implement unswitching of loops with switch stmts and selects in them
llvm-svn: 26114
This commit is contained in:
Chris Lattner
parent
f1b151684d
commit
fbadd7e1ee
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@ -66,10 +66,13 @@ namespace {
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}
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private:
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bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L);
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unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
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void VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2);
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void VersionLoop(Value *LIC, Constant *OnVal,
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Loop *L, Loop *&Out1, Loop *&Out2);
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BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To);
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void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
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void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,Constant *Val,
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bool isEqual);
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void UnswitchTrivialCondition(Loop *L, Value *Cond, bool EntersLoopOnCond,
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BasicBlock *ExitBlock);
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};
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@ -256,87 +259,88 @@ bool LoopUnswitch::visitLoop(Loop *L) {
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// loop.
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I) {
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TerminatorInst *TI = (*I)->getTerminator();
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if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
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// If this isn't branching on an invariant condition, we can't unswitch
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// it.
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if (BI->isConditional()) {
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// See if this, or some part of it, is loop invariant. If so, we can
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// unswitch on it if we desire.
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Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
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if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L))
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return true;
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}
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
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Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
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if (LoopCond && SI->getNumCases() > 1) {
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// Find a value to unswitch on:
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// FIXME: this should chose the most expensive case!
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Constant *UnswitchVal = SI->getCaseValue(1);
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if (UnswitchIfProfitable(LoopCond, UnswitchVal, L))
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return true;
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}
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}
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// Scan the instructions to check for unswitchable values.
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for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
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BBI != E; ++BBI)
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if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
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Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), L, Changed);
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if (LoopCond == 0) continue;
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//if (UnswitchIfProfitable(LoopCond,
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std::cerr << "LOOP INVARIANT SELECT: " << *SI;
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if (LoopCond && UnswitchIfProfitable(LoopCond, ConstantBool::True, L))
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return true;
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}
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TerminatorInst *TI = (*I)->getTerminator();
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// FIXME: Handle invariant select instructions.
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if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
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if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
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DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
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<< L->getHeader()->getName() << ", cost = "
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<< L->getBlocks().size() << "\n" << **I);
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continue;
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}
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BranchInst *BI = dyn_cast<BranchInst>(TI);
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if (!BI) continue;
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// If this isn't branching on an invariant condition, we can't unswitch it.
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if (!BI->isConditional())
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continue;
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// See if this, or some part of it, is loop invariant. If so, we can
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// unswitch on it if we desire.
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Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), L, Changed);
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if (LoopCond == 0) continue;
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// Check to see if it would be profitable to unswitch this loop.
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if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
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// FIXME: this should estimate growth by the amount of code shared by the
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// resultant unswitched loops. This should have no code growth:
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// for () { if (iv) {...} }
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// as one copy of the loop will be empty.
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//
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DEBUG(std::cerr << "NOT unswitching loop %"
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<< L->getHeader()->getName() << ", cost too high: "
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<< L->getBlocks().size() << "\n");
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continue;
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}
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// If this loop has live-out values, we can't unswitch it. We need something
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// like loop-closed SSA form in order to know how to insert PHI nodes for
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// these values.
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if (LoopValuesUsedOutsideLoop(L)) {
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DEBUG(std::cerr << "NOT unswitching loop %"
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<< L->getHeader()->getName()
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<< ", a loop value is used outside loop!\n");
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continue;
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}
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//std::cerr << "BEFORE:\n"; LI->dump();
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Loop *NewLoop1 = 0, *NewLoop2 = 0;
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// If this is a trivial condition to unswitch (which results in no code
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// duplication), do it now.
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bool EntersLoopOnCond;
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BasicBlock *ExitBlock;
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if (IsTrivialUnswitchCondition(L, LoopCond, &EntersLoopOnCond, &ExitBlock)){
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UnswitchTrivialCondition(L, LoopCond, EntersLoopOnCond, ExitBlock);
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NewLoop1 = L;
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} else {
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VersionLoop(LoopCond, L, NewLoop1, NewLoop2);
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}
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//std::cerr << "AFTER:\n"; LI->dump();
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// Try to unswitch each of our new loops now!
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if (NewLoop1) visitLoop(NewLoop1);
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if (NewLoop2) visitLoop(NewLoop2);
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return true;
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}
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return Changed;
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}
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/// UnswitchIfProfitable - We have found that we can unswitch L when
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/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
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/// unswitch the loop, reprocess the pieces, then return true.
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bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,Loop *L){
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// Check to see if it would be profitable to unswitch this loop.
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if (getLoopUnswitchCost(L, LoopCond) > Threshold) {
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// FIXME: this should estimate growth by the amount of code shared by the
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// resultant unswitched loops.
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//
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DEBUG(std::cerr << "NOT unswitching loop %"
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<< L->getHeader()->getName() << ", cost too high: "
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<< L->getBlocks().size() << "\n");
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return false;
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}
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// If this loop has live-out values, we can't unswitch it. We need something
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// like loop-closed SSA form in order to know how to insert PHI nodes for
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// these values.
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if (LoopValuesUsedOutsideLoop(L)) {
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DEBUG(std::cerr << "NOT unswitching loop %" << L->getHeader()->getName()
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<< ", a loop value is used outside loop!\n");
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return false;
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}
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//std::cerr << "BEFORE:\n"; LI->dump();
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Loop *NewLoop1 = 0, *NewLoop2 = 0;
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// If this is a trivial condition to unswitch (which results in no code
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// duplication), do it now.
