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Do not compute the predecessor list for a block unless we need it.

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This speeds up simplifycfg on this program, from 44.87s to 0.29s (with
a profiled build):

 #define CL0(a) case a: goto c;
 #define CL1(a) CL0(a##0) CL0(a##1) CL0(a##2) CL0(a##3) CL0(a##4) CL0(a##5) \
 CL0(a##6) CL0(a##7) CL0(a##8) CL0(a##9)
 #define CL2(a) CL1(a##0) CL1(a##1) CL1(a##2) CL1(a##3) CL1(a##4) CL1(a##5) \
 CL1(a##6) CL1(a##7) CL1(a##8) CL1(a##9)
 #define CL3(a) CL2(a##0) CL2(a##1) CL2(a##2) CL2(a##3) CL2(a##4) CL2(a##5) \
 CL2(a##6) CL2(a##7) CL2(a##8) CL2(a##9)
 #define CL4(a) CL3(a##0) CL3(a##1) CL3(a##2) CL3(a##3) CL3(a##4) CL3(a##5) \
 CL3(a##6) CL3(a##7) CL3(a##8) CL3(a##9)

 void f();

 void a() {
     int b;
  c: switch (b) {
         CL4(1)
     }
 }

This testcase is contrived to expose N^2 behavior, but this patch should speedup
simplifycfg on any programs that use large switch statements.  This testcase
comes from GCC PR17895.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17389 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-11-01 06:53:58 +00:00
parent e4cb90f41c
commit bfd3e52701
1 changed files with 24 additions and 27 deletions
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51
lib/Transforms/Utils/SimplifyCFG.cpp
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@ -608,31 +608,29 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
// to the successor.
succ_iterator SI(succ_begin(BB));
if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
while (isa<PHINode>(*BBI)) ++BBI;
if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
if (Succ != BB) { // Arg, don't hurt infinite loops!
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
// Be careful though, if this transformation fails (returns true) then
// we cannot do this transformation!
//
if (!PropagatePredecessorsForPHIs(BB, Succ)) {
DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
std::string OldName = BB->getName();
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor.
if (BBI->isTerminator() && // Terminator is the only non-phi instruction!
Succ != BB) { // Don't hurt infinite loops!
// If our successor has PHI nodes, then we need to update them to include
// entries for BB's predecessors, not for BB itself. Be careful though,
// if this transformation fails (returns true) then we cannot do this
// transformation!
//
if (!PropagatePredecessorsForPHIs(BB, Succ)) {
DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB);
if (isa<PHINode>(&BB->front())) {
std::vector<BasicBlock*>
OldSuccPreds(pred_begin(Succ), pred_end(Succ));
// Move all PHI nodes in BB to Succ if they are alive, otherwise
// delete them.
while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
if (PN->use_empty())
BB->getInstList().erase(BB->begin()); // Nuke instruction...
BB->getInstList().erase(BB->begin()); // Nuke instruction.
else {
// The instruction is alive, so this means that Succ must have
// *ONLY* had BB as a predecessor, and the PHI node is still valid
@ -640,7 +638,7 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
// strictly dominated Succ.
BB->getInstList().remove(BB->begin());
Succ->getInstList().push_front(PN);
// We need to add new entries for the PHI node to account for
// predecessors of Succ that the PHI node does not take into
// account. At this point, since we know that BB dominated succ,
@ -651,17 +649,16 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
if (OldSuccPreds[i] != BB)
PN->addIncoming(PN, OldSuccPreds[i]);
}
}
// Everything that jumped to BB now goes to Succ.
std::string OldName = BB->getName();
BB->replaceAllUsesWith(Succ);
BB->eraseFromParent(); // Delete the old basic block.
// Everything that jumped to BB now goes to Succ...
BB->replaceAllUsesWith(Succ);
// Delete the old basic block...
M->getBasicBlockList().erase(BB);
if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
Succ->setName(OldName);
return true;
}
if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
Succ->setName(OldName);
return true;
}
}
}