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[LoopRotate] Fix DomTree update logic for unreachable nodes. Fix PR33701. 

Summary:
LoopRotate manually updates the DoomTree by iterating over all predecessors of a basic block and computing the Nearest Common Dominator.

When a predecessor happens to be unreachable, `DT.findNearestCommonDominator` returns nullptr.

This patch teaches LoopRotate to handle this case and fixes [[ https://bugs.llvm.org/show_bug.cgi?id=33701 | PR33701 ]].

In the future, LoopRotate should be taught to use the new incremental API for updating the DomTree.

Reviewers: dberlin, davide, uabelho, grosser

Subscribers: efriedma, mzolotukhin

Differential Revision: https://reviews.llvm.org/D35074

llvm-svn: 307828
This commit is contained in:
Jakub Kuderski 2017-07-12 18:42:16 +00:00
parent 5ae1bfe813
commit b323f4f173
2 changed files with 43 additions and 4 deletions
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20
llvm/lib/Transforms/Scalar/LoopRotation.cpp
View File

@ -485,10 +485,22 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
DomTreeNode *Node = HeaderChildren[I];
BasicBlock *BB = Node->getBlock();
pred_iterator PI = pred_begin(BB);
BasicBlock *NearestDom = *PI;
for (pred_iterator PE = pred_end(BB); PI != PE; ++PI)
NearestDom = DT->findNearestCommonDominator(NearestDom, *PI);
BasicBlock *NearestDom = nullptr;
for (BasicBlock *Pred : predecessors(BB)) {
// Consider only reachable basic blocks.
if (!DT->getNode(Pred))
continue;
if (!NearestDom) {
NearestDom = Pred;
continue;
}
NearestDom = DT->findNearestCommonDominator(NearestDom, Pred);
assert(NearestDom && "No NearestCommonDominator found");
}
assert(NearestDom && "Nearest dominator not found");
// Remember if this changes the DomTree.
if (Node->getIDom()->getBlock() != NearestDom) {

27
llvm/test/Transforms/LoopRotate/pr33701.ll Normal file
View File

@ -0,0 +1,27 @@
; RUN: opt < %s -loop-rotate -verify-dom-info -verify-loop-info -disable-output
define void @func() {
bb0:
br label %bb1
bb1: ; preds = %bb4, %bb0
%0 = phi i16 [ %2, %bb4 ], [ 0, %bb0 ]
%1 = icmp sle i16 %0, 2
br i1 %1, label %bb2, label %bb5
bb2: ; preds = %bb1
br i1 undef, label %bb6, label %bb4
bb3: ; No predecessors!
br label %bb6
bb4: ; preds = %bb2
%2 = add i16 undef, 1
br label %bb1
bb5: ; preds = %bb1
br label %bb6
bb6: ; preds = %bb5, %bb3, %bb2
unreachable
}