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[Dominators] Teach LoopDeletion to use the new incremental API

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Summary:
This patch makes LoopDeletion use the incremental DominatorTree API.

We modify LoopDeletion to perform the deletion in 5 steps:
1. Create a new dummy edge from the preheader to the exit, by adding a conditional branch.
2. Inform the DomTree about the new edge.
3. Remove the conditional branch and replace it with an unconditional edge to the exit. This removes the edge to the loop header, making it unreachable.
4. Inform the DomTree about the deleted edge.
5. Remove the unreachable block from the function.

Creating the dummy conditional branch is necessary to perform incremental DomTree update.
We should consider using the batch updater when it's ready.

Reviewers: dberlin, davide, grosser, sanjoy

Reviewed By: dberlin, grosser

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D35391

llvm-svn: 309850
This commit is contained in:
Jakub Kuderski 2017-08-02 18:17:52 +00:00
parent 6d8320b465
commit 8f78266b9f
4 changed files with 136 additions and 25 deletions
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14
include/llvm/Support/GenericDomTree.h
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@ -481,11 +481,15 @@ class DominatorTreeBase {
/// Inform the dominator tree about a CFG edge deletion and update the tree.
///
/// This function has to be called just after making the update
/// on the actual CFG. There cannot be any other updates that the dominator
/// tree doesn't know about. The only exception is when the deletion that the
/// tree is informed about makes some (dominator) subtree unreachable -- in
/// this case, it is fine to perform deletions within this subtree.
/// This function has to be called just after making the update on the actual
/// CFG. An internal functions checks if the edge doesn't exist in the CFG in
/// DEBUG mode. There cannot be any other updates that the
/// dominator tree doesn't know about.
///
/// However, it is fine to perform multiple CFG deletions that make different
/// subtrees forward-unreachable and to inform the DomTree about them all at
/// the same time, as the incremental algorithm doesn't walk the tree above
/// the NearestCommonDominator of a deleted edge
///
/// Note that for postdominators it automatically takes care of deleting
/// a reverse edge internally (so there's no need to swap the parameters).

61
lib/Transforms/Scalar/LoopDeletion.cpp
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@ -239,17 +239,44 @@ static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
auto *ExitBlock = L->getUniqueExitBlock();
assert(ExitBlock && "Should have a unique exit block!");
assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
// Connect the preheader directly to the exit block.
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.
Preheader->getTerminator()->replaceUsesOfWith(L->getHeader(), ExitBlock);
IRBuilder<> Builder(OldBr);
Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
// Remove the old branch. The conditional branch becomes a new terminator.
OldBr->eraseFromParent();
// Update the dominator tree by informing it about the new edge from the
// preheader to the exit.
DT.insertEdge(Preheader, ExitBlock);
// Rewrite phis in the exit block to get their inputs from the Preheader
// instead of the exiting block.
@ -276,25 +303,19 @@ static void deleteDeadLoop(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
++BI;
}
// Update the dominator tree and remove the instructions and blocks that will
// be deleted from the reference counting scheme.
SmallVector<DomTreeNode*, 8> ChildNodes;
for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
LI != LE; ++LI) {
// Move all of the block's children to be children of the Preheader, which
// allows us to remove the domtree entry for the block.
ChildNodes.insert(ChildNodes.begin(), DT[*LI]->begin(), DT[*LI]->end());
for (DomTreeNode *ChildNode : ChildNodes) {
DT.changeImmediateDominator(ChildNode, DT[Preheader]);
}
// Disconnect the loop body by branching directly to its exit.
Builder.SetInsertPoint(Preheader->getTerminator());
Builder.CreateBr(ExitBlock);
// Remove the old branch.
Preheader->getTerminator()->eraseFromParent();
ChildNodes.clear();
DT.eraseNode(*LI);
// 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.
(*LI)->dropAllReferences();
}
// Remove the block from the reference counting scheme, so that we can
// delete it freely later.
for (auto *Block : L->blocks())
Block->dropAllReferences();
// Erase the instructions and the blocks without having to worry
// about ordering because we already dropped the references.

