[WebAssembly] Remove unnecessary instructions after TRY marker placement

aheejin · aheejin · commit 6596fa33d782 · 2019-02-27T00:50:53.000Z
Summary: This removes unnecessary instructions after TRY marker placement. There are two cases: - `end`/`end_block` can be removed if they overlap with `try`/`end_try` and they have the same return types. - `br` right before `catch` that branches to after `end_try` can be deleted. Reviewers: dschuff Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D58591 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354939 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -64,6 +64,7 @@ class WebAssemblyCFGStackify final : public MachineFunctionPass {
   void placeBlockMarker(MachineBasicBlock &MBB);
   void placeLoopMarker(MachineBasicBlock &MBB);
   void placeTryMarker(MachineBasicBlock &MBB);
+  void removeUnnecessaryInstrs(MachineFunction &MF);
   void rewriteDepthImmediates(MachineFunction &MF);
   void fixEndsAtEndOfFunction(MachineFunction &MF);
 
@@ -76,11 +77,12 @@ class WebAssemblyCFGStackify final : public MachineFunctionPass {
   // <EH pad, TRY marker> map
   DenseMap<const MachineBasicBlock *, MachineInstr *> EHPadToTry;
 
-  // Helper functions to register scope information created by marker
-  // instructions.
+  // Helper functions to register / unregister scope information created by
+  // marker instructions.
   void registerScope(MachineInstr *Begin, MachineInstr *End);
   void registerTryScope(MachineInstr *Begin, MachineInstr *End,
                         MachineBasicBlock *EHPad);
+  void unregisterScope(MachineInstr *Begin);
 
 public:
   static char ID; // Pass identification, replacement for typeid
@@ -175,6 +177,20 @@ void WebAssemblyCFGStackify::registerTryScope(MachineInstr *Begin,
   EHPadToTry[EHPad] = Begin;
 }
 
+void WebAssemblyCFGStackify::unregisterScope(MachineInstr *Begin) {
+  assert(BeginToEnd.count(Begin));
+  MachineInstr *End = BeginToEnd[Begin];
+  assert(EndToBegin.count(End));
+  BeginToEnd.erase(Begin);
+  EndToBegin.erase(End);
+  MachineBasicBlock *EHPad = TryToEHPad.lookup(Begin);
+  if (EHPad) {
+    assert(EHPadToTry.count(EHPad));
+    TryToEHPad.erase(Begin);
+    EHPadToTry.erase(EHPad);
+  }
+}
+
 /// Insert a BLOCK marker for branches to MBB (if needed).
 void WebAssemblyCFGStackify::placeBlockMarker(MachineBasicBlock &MBB) {
   // This should have been handled in placeTryMarker.
@@ -585,6 +601,76 @@ void WebAssemblyCFGStackify::placeTryMarker(MachineBasicBlock &MBB) {
     ScopeTops[Number] = Header;
 }
 
