[llvm] r347260 - Revert "[LoopSimplifyCFG] Teach LoopSimplifyCFG to constant-fold branches and switches"
Benjamin Kramer via llvm-commits
llvm-commits at lists.llvm.org
Mon Nov 19 12:01:20 PST 2018
Author: d0k
Date: Mon Nov 19 12:01:20 2018
New Revision: 347260
URL: http://llvm.org/viewvc/llvm-project?rev=347260&view=rev
Log:
Revert "[LoopSimplifyCFG] Teach LoopSimplifyCFG to constant-fold branches and switches"
This reverts commits r347183 & r347184. Crashes while building libxml.
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
llvm/trunk/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll
Modified: llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp?rev=347260&r1=347259&r2=347260&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopSimplifyCFG.cpp Mon Nov 19 12:01:20 2018
@@ -41,316 +41,6 @@ using namespace llvm;
#define DEBUG_TYPE "loop-simplifycfg"
-STATISTIC(NumTerminatorsFolded,
- "Number of terminators folded to unconditional branches");
-
-/// If \p BB is a switch or a conditional branch, but only one of its successors
-/// can be reached from this block in runtime, return this successor. Otherwise,
-/// return nullptr.
-static BasicBlock *getOnlyLiveSuccessor(BasicBlock *BB) {
- Instruction *TI = BB->getTerminator();
- if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
- if (BI->isUnconditional())
- return nullptr;
- if (BI->getSuccessor(0) == BI->getSuccessor(1))
- return BI->getSuccessor(0);
- ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition());
- if (!Cond)
- return nullptr;
- return Cond->isZero() ? BI->getSuccessor(1) : BI->getSuccessor(0);
- }
-
- if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
- auto *CI = dyn_cast<ConstantInt>(SI->getCondition());
- if (!CI)
- return nullptr;
- for (auto Case : SI->cases())
- if (Case.getCaseValue() == CI)
- return Case.getCaseSuccessor();
- return SI->getDefaultDest();
- }
-
- return nullptr;
-}
-
-/// Helper class that can turn branches and switches with constant conditions
-/// into unconditional branches.
-class ConstantTerminatorFoldingImpl {
-private:
- Loop &L;
- LoopInfo &LI;
- DominatorTree &DT;
-
- // Whether or not the current loop will still exist after terminator constant
- // folding will be done. In theory, there are two ways how it can happen:
- // 1. Loop's latch(es) become unreachable from loop header;
- // 2. Loop's header becomes unreachable from method entry.
- // In practice, the second situation is impossible because we only modify the
- // current loop and its preheader and do not affect preheader's reachibility
- // from any other block. So this variable set to true means that loop's latch
- // has become unreachable from loop header.
- bool DeleteCurrentLoop = false;
-
- // The blocks of the original loop that will still be reachable from entry
- // after the constant folding.
- SmallPtrSet<BasicBlock *, 8> LiveLoopBlocks;
- // The blocks of the original loop that will become unreachable from entry
- // after the constant folding.
- SmallPtrSet<BasicBlock *, 8> DeadLoopBlocks;
- // The exits of the original loop that will still be reachable from entry
- // after the constant folding.
- SmallPtrSet<BasicBlock *, 8> LiveExitBlocks;
- // The exits of the original loop that will become unreachable from entry
- // after the constant folding.
- SmallPtrSet<BasicBlock *, 8> DeadExitBlocks;
- // The blocks that will still be a part of the current loop after folding.
- SmallPtrSet<BasicBlock *, 8> BlocksInLoopAfterFolding;
- // The blocks that have terminators with constant condition that can be
- // folded. Note: fold candidates should be in L but not in any of its
- // subloops to avoid complex LI updates.
- SmallVector<BasicBlock *, 8> FoldCandidates;
-
- void dump() const {
- dbgs() << "Constant terminator folding for loop " << L << "\n";
- dbgs() << "After terminator constant-folding, the loop will";
- if (!DeleteCurrentLoop)
- dbgs() << " not";
- dbgs() << " be destroyed\n";
- dbgs() << "Blocks in which we can constant-fold terminator:\n";
- for (const BasicBlock *BB : FoldCandidates)
- dbgs() << "\t" << BB->getName() << "\n";
- auto PrintOutSet = [&](const char *Message,
- const SmallPtrSetImpl<BasicBlock *> &S) {
- dbgs() << Message << "\n";
- for (const BasicBlock *BB : S)
- dbgs() << "\t" << BB->getName() << "\n";
- };
- PrintOutSet("Live blocks from the original loop:", LiveLoopBlocks);
- PrintOutSet("Dead blocks from the original loop:", DeadLoopBlocks);
- PrintOutSet("Live exit blocks:", LiveExitBlocks);
- PrintOutSet("Dead exit blocks:", DeadExitBlocks);
- if (!DeleteCurrentLoop)
- PrintOutSet("The following blocks will still be part of the loop:",
- BlocksInLoopAfterFolding);
- }
-
- /// Fill all information about status of blocks and exits of the current loop
- /// if constant folding of all branches will be done.
