[SelectionDAG] Update the dominator after splitting critical edges.

Running `llc -verify-dom-info` on the attached testcase results in a
crash in the verifier, due to a stale dominator tree.

i.e.

  DominatorTree is not up to date!
  Computed:
  =============================--------------------------------
  Inorder Dominator Tree:
    [1] %safe_mod_func_uint8_t_u_u.exit.i.i.i {0,7}
      [2] %lor.lhs.false.i61.i.i.i {1,2}
      [2] %safe_mod_func_int8_t_s_s.exit.i.i.i {3,6}
        [3] %safe_div_func_int64_t_s_s.exit66.i.i.i {4,5}

  Actual:
  =============================--------------------------------
  Inorder Dominator Tree:
    [1] %safe_mod_func_uint8_t_u_u.exit.i.i.i {0,9}
      [2] %lor.lhs.false.i61.i.i.i {1,2}
      [2] %safe_mod_func_int8_t_s_s.exit.i.i.i {3,8}
        [3] %safe_div_func_int64_t_s_s.exit66.i.i.i {4,5}
        [3] %safe_mod_func_int8_t_s_s.exit.i.i.i.lor.lhs.false.i61.i.i.i_crit_edge {6,7}

This is because in `SelectionDAGIsel` we split critical edges without
updating the corresponding dominator for the function (and we claim
in `MachineFunctionPass::getAnalysisUsage()` that the domtree is preserved).

We could either stop preserving the domtree in `getAnalysisUsage`
or tell `splitCriticalEdge()` to update it.
As the second option is easy to implement, that's the one I chose.

Differential Revision:  https://reviews.llvm.org/D33800

llvm-svn: 304742

1 files changed, 8 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index b5ccd64ee76..d5fe24a6675 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -333,11 +333,12 @@ void SelectionDAGISel::getAnalysisUsage(AnalysisUsage &AU) const {
 /// SplitCriticalSideEffectEdges - Look for critical edges with a PHI value that
 /// may trap on it.  In this case we have to split the edge so that the path
 /// through the predecessor block that doesn't go to the phi block doesn't
-/// execute the possibly trapping instruction.
-///
+/// execute the possibly trapping instruction. If available, we pass a
+/// dominator tree to be updated when we split critical edges. This is because
+/// SelectionDAGISel preserves the DominatorTree.
 /// This is required for correctness, so it must be done at -O0.
 ///
-static void SplitCriticalSideEffectEdges(Function &Fn) {
+static void SplitCriticalSideEffectEdges(Function &Fn, DominatorTree *DT) {
   // Loop for blocks with phi nodes.
   for (BasicBlock &BB : Fn) {
     PHINode *PN = dyn_cast<PHINode>(BB.begin());
@@ -363,7 +364,7 @@ static void SplitCriticalSideEffectEdges(Function &Fn) {
         // Okay, we have to split this edge.
         SplitCriticalEdge(
             Pred->getTerminator(), GetSuccessorNumber(Pred, &BB),
-            CriticalEdgeSplittingOptions().setMergeIdenticalEdges());
+            CriticalEdgeSplittingOptions(DT).setMergeIdenticalEdges());
         goto ReprocessBlock;
       }
   }
@@ -399,10 +400,12 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
   LibInfo = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
   GFI = Fn.hasGC() ? &getAnalysis<GCModuleInfo>().getFunctionInfo(Fn) : nullptr;
   ORE = make_unique<OptimizationRemarkEmitter>(&Fn);
+  auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
+  DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
 
   DEBUG(dbgs() << "\n\n\n=== " << Fn.getName() << "\n");
 
-  SplitCriticalSideEffectEdges(const_cast<Function &>(Fn));
+  SplitCriticalSideEffectEdges(const_cast<Function &>(Fn), DT);
 
   CurDAG->init(*MF, *ORE);
   FuncInfo->set(Fn, *MF, CurDAG);