[WebAssembly] Split BBs after throw instructions

Summary:
`throw` instruction is a terminator in wasm, but BBs were not splitted
after `throw` instructions, causing machine instruction verifier to
fail.

This patch
- Splits BBs after `throw` instructions in WasmEHPrepare and adding an
  unreachable instruction after `throw`, which will be deleted in
  LateEHPrepare pass
- Refactors WasmEHPrepare into two member functions
- Changes the semantics of `eraseBBsAndChildren` in LateEHPrepare pass
  to match that of WasmEHPrepare pass, which is newly added. Now
  `eraseBBsAndChildren` does not delete BBs with remaining predecessors.
- Fixes style nits, making static function names conform to clang-tidy
- Re-enables the test temporarily disabled by rL346840 && rL346845

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D54571

llvm-svn: 347003

1 files changed, 44 insertions, 14 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp
index 98953f09482..871e92082a4 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp
@@ -31,6 +31,7 @@ class WebAssemblyLateEHPrepare final : public MachineFunctionPass {
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
+  bool removeUnnecessaryUnreachables(MachineFunction &MF);
   bool replaceFuncletReturns(MachineFunction &MF);
   bool hoistCatches(MachineFunction &MF);
   bool addCatchAlls(MachineFunction &MF);
@@ -59,7 +60,7 @@ FunctionPass *llvm::createWebAssemblyLateEHPrepare() {
 // possible search paths should be the same.
 // Returns nullptr in case it does not find any EH pad in the search, or finds
 // multiple different EH pads.
-static MachineBasicBlock *GetMatchingEHPad(MachineInstr *MI) {
+static MachineBasicBlock *getMatchingEHPad(MachineInstr *MI) {
   MachineFunction *MF = MI->getParent()->getParent();
   SmallVector<MachineBasicBlock *, 2> WL;
   SmallPtrSet<MachineBasicBlock *, 2> Visited;
@@ -83,18 +84,15 @@ static MachineBasicBlock *GetMatchingEHPad(MachineInstr *MI) {
   return EHPad;
 }
 
-// Erases the given BBs and all their children from the function. If other BBs
-// have the BB as a successor, the successor relationships will be deleted as
-// well.
+// Erase the specified BBs if the BB does not have any remaining predecessors,
+// and also all its dead children.
 template <typename Container>
-static void EraseBBsAndChildren(const Container &MBBs) {
+static void eraseDeadBBsAndChildren(const Container &MBBs) {
   SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
-    SmallVector<MachineBasicBlock *, 4> Preds(MBB->pred_begin(),
-                                              MBB->pred_end());
-    for (auto *Pred : Preds)
-      Pred->removeSuccessor(MBB);
+    if (!MBB->pred_empty())
+      continue;
     SmallVector<MachineBasicBlock *, 4> Succs(MBB->succ_begin(),
                                               MBB->succ_end());
     WL.append(MBB->succ_begin(), MBB->succ_end());
@@ -110,6 +108,7 @@ bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
     return false;
 
   bool Changed = false;
+  Changed |= removeUnnecessaryUnreachables(MF);
   Changed |= addRethrows(MF);
   if (!MF.getFunction().hasPersonalityFn())
     return Changed;
@@ -122,6 +121,31 @@ bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
   return Changed;
 }
 
+bool WebAssemblyLateEHPrepare::removeUnnecessaryUnreachables(
+    MachineFunction &MF) {
+  bool Changed = false;
+  for (auto &MBB : MF) {
+    for (auto &MI : MBB) {
+      if (!WebAssembly::isThrow(MI))
+        continue;
+      Changed = true;
+
+      // The instruction after the throw should be an unreachable or a branch to
+      // another BB that should eventually lead to an unreachable. Delete it
+      // because throw itself is a terminator, and also delete successors if
+      // any.
+      MBB.erase(std::next(MachineBasicBlock::iterator(MI)), MBB.end());
+      SmallVector<MachineBasicBlock *, 8> Succs(MBB.succ_begin(),
+                                                MBB.succ_end());
+      for (auto *Succ : Succs)
+        MBB.removeSuccessor(Succ);
+      eraseDeadBBsAndChildren(Succs);
+    }
+  }
+
+  return Changed;
+}
+
 bool WebAssemblyLateEHPrepare::replaceFuncletReturns(MachineFunction &MF) {
   bool Changed = false;
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
@@ -183,7 +207,7 @@ bool WebAssemblyLateEHPrepare::hoistCatches(MachineFunction &MF) {
         Catches.push_back(&MI);
 
   for (auto *Catch : Catches) {
-    MachineBasicBlock *EHPad = GetMatchingEHPad(Catch);
+    MachineBasicBlock *EHPad = getMatchingEHPad(Catch);
     assert(EHPad && "No matching EH pad for catch");
     if (EHPad->begin() == Catch)
       continue;
@@ -242,7 +266,7 @@ bool WebAssemblyLateEHPrepare::addRethrows(MachineFunction &MF) {
         Rethrow = BuildMI(MBB, InsertPt, MI.getDebugLoc(),
                           TII.get(WebAssembly::RETHROW_TO_CALLER));
 
-      // Becasue __cxa_rethrow does not return, the instruction after the
+      // Because __cxa_rethrow does not return, the instruction after the
       // rethrow should be an unreachable or a branch to another BB that should
       // eventually lead to an unreachable. Delete it because rethrow itself is
       // a terminator, and also delete non-EH pad successors if any.
@@ -251,7 +275,9 @@ bool WebAssemblyLateEHPrepare::addRethrows(MachineFunction &MF) {
       for (auto *Succ : MBB.successors())
         if (!Succ->isEHPad())
           NonPadSuccessors.push_back(Succ);
-      EraseBBsAndChildren(NonPadSuccessors);
+      for (auto *Succ : NonPadSuccessors)
+        MBB.removeSuccessor(Succ);
+      eraseDeadBBsAndChildren(NonPadSuccessors);
     }
   return Changed;
 }
@@ -283,7 +309,7 @@ bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
 
   bool Changed = false;
   for (auto *Call : ClangCallTerminateCalls) {
-    MachineBasicBlock *EHPad = GetMatchingEHPad(Call);
+    MachineBasicBlock *EHPad = getMatchingEHPad(Call);
     assert(EHPad && "No matching EH pad for catch");
 
     // If it is already the form we want, skip it
@@ -308,7 +334,11 @@ bool WebAssemblyLateEHPrepare::ensureSingleBBTermPads(MachineFunction &MF) {
     BuildMI(*EHPad, InsertPos, Call->getDebugLoc(),
             TII.get(WebAssembly::UNREACHABLE));
     EHPad->erase(InsertPos, EHPad->end());
-    EraseBBsAndChildren(EHPad->successors());
+    SmallVector<MachineBasicBlock *, 8> Succs(EHPad->succ_begin(),
+                                              EHPad->succ_end());
+    for (auto *Succ : Succs)
+      EHPad->removeSuccessor(Succ);
+    eraseDeadBBsAndChildren(Succs);
   }
   return Changed;
 }