- void analyze() {
- LoopBlocksDFS DFS(&L);
- DFS.perform(&LI);
- assert(DFS.isComplete() && "DFS is expected to be finished");
-
- // Collect live and dead loop blocks and exits.
- SmallPtrSet<BasicBlock *, 8> ExitBlocks;
- LiveLoopBlocks.insert(L.getHeader());
- for (auto I = DFS.beginRPO(), E = DFS.endRPO(); I != E; ++I) {
- BasicBlock *BB = *I;
-
- // If a loop block wasn't marked as live so far, then it's dead.
- if (!LiveLoopBlocks.count(BB)) {
- DeadLoopBlocks.insert(BB);
- continue;
- }
-
- BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
-
- // If a block has only one live successor, it's a candidate on constant
- // folding. Only handle blocks from current loop: branches in child loops
- // are skipped because if they can be folded, they should be folded during
- // the processing of child loops.
- if (TheOnlySucc && LI.getLoopFor(BB) == &L)
- FoldCandidates.push_back(BB);
-
- // Handle successors.
- auto ProcessSuccessor = [&](BasicBlock *Succ, bool IsLive) {
- if (!L.contains(Succ)) {
- if (IsLive)
- LiveExitBlocks.insert(Succ);
- ExitBlocks.insert(Succ);
- } else if (IsLive)
- LiveLoopBlocks.insert(Succ);
- };
- for (BasicBlock *Succ : successors(BB))
- ProcessSuccessor(Succ, !TheOnlySucc || TheOnlySucc == Succ);
- }
-
- // Sanity check: amount of dead and live loop blocks should match the total
- // number of blocks in loop.
- assert(L.getNumBlocks() == LiveLoopBlocks.size() + DeadLoopBlocks.size() &&
- "Malformed block sets?");
-
- // Now, all exit blocks that are not marked as live are dead.
- for (auto *ExitBlock : ExitBlocks)
- if (!LiveExitBlocks.count(ExitBlock))
- DeadExitBlocks.insert(ExitBlock);
-
- // Whether or not the edge From->To will still be present in graph after the
- // folding.
- auto IsEdgeLive = [&](BasicBlock *From, BasicBlock *To) {
- if (!LiveLoopBlocks.count(From))
- return false;
- BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(From);
- return !TheOnlySucc || TheOnlySucc == To;
- };
-
- // The loop will not be destroyed if its latch is live.
- DeleteCurrentLoop = !IsEdgeLive(L.getLoopLatch(), L.getHeader());
-
- // If we are going to delete the current loop completely, no extra analysis
- // is needed.
- if (DeleteCurrentLoop)
- return;
-
- // Otherwise, we should check which blocks will still be a part of the
- // current loop after the transform.
- BlocksInLoopAfterFolding.insert(L.getLoopLatch());
- // If the loop is live, then we should compute what blocks are still in
- // loop after all branch folding has been done. A block is in loop if
- // it has a live edge to another block that is in the loop; by definition,
- // latch is in the loop.
- auto BlockIsInLoop = [&](BasicBlock *BB) {
- return any_of(successors(BB), [&](BasicBlock *Succ) {
- return BlocksInLoopAfterFolding.count(Succ) && IsEdgeLive(BB, Succ);
- });
- };
- for (auto I = DFS.beginPostorder(), E = DFS.endPostorder(); I != E; ++I) {
- BasicBlock *BB = *I;
- if (BlockIsInLoop(BB))
- BlocksInLoopAfterFolding.insert(BB);
- }
-
- // Sanity check: header must be in loop.
- assert(BlocksInLoopAfterFolding.count(L.getHeader()) &&
- "Header not in loop?");
- }
-
- /// Constant-fold terminators of blocks acculumated in FoldCandidates into the
- /// unconditional branches.
