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Implement trivial unswitching for switch stmts. This allows us to trivial

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unswitch this loop on 2 before sweating to unswitch on 1/3.

void test4(int N, int i, int C, int*P, int*Q) {
  int j;
  for (j = 0; j < N; ++j) {
    switch (C) {                // general unswitching.
    default: P[i+j] = 0; break;
    case 1: Q[i+j] = 0; break;
    case 3: P[i+j] = Q[i+j]; break;
    case 2: break;              //  TRIVIAL UNSWITCH on C==2
    }
  }
}


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26223 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-02-15 22:52:05 +00:00
parent fe1c31204e
commit a48654ef23
1 changed files with 51 additions and 27 deletions
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78
lib/Transforms/Scalar/LoopUnswitch.cpp
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@ -79,7 +79,7 @@ namespace {
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val, void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
bool isEqual); bool isEqual);
void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
BasicBlock *ExitBlock); bool EntersWhenTrue, BasicBlock *ExitBlock);
}; };
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops"); RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
} }
@ -152,7 +152,7 @@ static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
// Okay, everything after this looks good, check to make sure that this block // Okay, everything after this looks good, check to make sure that this block
// doesn't include any side effects. // doesn't include any side effects.
for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
if (I->mayWriteToMemory()) if (I->mayWriteToMemory())
return false; return false;
@ -180,31 +180,48 @@ static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
/// condition is false. Otherwise, return null to indicate a complex condition. /// condition is false. Otherwise, return null to indicate a complex condition.
static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond, static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
Constant **Val = 0, Constant **Val = 0,
bool *EntersWhenTrue = 0,
BasicBlock **LoopExit = 0) { BasicBlock **LoopExit = 0) {
BasicBlock *Header = L->getHeader(); BasicBlock *Header = L->getHeader();
BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator()); TerminatorInst *HeaderTerm = Header->getTerminator();
BasicBlock *LoopExitBB = 0;
if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
// If the header block doesn't end with a conditional branch on Cond, we
// can't handle it.
if (!BI->isConditional() || BI->getCondition() != Cond)
return false;
// If the header block doesn't end with a conditional branch on Cond, we can't // Check to see if a successor of the branch is guaranteed to go to the
// handle it. // latch block or exit through a one exit block without having any
if (!HeaderTerm || !HeaderTerm->isConditional() || // side-effects. If so, determine the value of Cond that causes it to do
HeaderTerm->getCondition() != Cond) // this.
return false; if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(0)))) {
if (Val) *Val = ConstantBool::True;
// Check to see if a successor of the branch is guaranteed to go to the latch } else if ((LoopExitBB = isTrivialLoopExitBlock(L, BI->getSuccessor(1)))) {
// block or exit through a one exit block without having any side-effects. If if (Val) *Val = ConstantBool::False;
// so, determine the value of Cond that causes it to do this. }
BasicBlock *LoopExitBlock = 0; } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
if ((LoopExitBlock = isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(0)))){ // If this isn't a switch on Cond, we can't handle it.
if (Val) *Val = ConstantBool::True; if (SI->getCondition() != Cond) return false;
} else if ((LoopExitBlock =
isTrivialLoopExitBlock(L, HeaderTerm->getSuccessor(1)))) { // Check to see if a successor of the switch is guaranteed to go to the
if (Val) *Val = ConstantBool::False; // latch block or exit through a one exit block without having any
// side-effects. If so, determine the value of Cond that causes it to do
// this. Note that we can't trivially unswitch on the default case.
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
if ((LoopExitBB = isTrivialLoopExitBlock(L, SI->getSuccessor(i)))) {
// Okay, we found a trivial case, remember the value that is trivial.
if (Val) *Val = SI->getCaseValue(i);
if (EntersWhenTrue) *EntersWhenTrue = false;
break;
}
} }
if (!LoopExitBlock) if (!LoopExitBB)
return false; // Can't handle this. return false; // Can't handle this.
if (LoopExit) *LoopExit = LoopExitBlock; if (LoopExit) *LoopExit = LoopExitBB;
// We already know that nothing uses any scalar values defined inside of this // We already know that nothing uses any scalar values defined inside of this
// loop. As such, we just have to check to see if this loop will execute any // loop. As such, we just have to check to see if this loop will execute any
@ -356,9 +373,11 @@ bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
// If this is a trivial condition to unswitch (which results in no code // If this is a trivial condition to unswitch (which results in no code
// duplication), do it now. // duplication), do it now.
Constant *CondVal; Constant *CondVal;
bool EntersWhenTrue = true;
BasicBlock *ExitBlock; BasicBlock *ExitBlock;
if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal, &ExitBlock)){ if (IsTrivialUnswitchCondition(L, LoopCond, &CondVal,
UnswitchTrivialCondition(L, LoopCond, CondVal, ExitBlock); &EntersWhenTrue, &ExitBlock)) {
UnswitchTrivialCondition(L, LoopCond, CondVal, EntersWhenTrue, ExitBlock);
NewLoop1 = L; NewLoop1 = L;
} else { } else {
VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2); VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
@ -490,12 +509,13 @@ static void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
/// side-effects), unswitch it. This doesn't involve any code duplication, just /// side-effects), unswitch it. This doesn't involve any code duplication, just
/// moving the conditional branch outside of the loop and updating loop info. /// moving the conditional branch outside of the loop and updating loop info.
void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
Constant *Val, Constant *Val, bool EntersWhenTrue,
BasicBlock *ExitBlock) { BasicBlock *ExitBlock) {
DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %" DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size() << L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << L->getHeader()->getParent()->getName() << " blocks] in Function " << L->getHeader()->getParent()->getName()
<< " on cond:" << *Cond << "\n"); << " on cond: " << *Val << (EntersWhenTrue ? " == " : " != ") <<
*Cond << "\n");
// First step, split the preheader, so that we know that there is a safe place // First step, split the preheader, so that we know that there is a safe place
// to insert the conditional branch. We will change 'OrigPH' to have a // to insert the conditional branch. We will change 'OrigPH' to have a
@ -516,14 +536,18 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
// Okay, now we have a position to branch from and a position to branch to, // Okay, now we have a position to branch from and a position to branch to,
// insert the new conditional branch. // insert the new conditional branch.
EmitPreheaderBranchOnCondition(Cond, Val, NewPH, NewExit, {
OrigPH->getTerminator()); BasicBlock *TrueDest = NewPH, *FalseDest = NewExit;
if (!EntersWhenTrue) std::swap(TrueDest, FalseDest);
EmitPreheaderBranchOnCondition(Cond, Val, TrueDest, FalseDest,
OrigPH->getTerminator());
}
OrigPH->getTerminator()->eraseFromParent(); OrigPH->getTerminator()->eraseFromParent();
// Now that we know that the loop is never entered when this condition is a // Now that we know that the loop is never entered when this condition is a
// particular value, rewrite the loop with this info. We know that this will // particular value, rewrite the loop with this info. We know that this will
// at least eliminate the old branch. // at least eliminate the old branch.
RewriteLoopBodyWithConditionConstant(L, Cond, Val, true); RewriteLoopBodyWithConditionConstant(L, Cond, Val, EntersWhenTrue);
++NumTrivial; ++NumTrivial;
} }