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bool EntersLoopOnCond;
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BasicBlock *ExitBlock;
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if (IsTrivialUnswitchCondition(L, LoopCond, &EntersLoopOnCond, &ExitBlock)){
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UnswitchTrivialCondition(L, LoopCond, EntersLoopOnCond, ExitBlock);
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NewLoop1 = L;
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} else {
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VersionLoop(LoopCond, Val, L, NewLoop1, NewLoop2);
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}
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++NumUnswitched;
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//std::cerr << "AFTER:\n"; LI->dump();
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// Try to unswitch each of our new loops now!
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if (NewLoop1) visitLoop(NewLoop1);
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if (NewLoop2) visitLoop(NewLoop2);
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return true;
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}
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BasicBlock *LoopUnswitch::SplitEdge(BasicBlock *BB, BasicBlock *Succ) {
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TerminatorInst *LatchTerm = BB->getTerminator();
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unsigned SuccNum = 0;
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@ -456,23 +460,22 @@ void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
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// Now that we know that the loop is never entered when this condition is a
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// particular value, rewrite the loop with this info. We know that this will
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// at least eliminate the old branch.
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RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnCond);
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++NumUnswitched;
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RewriteLoopBodyWithConditionConstant(L, Cond, ConstantBool::get(EnterOnCond),
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true);
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}
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/// VersionLoop - We determined that the loop is profitable to unswitch and
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/// contains a branch on a loop invariant condition. Split it into loop
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/// versions and test the condition outside of either loop. Return the loops
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/// created as Out1/Out2.
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void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
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/// VersionLoop - We determined that the loop is profitable to unswitch when LIC
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/// equal Val. Split it into loop versions and test the condition outside of
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/// either loop. Return the loops created as Out1/Out2.
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void LoopUnswitch::VersionLoop(Value *LIC, Constant *Val, Loop *L,
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Loop *&Out1, Loop *&Out2) {
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Function *F = L->getHeader()->getParent();
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DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
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<< L->getHeader()->getName() << " [" << L->getBlocks().size()
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<< " blocks] in Function " << F->getName()
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<< " on cond:" << *LIC << "\n");
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<< L->getHeader()->getName() << " [" << L->getBlocks().size()
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<< " blocks] in Function " << F->getName()
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<< " when '" << *Val << "' == " << *LIC << "\n");
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// LoopBlocks contains all of the basic blocks of the loop, including the
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// preheader of the loop, the body of the loop, and the exit blocks of the
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@ -572,41 +575,79 @@ void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
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cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
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OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
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"Preheader splitting did not work correctly!");
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// Remove the unconditional branch to LoopBlocks[0].
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OrigPreheader->getInstList().pop_back();
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// Insert a conditional branch on LIC to the two preheaders. The original
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// code is the true version and the new code is the false version.
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new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
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Value *BranchVal = LIC;
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if (!isa<ConstantBool>(BranchVal)) {
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BranchVal = BinaryOperator::createSetEQ(LIC, Val, "tmp",
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OrigPreheader->getTerminator());
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} else if (Val != ConstantBool::True) {
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// We want to enter the new loop when the condition is true.
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BranchVal = BinaryOperator::createNot(BranchVal, "tmp",
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OrigPreheader->getTerminator());
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}
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// Remove the unconditional branch to LoopBlocks[0] and insert the new branch.
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OrigPreheader->getInstList().pop_back();
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new BranchInst(NewBlocks[0], LoopBlocks[0], BranchVal, OrigPreheader);
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// Now we rewrite the original code to know that the condition is true and the
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// new code to know that the condition is false.
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RewriteLoopBodyWithConditionConstant(L, LIC, true);
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RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
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++NumUnswitched;
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RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
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RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
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Out1 = L;
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Out2 = NewLoop;
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}
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// RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
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// the value specified by Val in the specified loop. Rewrite any uses of LIC or
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// of properties correlated to it.
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// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
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// the value specified by Val in the specified loop, or we know it does NOT have
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// that value. Rewrite any uses of LIC or of properties correlated to it.
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void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
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bool Val) {
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Constant *Val,
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bool IsEqual) {
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assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
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// FIXME: Support correlated properties, like:
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// for (...)
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// if (li1 < li2)
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// ...
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// if (li1 > li2)
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// ...
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ConstantBool *BoolVal = ConstantBool::get(Val);
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// NotVal - If Val is a bool, this contains its inverse.
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Constant *NotVal = 0;
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if (ConstantBool *CB = dyn_cast<ConstantBool>(Val))
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NotVal = ConstantBool::get(!CB->getValue());
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// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
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// selects, switches.
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std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
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// Haha, this loop could be unswitched. Get it? The unswitch pass could
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// unswitch itself. Amazing.
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for (unsigned i = 0, e = Users.size(); i != e; ++i)
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if (Instruction *U = cast<Instruction>(Users[i]))
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if (L->contains(U->getParent()))
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U->replaceUsesOfWith(LIC, BoolVal);
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if (IsEqual) {
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U->replaceUsesOfWith(LIC, Val);
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} else if (NotVal) {
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U->replaceUsesOfWith(LIC, NotVal);
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} else {
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// If we know that LIC is not Val, use this info to simplify code.
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if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
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for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
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if (SI->getCaseValue(i) == Val) {
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// Found a dead case value. Don't remove PHI nodes in the
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// successor if they become single-entry, those PHI nodes may
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// be in the Users list.
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SI->getSuccessor(i)->removePredecessor(SI->getParent(), true);
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SI->removeCase(i);
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break;
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}
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}
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}
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// TODO: We could simplify stuff like X == C.
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}
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}
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