- void foldTerminators() {
- DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
-
- for (BasicBlock *BB : FoldCandidates) {
- assert(LI.getLoopFor(BB) == &L && "Should be a loop block!");
- BasicBlock *TheOnlySucc = getOnlyLiveSuccessor(BB);
- assert(TheOnlySucc && "Should have one live successor!");
-
- LLVM_DEBUG(dbgs() << "Replacing terminator of " << BB->getName()
- << " with an unconditional branch to the block "
- << TheOnlySucc->getName() << "\n");
-
- SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
- // Remove all BB's successors except for the live one.
- for (auto *Succ : successors(BB))
- if (Succ != TheOnlySucc)
- DeadSuccessors.insert(Succ);
-
- IRBuilder<> Builder(BB->getContext());
- Instruction *Term = BB->getTerminator();
- Builder.SetInsertPoint(Term);
- Builder.CreateBr(TheOnlySucc);
- Term->eraseFromParent();
-
- for (auto *DeadSucc : DeadSuccessors)
- DTU.deleteEdge(BB, DeadSucc);
-
- ++NumTerminatorsFolded;
- }
- }
-
-public:
- ConstantTerminatorFoldingImpl(Loop &L, LoopInfo &LI, DominatorTree &DT)
- : L(L), LI(LI), DT(DT) {}
- bool run() {
- assert(L.getLoopLatch() && "Should be single latch!");
-
- // Collect all available information about status of blocks after constant
- // folding.
- analyze();
-
- LLVM_DEBUG(dbgs() << "In function " << L.getHeader()->getParent()->getName()
- << ": ");
-
- // Nothing to constant-fold.
- if (FoldCandidates.empty()) {
- LLVM_DEBUG(
- dbgs() << "No constant terminator folding candidates found in loop "
- << L.getHeader()->getName() << "\n");
- return false;
- }
-
- // TODO: Support deletion of the current loop.
- if (DeleteCurrentLoop) {
- LLVM_DEBUG(
- dbgs()
- << "Give up constant terminator folding in loop "
- << L.getHeader()->getName()
- << ": we don't currently support deletion of the current loop.\n");
- return false;
- }
-
- // TODO: Support deletion of dead loop blocks.
- if (!DeadLoopBlocks.empty()) {
- LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
- << L.getHeader()->getName()
- << ": we don't currently"
- " support deletion of dead in-loop blocks.\n");
- return false;
- }
-
- // TODO: Support dead loop exits.
- if (!DeadExitBlocks.empty()) {
- LLVM_DEBUG(dbgs() << "Give up constant terminator folding in loop "
- << L.getHeader()->getName()
- << ": we don't currently support dead loop exits.\n");
- return false;
- }
-
- // TODO: Support blocks that are not dead, but also not in loop after the
- // folding.
- if (BlocksInLoopAfterFolding.size() != L.getNumBlocks()) {
- LLVM_DEBUG(
- dbgs() << "Give up constant terminator folding in loop "
- << L.getHeader()->getName()
- << ": we don't currently"
- " support blocks that are not dead, but will stop "
- "being a part of the loop after constant-folding.\n");
- return false;
- }
-
- // Dump analysis results.
- LLVM_DEBUG(dump());
-
- LLVM_DEBUG(dbgs() << "Constant-folding " << FoldCandidates.size()
- << " terminators in loop " << L.getHeader()->getName()
- << "\n");
-
- // Make the actual transforms.
- foldTerminators();
-
-#ifndef NDEBUG
- // Make sure that we have preserved all data structures after the transform.
- DT.verify();
- assert(DT.isReachableFromEntry(L.getHeader()));
- LI.verify(DT);
-#endif
-
- return true;
- }
-};
-
-/// Turn branches and switches with known constant conditions into unconditional
-/// branches.
-static bool constantFoldTerminators(Loop &L, DominatorTree &DT, LoopInfo &LI) {
- // To keep things simple, only process loops with single latch. We
- // canonicalize most loops to this form. We can support multi-latch if needed.
- if (!L.getLoopLatch())
- return false;
-
- ConstantTerminatorFoldingImpl BranchFolder(L, LI, DT);
- return BranchFolder.run();
-}
-
static bool mergeBlocksIntoPredecessors(Loop &L, DominatorTree &DT,
LoopInfo &LI, MemorySSAUpdater *MSSAU) {
bool Changed = false;
@@ -383,9 +73,6 @@ static bool simplifyLoopCFG(Loop &L, Dom
ScalarEvolution &SE, MemorySSAUpdater *MSSAU) {
bool Changed = false;
- // Constant-fold terminators with known constant conditions.
- Changed |= constantFoldTerminators(L, DT, LI);
-
// Eliminate unconditional branches by merging blocks into their predecessors.