+void WebAssemblyCFGStackify::removeUnnecessaryInstrs(MachineFunction &MF) {
+  const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
+
+  // When there is an unconditional branch right before a catch instruction and
+  // it branches to the end of end_try marker, we don't need the branch, because
+  // it there is no exception, the control flow transfers to that point anyway.
+  // bb0:
+  //   try
+  //     ...
+  //     br bb2      <- Not necessary
+  // bb1:
+  //   catch
+  //     ...
+  // bb2:
+  //   end
+  for (auto &MBB : MF) {
+    if (!MBB.isEHPad())
+      continue;
+
+    MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
+    SmallVector<MachineOperand, 4> Cond;
+    MachineBasicBlock *EHPadLayoutPred =
+        &*std::prev(MachineFunction::iterator(&MBB));
+    MachineBasicBlock *Cont = BeginToEnd[EHPadToTry[&MBB]]->getParent();
+    bool Analyzable = !TII.analyzeBranch(*EHPadLayoutPred, TBB, FBB, Cond);
+    if (Analyzable && ((Cond.empty() && TBB && TBB == Cont) ||
+                       (!Cond.empty() && FBB && FBB == Cont)))
+      TII.removeBranch(*EHPadLayoutPred);
+  }
+
+  // When there are block / end_block markers that overlap with try / end_try
+  // markers, and the block and try markers' return types are the same, the
+  // block /end_block markers are not necessary, because try / end_try markers
+  // also can serve as boundaries for branches.
+  // block         <- Not necessary
+  //   try
+  //     ...
+  //   catch
+  //     ...
+  //   end
+  // end           <- Not necessary
+  SmallVector<MachineInstr *, 32> ToDelete;
+  for (auto &MBB : MF) {
+    for (auto &MI : MBB) {
+      if (MI.getOpcode() != WebAssembly::TRY)
+        continue;
+
+      MachineInstr *Try = &MI, *EndTry = BeginToEnd[Try];
+      MachineBasicBlock *TryBB = Try->getParent();
+      MachineBasicBlock *Cont = EndTry->getParent();
+      int64_t RetType = Try->getOperand(0).getImm();
+      for (auto B = MachineBasicBlock::iterator(Try),
+                E = std::next(MachineBasicBlock::iterator(EndTry));
+           B != TryBB->begin() && E != Cont->end() &&
+           std::prev(B)->getOpcode() == WebAssembly::BLOCK &&
+           E->getOpcode() == WebAssembly::END_BLOCK &&
+           std::prev(B)->getOperand(0).getImm() == RetType;
+           --B, ++E) {
+        ToDelete.push_back(&*std::prev(B));
+        ToDelete.push_back(&*E);
+      }
+    }
+  }
+  for (auto *MI : ToDelete) {
+    if (MI->getOpcode() == WebAssembly::BLOCK)
+      unregisterScope(MI);
+    MI->eraseFromParent();
+  }
+}
+
 static unsigned
 getDepth(const SmallVectorImpl<const MachineBasicBlock *> &Stack,
          const MachineBasicBlock *MBB) {
@@ -747,6 +833,7 @@ bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
   LLVM_DEBUG(dbgs() << "********** CFG Stackifying **********\n"
                        "********** Function: "
                     << MF.getName() << '\n');
+  const MCAsmInfo *MCAI = MF.getTarget().getMCAsmInfo();
 
   releaseMemory();
 
@@ -756,6 +843,11 @@ bool WebAssemblyCFGStackify::runOnMachineFunction(MachineFunction &MF) {
   // Place the BLOCK/LOOP/TRY markers to indicate the beginnings of scopes.
   placeMarkers(MF);
 
+  if (MCAI->getExceptionHandlingType() == ExceptionHandling::Wasm &&
+      MF.getFunction().hasPersonalityFn())
+    // Remove unnecessary instructions.
+    removeUnnecessaryInstrs(MF);
+
   // Convert MBB operands in terminators to relative depth immediates.
   rewriteDepthImmediates(MF);
 
diff --git a/test/CodeGen/WebAssembly/cfg-stackify-eh.ll b/test/CodeGen/WebAssembly/cfg-stackify-eh.ll
@@ -18,26 +18,23 @@ target triple = "wasm32-unknown-unknown"
 ; }
 
