum;
Loop *L = LI.getLoopFor(BB);
if (!L)
return false;
if (!L) {
SccNum = getSCCNum(BB, SccI);
if (SccNum < 0)
return false;
}
SmallVector<unsigned, 8> BackEdges;
SmallVector<unsigned, 8> ExitingEdges;
SmallVector<unsigned, 8> InEdges; // Edges from header to the loop.
for (succ_const_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I) {
if (!L->contains(*I))
ExitingEdges.push_back(I.getSuccessorIndex());
else if (L->getHeader() == *I)
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
// Use LoopInfo if we have it, otherwise fall-back to SCC info to catch
// irreducible loops.
if (L) {
if (!L->contains(*I))
ExitingEdges.push_back(I.getSuccessorIndex());
else if (L->getHeader() == *I)
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
} else {
if (getSCCNum(*I, SccI) != SccNum)
ExitingEdges.push_back(I.getSuccessorIndex());
else if (isSCCHeader(*I, SccNum, SccI))
BackEdges.push_back(I.getSuccessorIndex());
else
InEdges.push_back(I.getSuccessorIndex());
}
}
if (BackEdges.empty() && ExitingEdges.empty())
@@ -771,6 +820,27 @@ void BranchProbabilityInfo::calculate(const Function &F, const LoopInfo &LI,
assert(PostDominatedByUnreachable.empty());
assert(PostDominatedByColdCall.empty());
// Record SCC numbers of blocks in the CFG to identify irreducible loops.
// FIXME: We could only calculate this if the CFG is known to be irreducible
// (perhaps cache this info in LoopInfo if we can easily calculate it there?).
int SccNum = 0;
SccInfo SccI;
for (scc_iterator<const Function *> It = scc_begin(&F); !It.isAtEnd();
++It, ++SccNum) {
// Ignore single-block SCCs since they either aren't loops or LoopInfo will
// catch them.
const std::vector<const BasicBlock *> &Scc = *It;
if (Scc.size() == 1)
continue;
DEBUG(dbgs() << "BPI: SCC " << SccNum << ":");
for (auto *BB : Scc) {
DEBUG(dbgs() << " " << BB->getName());
SccI.SccNums[BB] = SccNum;
}
DEBUG(dbgs() << "\n");
}
// Walk the basic blocks in post-order so that we can build up state about
// the successors of a block iteratively.
for (auto BB : post_order(&F.getEntryBlock())) {
@@ -786,7 +856,7 @@ void BranchProbabilityInfo::calculate(const Function &F, const LoopInfo &LI,
continue;
if (calcColdCallHeuristics(BB))
continue;
if (calcLoopBranchHeuristics(BB, LI))
if (calcLoopBranchHeuristics(BB, LI, SccI))
continue;
if (calcPointerHeuristics(BB))
continue;