[LoopDeletion] Move deleteDeadLoop to to LoopUtils. NFC
llvm-svn: 314934
diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index bd89b6b..c8a81ab 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -1137,6 +1137,128 @@
return Worklist;
}
+void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT = nullptr,
+ ScalarEvolution *SE = nullptr,
+ LoopInfo *LI = nullptr) {
+ assert(!DT || L->isLCSSAForm(*DT) && "Expected LCSSA!");
+ auto *Preheader = L->getLoopPreheader();
+ assert(Preheader && "Preheader should exist!");
+
+ // Now that we know the removal is safe, remove the loop by changing the
+ // branch from the preheader to go to the single exit block.
+ //
+ // Because we're deleting a large chunk of code at once, the sequence in which
+ // we remove things is very important to avoid invalidation issues.
+
+ // Tell ScalarEvolution that the loop is deleted. Do this before
+ // deleting the loop so that ScalarEvolution can look at the loop
+ // to determine what it needs to clean up.
+ if (SE)
+ SE->forgetLoop(L);
+
+ auto *ExitBlock = L->getUniqueExitBlock();
+ assert(ExitBlock && "Should have a unique exit block!");
+ assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
+
+ auto *OldBr = dyn_cast<BranchInst>(Preheader->getTerminator());
+ assert(OldBr && "Preheader must end with a branch");
+ assert(OldBr->isUnconditional() && "Preheader must have a single successor");
+ // Connect the preheader to the exit block. Keep the old edge to the header
+ // around to perform the dominator tree update in two separate steps
+ // -- #1 insertion of the edge preheader -> exit and #2 deletion of the edge
+ // preheader -> header.
+ //
+ //
+ // 0. Preheader 1. Preheader 2. Preheader
+ // | | | |
+ // V | V |
+ // Header <--\ | Header <--\ | Header <--\
+ // | | | | | | | | | | |
+ // | V | | | V | | | V |
+ // | Body --/ | | Body --/ | | Body --/
+ // V V V V V
+ // Exit Exit Exit
+ //
+ // By doing this is two separate steps we can perform the dominator tree
+ // update without using the batch update API.
+ //
+ // Even when the loop is never executed, we cannot remove the edge from the
+ // source block to the exit block. Consider the case where the unexecuted loop
+ // branches back to an outer loop. If we deleted the loop and removed the edge
+ // coming to this inner loop, this will break the outer loop structure (by
+ // deleting the backedge of the outer loop). If the outer loop is indeed a
+ // non-loop, it will be deleted in a future iteration of loop deletion pass.
+ IRBuilder<> Builder(OldBr);
+ Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
+ // Remove the old branch. The conditional branch becomes a new terminator.
+ OldBr->eraseFromParent();
+
+ // Rewrite phis in the exit block to get their inputs from the Preheader
+ // instead of the exiting block.
+ BasicBlock::iterator BI = ExitBlock->begin();
+ while (PHINode *P = dyn_cast<PHINode>(BI)) {
+ // Set the zero'th element of Phi to be from the preheader and remove all
+ // other incoming values. Given the loop has dedicated exits, all other
+ // incoming values must be from the exiting blocks.
+ int PredIndex = 0;
+ P->setIncomingBlock(PredIndex, Preheader);
+ // Removes all incoming values from all other exiting blocks (including
+ // duplicate values from an exiting block).
+ // Nuke all entries except the zero'th entry which is the preheader entry.
+ // NOTE! We need to remove Incoming Values in the reverse order as done
+ // below, to keep the indices valid for deletion (removeIncomingValues
+ // updates getNumIncomingValues and shifts all values down into the operand
+ // being deleted).
+ for (unsigned i = 0, e = P->getNumIncomingValues() - 1; i != e; ++i)
+ P->removeIncomingValue(e - i, false);
+
+ assert((P->getNumIncomingValues() == 1 &&
+ P->getIncomingBlock(PredIndex) == Preheader) &&
+ "Should have exactly one value and that's from the preheader!");
+ ++BI;
+ }
+
+ // Disconnect the loop body by branching directly to its exit.
+ Builder.SetInsertPoint(Preheader->getTerminator());
+ Builder.CreateBr(ExitBlock);
+ // Remove the old branch.
+ Preheader->getTerminator()->eraseFromParent();
+
+ if (DT) {
+ // Update the dominator tree by informing it about the new edge from the
+ // preheader to the exit.
+ DT->insertEdge(Preheader, ExitBlock);
+ // Inform the dominator tree about the removed edge.
+ DT->deleteEdge(Preheader, L->getHeader());
+ }
+
+ // Remove the block from the reference counting scheme, so that we can
+ // delete it freely later.
+ for (auto *Block : L->blocks())
+ Block->dropAllReferences();
+
+ if (LI) {
+ // Erase the instructions and the blocks without having to worry
+ // about ordering because we already dropped the references.
+ // NOTE: This iteration is safe because erasing the block does not remove
+ // its entry from the loop's block list. We do that in the next section.
+ for (Loop::block_iterator LpI = L->block_begin(), LpE = L->block_end();
+ LpI != LpE; ++LpI)
+ (*LpI)->eraseFromParent();
+
+ // Finally, the blocks from loopinfo. This has to happen late because
+ // otherwise our loop iterators won't work.
+
+ SmallPtrSet<BasicBlock *, 8> blocks;
+ blocks.insert(L->block_begin(), L->block_end());
+ for (BasicBlock *BB : blocks)
+ LI->removeBlock(BB);
+
+ // The last step is to update LoopInfo now that we've eliminated this loop.
+ LI->erase(L);
+ }
+}
+
/// Returns true if the instruction in a loop is guaranteed to execute at least
/// once.
bool llvm::isGuaranteedToExecute(const Instruction &Inst,