Changed |= mergeBlocksIntoPredecessors(L, DT, LI, MSSAU);
Modified: llvm/trunk/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll?rev=347260&r1=347259&r2=347260&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll (original)
+++ llvm/trunk/test/Transforms/LoopSimplifyCFG/constant-fold-branch.ll Mon Nov 19 12:01:20 2018
@@ -1,54 +1,10 @@
-; REQUIRES: asserts
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -loop-simplifycfg -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
-; RUN: opt -S -passes='require<domtree>,loop(simplify-cfg)' -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
-; RUN: opt -S -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa -debug-only=loop-simplifycfg 2>&1 < %s | FileCheck %s
+; RUN: opt -S -loop-simplifycfg < %s | FileCheck %s
+; RUN: opt -S -passes='require<domtree>,loop(simplify-cfg)' < %s | FileCheck %s
+; RUN: opt -S -loop-simplifycfg -enable-mssa-loop-dependency=true -verify-memoryssa < %s | FileCheck %s
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
-; CHECK-LABEL: In function dead_backedge_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support blocks that are not dead, but will stop being a part of the loop after constant-folding.
-; CHECK-LABEL: In function dead_backedge_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support blocks that are not dead, but will stop being a part of the loop after constant-folding.
-; CHECK-LABEL: In function dead_block_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function dead_block_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function dead_block_propogate_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function dead_block_propogate_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function dead_exit_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
-; CHECK-LABEL: In function dead_exit_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
-; CHECK-LABEL: In function dead_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function dead_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop dead_loop
-; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop live_loop
-; CHECK-LABEL: In function dead_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop live_loop
-; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop dead_loop
-; CHECK-LABEL: In function dead_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function inf_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function inf_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function live_block_test_branch_loop: Constant terminator folding for loop Loop at depth 1 containing: %header<header>,%check,%live,%backedge<latch><exiting>
-; CHECK: Replacing terminator of check with an unconditional branch to the block backedge
-; CHECK-LABEL: In function live_block_test_switch_loop: Constant terminator folding for loop Loop at depth 1 containing: %header<header>,%check,%live,%backedge<latch><exiting>
-; CHECK: Replacing terminator of check with an unconditional branch to the block backedge
-; CHECK-LABEL: In function partial_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function partial_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function partial_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function partial_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_1: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_1: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_1: Give up constant terminator folding in loop header: we don't currently support deletion of the current loop.
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_1: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_2: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_2: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_2: Give up constant terminator folding in loop header: we don't currently support dead loop exits.
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_2: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_3: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function full_sub_loop_test_branch_loop_inverse_3: No constant terminator folding candidates found in loop outer_header
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_3: Give up constant terminator folding in loop header: we don't currently support deletion of dead in-loop blocks.
-; CHECK-LABEL: In function full_sub_loop_test_switch_loop_inverse_3: No constant terminator folding candidates found in loop outer_header
-
; Make sure that we can eliminate a provably dead backedge.
define i32 @dead_backedge_test_branch_loop(i32 %end) {
; CHECK-LABEL: @dead_backedge_test_branch_loop(
@@ -751,7 +707,7 @@ define i32 @live_block_test_branch_loop(
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
; CHECK-NEXT: br i1 [[C:%.*]], label [[CHECK:%.*]], label [[LIVE:%.*]]
; CHECK: check:
-; CHECK-NEXT: br label [[BACKEDGE]]
+; CHECK-NEXT: br i1 true, label [[BACKEDGE]], label [[LIVE]]
; CHECK: live:
; CHECK-NEXT: [[I_2:%.*]] = add i32 [[I]], 1
; CHECK-NEXT: br label [[BACKEDGE]]
@@ -796,7 +752,11 @@ define i32 @live_block_test_switch_loop(
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[PREHEADER:%.*]] ], [ [[I_INC:%.*]], [[BACKEDGE:%.*]] ]
; CHECK-NEXT: br i1 [[C:%.*]], label [[CHECK:%.*]], label [[LIVE:%.*]]
; CHECK: check:
-; CHECK-NEXT: br label [[BACKEDGE]]
+; CHECK-NEXT: switch i32 1, label [[LIVE]] [
+; CHECK-NEXT: i32 0, label [[LIVE]]
+; CHECK-NEXT: i32 1, label [[BACKEDGE]]
+; CHECK-NEXT: i32 2, label [[LIVE]]
+; CHECK-NEXT: ]
; CHECK: live:
; CHECK-NEXT: [[I_2:%.*]] = add i32 [[I]], 1
; CHECK-NEXT: br label [[BACKEDGE]]
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