56
test/Transforms/LoopDeletion/2017-07-11-incremental-dt.ll Normal file
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@ -0,0 +1,56 @@
; RUN: opt < %s -loop-deletion -S
; RUN: opt < %s -loop-deletion -analyze -domtree 2>&1 | FileCheck -check-prefix=DT %s
; RUN: opt < %s -loop-deletion -analyze -verify-dom-info
; CHECK: for.body
; CHECK-NOT: for.cond1
; Verify only the important parts of the DomTree.
; DT: [1] %entry
; DT: [2] %for.cond
; DT: [3] %lbl63A679E5
; DT: [3] %for.cond9
; DT: [3] %lbl64774A9B
; DT: [3] %for.body
; DT: [4] %for.cond3.loopexit
define i32 @fn1() {
entry:
br label %for.cond
for.cond: ; preds = %entry
br i1 undef, label %lbl63A679E5, label %for.body
for.body: ; preds = %for.cond
br label %for.cond1
for.cond1: ; preds = %for.cond1, %for.body
br i1 undef, label %for.cond1, label %for.cond3.loopexit
for.cond3.loopexit: ; preds = %for.cond1
br label %for.cond3
for.cond3: ; preds = %for.cond9, %for.cond3.loopexit
br i1 undef, label %for.body4, label %for.cond17
for.body4: ; preds = %for.cond3
br label %for.cond5
for.cond5: ; preds = %lbl63A679E5, %for.body4
br label %for.cond9
lbl63A679E5: ; preds = %for.cond
br label %for.cond5
for.cond9: ; preds = %for.end14.split, %for.cond5
br i1 undef, label %for.cond3, label %lbl64774A9B
lbl64774A9B: ; preds = %for.cond17, %for.cond9
br label %for.end14.split
for.end14.split: ; preds = %lbl64774A9B
br label %for.cond9
for.cond17: ; preds = %for.cond3
br label %lbl64774A9B
}

30
unittests/IR/DominatorTreeTest.cpp
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@ -797,6 +797,36 @@ TEST(DominatorTree, DeleteUnreachable) {
}
}
TEST(DominatorTree, DeletionsInSubtrees) {
CFGHolder Holder;
std::vector<CFGBuilder::Arc> Arcs = {{"0", "1"}, {"1", "2"}, {"1", "3"},
{"1", "6"}, {"3", "4"}, {"2", "5"},
{"5", "2"}};
// It is possible to perform multiple deletions and inform the
// DominatorTree about them at the same time, if the all of the
// deletions happen in different subtrees.
std::vector<CFGBuilder::Update> Updates = {{Delete, {"1", "2"}},
{Delete, {"1", "3"}}};
CFGBuilder B(Holder.F, Arcs, Updates);
DominatorTree DT(*Holder.F);
EXPECT_TRUE(DT.verify());
Optional<CFGBuilder::Update> LastUpdate;
while ((LastUpdate = B.applyUpdate()))
;
DT.deleteEdge(B.getOrAddBlock("1"), B.getOrAddBlock("2"));
DT.deleteEdge(B.getOrAddBlock("1"), B.getOrAddBlock("3"));
EXPECT_TRUE(DT.verify());
EXPECT_EQ(DT.getNode(B.getOrAddBlock("2")), nullptr);
EXPECT_EQ(DT.getNode(B.getOrAddBlock("3")), nullptr);
EXPECT_EQ(DT.getNode(B.getOrAddBlock("4")), nullptr);
EXPECT_EQ(DT.getNode(B.getOrAddBlock("5")), nullptr);
EXPECT_NE(DT.getNode(B.getOrAddBlock("6")), nullptr);
}
TEST(DominatorTree, InsertDelete) {
std::vector<CFGBuilder::Arc> Arcs = {
{"1", "2"}, {"2", "3"}, {"3", "4"}, {"4", "5"}, {"5", "6"}, {"5", "7"},