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split critical edges more carefully and intelligently. In particular, critical

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edges whose destinations are not phi nodes don't bother us.  Also, share
split edges, since the split edge can't have a phi.  This significantly
reduces the complexity of generated code in some cases.

llvm-svn: 31274
This commit is contained in:
Chris Lattner 2006-10-28 19:22:10 +00:00
parent 95eed3bd09
commit 776740f897
1 changed files with 61 additions and 4 deletions
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65
lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
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@ -44,8 +44,6 @@
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Compiler.h"
#include <map>
#include <set>
#include <iostream>
#include <algorithm>
using namespace llvm;
@ -3490,6 +3488,63 @@ static bool OptimizeGEPExpression(GetElementPtrInst *GEPI,
return true;
}
/// SplitEdgeNicely - Split the critical edge from TI to it's specified
/// successor if it will improve codegen. We only do this if the successor has
/// phi nodes (otherwise critical edges are ok). If there is already another
/// predecessor of the succ that is empty (and thus has no phi nodes), use it
/// instead of introducing a new block.
static void SplitEdgeNicely(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
BasicBlock *TIBB = TI->getParent();
BasicBlock *Dest = TI->getSuccessor(SuccNum);
assert(isa<PHINode>(Dest->begin()) &&
"This should only be called if Dest has a PHI!");
/// TIPHIValues - This array is lazily computed to determine the values of
/// PHIs in Dest that TI would provide.
std::vector<Value*> TIPHIValues;
// Check to see if Dest has any blocks that can be used as a split edge for
// this terminator.
for (pred_iterator PI = pred_begin(Dest), E = pred_end(Dest); PI != E; ++PI) {
BasicBlock *Pred = *PI;
// To be usable, the pred has to end with an uncond branch to the dest.
BranchInst *PredBr = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredBr || !PredBr->isUnconditional() ||
// Must be empty other than the branch.
&Pred->front() != PredBr)
continue;
// Finally, since we know that Dest has phi nodes in it, we have to make
// sure that jumping to Pred will have the same affect as going to Dest in
// terms of PHI values.
PHINode *PN;
unsigned PHINo = 0;
bool FoundMatch = true;
for (BasicBlock::iterator I = Dest->begin();
(PN = dyn_cast<PHINode>(I)); ++I, ++PHINo) {
if (PHINo == TIPHIValues.size())
TIPHIValues.push_back(PN->getIncomingValueForBlock(TIBB));
// If the PHI entry doesn't work, we can't use this pred.
if (TIPHIValues[PHINo] != PN->getIncomingValueForBlock(Pred)) {
FoundMatch = false;
break;
}
}
// If we found a workable predecessor, change TI to branch to Succ.
if (FoundMatch) {
Dest->removePredecessor(TIBB);
TI->setSuccessor(SuccNum, Pred);
return;
}
}
SplitCriticalEdge(TI, SuccNum, P, true);
}
bool SelectionDAGISel::runOnFunction(Function &Fn) {
MachineFunction &MF = MachineFunction::construct(&Fn, TLI.getTargetMachine());
RegMap = MF.getSSARegMap();
@ -3505,11 +3560,13 @@ bool SelectionDAGISel::runOnFunction(Function &Fn) {
while (MadeChange) {
MadeChange = false;
for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
// Split all critical edges.
// Split all critical edges where the dest block has a PHI.
TerminatorInst *BBTI = BB->getTerminator();
if (BBTI->getNumSuccessors() > 1) {
for (unsigned i = 0, e = BBTI->getNumSuccessors(); i != e; ++i)
SplitCriticalEdge(BBTI, i, this, true);
if (isa<PHINode>(BBTI->getSuccessor(i)->begin()) &&
isCriticalEdge(BBTI, i, true))
SplitEdgeNicely(BBTI, i, this);
}