Teach PHIElimination to split critical edges when -split-phi-edges is enabled.
Critical edges leading to a PHI node are split when the PHI source variable is
live out from the predecessor block. This help the coalescer eliminate more
PHI joins.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86725 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/PHIElimination.cpp b/lib/CodeGen/PHIElimination.cpp
index bf15bac..7c9c18c 100644
--- a/lib/CodeGen/PHIElimination.cpp
+++ b/lib/CodeGen/PHIElimination.cpp
@@ -27,12 +27,20 @@
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
+#include "llvm/Support/Debug.h"
#include <algorithm>
#include <map>
using namespace llvm;
STATISTIC(NumAtomic, "Number of atomic phis lowered");
+STATISTIC(NumSplits, "Number of critical edges split on demand");
+
+static cl::opt<bool>
+SplitEdges("split-phi-edges",
+ cl::desc("Split critical edges during phi elimination"),
+ cl::init(false), cl::Hidden);
char PHIElimination::ID = 0;
static RegisterPass<PHIElimination>
@@ -41,10 +49,14 @@
const PassInfo *const llvm::PHIEliminationID = &X;
void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
- AU.setPreservesCFG();
AU.addPreserved<LiveVariables>();
- AU.addPreservedID(MachineLoopInfoID);
- AU.addPreservedID(MachineDominatorsID);
+ if (SplitEdges) {
+ AU.addRequired<LiveVariables>();
+ } else {
+ AU.setPreservesCFG();
+ AU.addPreservedID(MachineLoopInfoID);
+ AU.addPreservedID(MachineDominatorsID);
+ }
MachineFunctionPass::getAnalysisUsage(AU);
}
@@ -84,6 +96,9 @@
if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
return false; // Quick exit for basic blocks without PHIs.
+ if (SplitEdges)
+ SplitPHIEdges(MF, MBB);
+
// Get an iterator to the first instruction after the last PHI node (this may
// also be the end of the basic block).
MachineBasicBlock::iterator AfterPHIsIt = SkipPHIsAndLabels(MBB, MBB.begin());
@@ -277,7 +292,8 @@
// Okay, if we now know that the value is not live out of the block, we can
// add a kill marker in this block saying that it kills the incoming value!
- if (!ValueIsUsed && !isLiveOut(SrcReg, opBlock, *LV)) {
+ // When SplitEdges is enabled, the value is never live out.
+ if (!ValueIsUsed && (SplitEdges || !isLiveOut(SrcReg, opBlock, *LV))) {
// In our final twist, we have to decide which instruction kills the
// register. In most cases this is the copy, however, the first
// terminator instruction at the end of the block may also use the value.
@@ -329,6 +345,22 @@
BBI->getOperand(i).getReg())];
}
+void llvm::PHIElimination::SplitPHIEdges(MachineFunction &MF,
+ MachineBasicBlock &MBB) {
+ LiveVariables &LV = getAnalysis<LiveVariables>();
+ for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
+ BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) {
+ for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
+ unsigned Reg = BBI->getOperand(i).getReg();
+ MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB();
+ // We break edges when registers are live out from the predecessor block
+ // (not considering PHI nodes).
+ if (isLiveOut(Reg, *PreMBB, LV))
+ SplitCriticalEdge(PreMBB, &MBB);
+ }
+ }
+}
+
bool llvm::PHIElimination::isLiveOut(unsigned Reg, const MachineBasicBlock &MBB,
LiveVariables &LV) {
LiveVariables::VarInfo &InRegVI = LV.getVarInfo(Reg);
@@ -376,3 +408,43 @@
}
return false;
}
+
+MachineBasicBlock *PHIElimination::SplitCriticalEdge(MachineBasicBlock *A,
+ MachineBasicBlock *B) {
+ assert(A && B && "Missing MBB end point");
+ ++NumSplits;
+
+ MachineFunction *MF = A->getParent();
+ MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock(B->getBasicBlock());
+ MF->push_back(NMBB);
+ const unsigned NewNum = NMBB->getNumber();
+ DEBUG(errs() << "PHIElimination splitting critical edge:"
+ " BB#" << A->getNumber()
+ << " -- BB#" << NewNum
+ << " -- BB#" << B->getNumber() << '\n');
+
+ A->ReplaceUsesOfBlockWith(B, NMBB);
+ NMBB->addSuccessor(B);
+
+ // Insert unconditional "jump B" instruction in NMBB.
+ SmallVector<MachineOperand, 4> Cond;
+ MF->getTarget().getInstrInfo()->InsertBranch(*NMBB, B, NULL, Cond);
+
+ LiveVariables *LV = getAnalysisIfAvailable<LiveVariables>();
+ if (LV)
+ LV->addNewBlock(NMBB, B);
+
+ // Fix PHI nodes in B so they refer to NMBB instead of A
+ for (MachineBasicBlock::iterator i = B->begin(), e = B->end();
+ i != e && i->getOpcode() == TargetInstrInfo::PHI; ++i)
+ for (unsigned ni = 1, ne = i->getNumOperands(); ni != ne; ni += 2)
+ if (i->getOperand(ni+1).getMBB() == A) {
+ i->getOperand(ni+1).setMBB(NMBB);
+ // Mark PHI sources as passing live through NMBB
+ if (LV)
+ LV->getVarInfo(i->getOperand(ni).getReg()).AliveBlocks.set(NewNum);
+ }
+ return NMBB;
+}
+
+