 ; CHECK-LABEL: test0
-; CHECK: block
-; CHECK:   try
-; CHECK:     call      foo
-; CHECK:     br        0                               # 0: down to label1
-; CHECK:   catch
-; CHECK:     block
-; CHECK:       br_if     0, {{.*}}                     # 0: down to label3
-; CHECK:       i32.call  $drop=, __cxa_begin_catch
-; CHECK:       call      __cxa_end_catch
-; CHECK:       br        1                             # 1: down to label1
-; CHECK:     end_block                                 # label3:
-; CHECK:     block
-; CHECK:       br_if     0, {{.*}}                     # 0: down to label4
-; CHECK:       i32.call  $drop=, __cxa_begin_catch
-; CHECK:       call      __cxa_end_catch
-; CHECK:       br        1                             # 1: down to label1
-; CHECK:     end_block                                 # label4:
-; CHECK:     call      __cxa_rethrow
-; CHECK:   end_try                                     # label1:
-; CHECK: end_block
+; CHECK: try
+; CHECK:   call      foo
+; CHECK: catch
+; CHECK:   block
+; CHECK:     br_if     0, {{.*}}                       # 0: down to label2
+; CHECK:     i32.call  $drop=, __cxa_begin_catch
+; CHECK:     call      __cxa_end_catch
+; CHECK:     br        1                               # 1: down to label0
+; CHECK:   end_block                                   # label2:
+; CHECK:   block
+; CHECK:     br_if     0, {{.*}}                       # 0: down to label3
+; CHECK:     i32.call  $drop=, __cxa_begin_catch
+; CHECK:     call      __cxa_end_catch
+; CHECK:     br        1                               # 1: down to label0
+; CHECK:   end_block                                   # label3:
+; CHECK:   call      __cxa_rethrow
+; CHECK: end_try                                       # label0:
 define void @test0() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
 entry:
   invoke void @foo()
@@ -177,39 +174,33 @@ unreachable:                                      ; preds = %rethrow5
 ; }
 
 ; CHECK-LABEL: test2
-; CHECK: block
-; CHECK:   try
-; CHECK:     call      foo
-; CHECK:     br        0                               # 0: down to label17
-; CHECK:   catch
-; CHECK:     i32.call  $drop=, __cxa_begin_catch
-; CHECK:     loop                                      # label19:
+; CHECK: try
+; CHECK:   call      foo
+; CHECK: catch
+; CHECK:   i32.call  $drop=, __cxa_begin_catch
+; CHECK:   loop                                        # label15:
+; CHECK:     block
 ; CHECK:       block
-; CHECK:         block
-; CHECK:           br_if     0, {{.*}}                 # 0: down to label21
+; CHECK:         br_if     0, {{.*}}                   # 0: down to label17
+; CHECK:         try
+; CHECK:           call      foo
+; CHECK:           br        2                         # 2: down to label16
+; CHECK:         catch
 ; CHECK:           try
-; CHECK:             call      foo
-; CHECK:             br        2                       # 2: down to label20
+; CHECK:             call      __cxa_end_catch
 ; CHECK:           catch
-; CHECK:             block
-; CHECK:               try
-; CHECK:                 call      __cxa_end_catch
-; CHECK:                 br        0                   # 0: down to label24
-; CHECK:               catch
-; CHECK:                 call      __clang_call_terminate
-; CHECK:                 unreachable
-; CHECK:               end_try                         # label24:
-; CHECK:             end_block
-; CHECK:             rethrow                           # to caller
+; CHECK:             call      __clang_call_terminate
+; CHECK:             unreachable
 ; CHECK:           end_try
-; CHECK:         end_block                             # label21:
-; CHECK:         call      __cxa_end_catch
-; CHECK:         br        2                           # 2: down to label17
-; CHECK:       end_block                               # label20:
-; CHECK:       br        0                             # 0: up to label19
-; CHECK:     end_loop
-; CHECK:   end_try                                     # label17:
-; CHECK: end_block
+; CHECK:           rethrow                             # to caller
+; CHECK:         end_try
+; CHECK:       end_block                               # label17:
+; CHECK:       call      __cxa_end_catch
+; CHECK:       br        2                             # 2: down to label13
+; CHECK:     end_block                                 # label16:
+; CHECK:     br        0                               # 0: up to label15
+; CHECK:   end_loop
+; CHECK: end_try                                       # label13:
 define void @test2() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
 entry:
   invoke void @foo()
diff --git a/test/CodeGen/WebAssembly/exception.ll b/test/CodeGen/WebAssembly/exception.ll
