[WebAssembly] Fix a bug deleting instruction in a ranged for loop

[aheejin]

Summary

We shouldn't delete elements while iterating a ranged for loop.

[aheejin]

rebase

lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

 // values (in C++'s case a single i32, which represents an exception pointer)
 // are placed on top of the wasm stack. Because we can't model wasm stack in
 // LLVM instruction, we use 'extract_exception' pseudo instruction to retrieve
 // it. The pseudo instruction will be deleted later.
 bool WebAssemblyLateEHPrepare::addExceptionExtraction(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   auto *EHInfo = MF.getWasmEHFuncInfo();
   SmallVector<MachineInstr *, 16> ExtractInstrs;
+  SmallVector<MachineInstr *, 8> ToDelete;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
         if (MI.getOperand(0).isDead())
-          MI.eraseFromParent();
+          ToDelete.push_back(&MI);
         else
           ExtractInstrs.push_back(&MI);
       }
     }
   }
+  bool Changed = !ToDelete.empty() || !ExtractInstrs.empty();
+  for (auto *MI : ToDelete)
+    MI->eraseFromParent();
   if (ExtractInstrs.empty())
-    return false;
+    return Changed;
 
   // Find terminate pads.
   SmallSet<MachineBasicBlock *, 8> TerminatePads;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.isCall()) {
         const MachineOperand &CalleeOp = MI.getOperand(0);
         if (CalleeOp.isGlobal() && CalleeOp.getGlobal()->getName() ==

test/CodeGen/WebAssembly/exception.ll

catch.start:                                      ; preds = %catch.dispatch
   %4 = call i8* @__cxa_begin_catch(i8* %2) #2 [ "funclet"(token %1) ]
   call void @__cxa_end_catch() [ "funclet"(token %1) ]
   catchret from %1 to label %try.cont
 
 try.cont:                                         ; preds = %entry, %catch.start
   ret void
 }
 
+; When the result of @llvm.wasm.get.exception is not used. This is created to
+; fix a bug in LateEHPrepare and this should not crash.
+define void @test_get_exception_wo_use() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
+entry:
+  invoke void @foo()
+          to label %try.cont unwind label %catch.dispatch
+
+catch.dispatch:                                   ; preds = %entry
+  %0 = catchswitch within none [label %catch.start] unwind to caller
+
+catch.start:                                      ; preds = %catch.dispatch
+  %1 = catchpad within %0 [i8* null]
+  %2 = call i8* @llvm.wasm.get.exception(token %1)
+  %3 = call i32 @llvm.wasm.get.ehselector(token %1)
+  catchret from %1 to label %try.cont
+
+try.cont:                                         ; preds = %entry, %catch.start
+  ret void
+}
+
 declare void @foo()
 declare void @bar(i32*)
 declare i32 @__gxx_wasm_personality_v0(...)
 declare void @llvm.wasm.throw(i32, i8*)
 declare void @llvm.wasm.rethrow()
 declare i8* @llvm.wasm.get.exception(token)
 declare i32 @llvm.wasm.get.ehselector(token)
 declare i32 @llvm.eh.typeid.for(i8*)
 declare i8* @__cxa_begin_catch(i8*)
 declare void @__cxa_end_catch()
 declare void @__cxa_rethrow()
 declare void @__clang_call_terminate(i8*)
 declare %struct.Temp* @_ZN4TempD2Ev(%struct.Temp* returned)
 
 ; CHECK: __cpp_exception:
 ; CHECK: .eventtype  __cpp_exception i32