[PATCH] D33800: [SelectionDAG] Update the dominator after splitting critical edges

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.

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.

Side note: not sure why this went unnoticed for so long.


https://reviews.llvm.org/D33800

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  test/CodeGen/X86/O0-pipeline.ll
  test/CodeGen/X86/pr33266.ll


Index: test/CodeGen/X86/pr33266.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/pr33266.ll
@@ -0,0 +1,29 @@
+; Make sure we don't crash because we have a stale dominator tree.
+; REQUIRES: asserts
+; RUN: llc -verify-dom-info %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+ at global = external global [8 x [8 x [4 x i8]]], align 2
+ at global.1 = external global { i8, [3 x i8] }, align 4
+
+define void @patatino() local_unnamed_addr {
+bb:
+  br label %bb1
+
+bb1:
+  br label %bb2
+
+bb2:
+  br i1 icmp ne (i8* getelementptr inbounds ({ i8, [3 x i8] }, { i8, [3 x i8] }* @global.1, i64 0, i32 0), i8* getelementptr inbounds ([8 x [8 x [4 x i8]]], [8 x [8 x [4 x i8]]]* @global, i64 0, i64 6, i64 6, i64 2)), label %bb4, label %bb3
+
+bb3:
+  br i1 icmp eq (i64 ashr (i64 shl (i64 zext (i32 srem (i32 7, i32 zext (i1 icmp eq (i8* getelementptr inbounds ({ i8, [3 x i8] }, { i8, [3 x i8] }* @global.1, i64 0, i32 0), i8* getelementptr inbounds ([8 x [8 x [4 x i8]]], [8 x [8 x [4 x i8]]]* @global, i64 0, i64 6, i64 6, i64 2)) to i32)) to i64), i64 56), i64 56), i64 0), label %bb5, label %bb4
+
+bb4:
+  %tmp = phi i64 [ ashr (i64 shl (i64 zext (i32 srem (i32 7, i32 zext (i1 icmp eq (i8* getelementptr inbounds ({ i8, [3 x i8] }, { i8, [3 x i8] }* @global.1, i64 0, i32 0), i8* getelementptr inbounds ([8 x [8 x [4 x i8]]], [8 x [8 x [4 x i8]]]* @global, i64 0, i64 6, i64 6, i64 2)) to i32)) to i64), i64 56), i64 56), %bb3 ], [ 7, %bb2 ]
+  ret void
+
+bb5:
+  ret void
+}
Index: test/CodeGen/X86/O0-pipeline.ll
===================================================================
--- test/CodeGen/X86/O0-pipeline.ll
+++ test/CodeGen/X86/O0-pipeline.ll
@@ -32,6 +32,7 @@
 ; CHECK-NEXT:       Safe Stack instrumentation pass
 ; CHECK-NEXT:       Insert stack protectors
 ; CHECK-NEXT:       Module Verifier
+; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       X86 DAG->DAG Instruction Selection
 ; CHECK-NEXT:       X86 PIC Global Base Reg Initialization
 ; CHECK-NEXT:       Expand ISel Pseudo-instructions
Index: lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -328,6 +328,9 @@
   if (UseMBPI && OptLevel != CodeGenOpt::None)
     AU.addRequired<BranchProbabilityInfoWrapperPass>();
   MachineFunctionPass::getAnalysisUsage(AU);
+
+  // The dominator tree is preserved in MachineFunctionPass::getAnalysisUsage()
+  AU.addRequired<DominatorTreeWrapperPass>();
 }
 
 /// SplitCriticalSideEffectEdges - Look for critical edges with a PHI value that
@@ -337,7 +340,7 @@
 ///
 /// 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 +366,7 @@
         // Okay, we have to split this edge.
         SplitCriticalEdge(
             Pred->getTerminator(), GetSuccessorNumber(Pred, &BB),
-            CriticalEdgeSplittingOptions().setMergeIdenticalEdges());
+            CriticalEdgeSplittingOptions(DT).setMergeIdenticalEdges());
         goto ReprocessBlock;
       }
   }
@@ -399,10 +402,11 @@
   LibInfo = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
   GFI = Fn.hasGC() ? &getAnalysis<GCModuleInfo>().getFunctionInfo(Fn) : nullptr;
   ORE = make_unique<OptimizationRemarkEmitter>(&Fn);
+  DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 
   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);


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