[WebAssembly] Fix bugs in rethrow depth counting and InstPrinter

[aheejin]

Summary

EH stack depth is incremented at try and decremented at catch. When
there are more than two catch instructions for a try instruction, we
shouldn't count non-first catches when calculating EH stack depths.

This patch fixes two bugs:

• CFGStackify: Exclude catch_all in the terminate catch pad when calculating EH pad stack, because when we have multiple catches for a try we should count only the first catch instruction when calculating EH pad stack.
• InstPrinter: The initial intention was also to exclude non-first catches, but it didn't account nested try-catches, so it failed on this case:

try
  try
  catch
  end
catch    <-- (1)
end

In the example, when we are at the catch (1), the last seen EH
instruction is not try but end_try, violating the wrong assumption.

We don't need these after we switch to the second proposal because there
is gonna be only one catch instruction. But anyway before then these
bugfixes are necessary for keep trunk in working state.

[aheejin]

Ping :)

[dschuff]

By "Multiple catches for a try" you mean catches with different tags?

[aheejin]

Yes, try with catches with different tags (which we don't generate currently) or a try with both catch and catch_all (which we generate for terminate lpads). Anyway this is just to keep the trunk working and gonna go away soon ༼ ༎ຶ ෴ ༎ຶ༽

lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp

case WebAssembly::END_TRY_S:
       break;
 
     case WebAssembly::CATCH_I32:
     case WebAssembly::CATCH_I32_S:
     case WebAssembly::CATCH_I64:
     case WebAssembly::CATCH_I64_S:
     case WebAssembly::CATCH_ALL:
     case WebAssembly::CATCH_ALL_S:
-      assert(LastSeenEHInst != END_TRY);
       // There can be multiple catch instructions for one try instruction, so we
-      // only print 'catch' label when the last seen EH instruction was 'try'.
-      if (LastSeenEHInst == TRY) {
+      // print a label only for the first 'catch' label.
+      if (LastSeenEHInst != CATCH) {
         assert(!EHPadStack.empty() && "try-catch mismatch!");
         printAnnotation(OS, "catch" + utostr(EHPadStack.pop_back_val()) + ':');
       }
       LastSeenEHInst = CATCH;
       break;
     }
 
     // Annotate any control flow label references.

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

for (auto I = MBB.rbegin(), E = MBB.rend(); I != E; ++I) {
                "Block/try marker should be balanced");
         Stack.pop_back();
         EHPadStack.pop_back();
         break;
 
       case WebAssembly::CATCH_I32:
       case WebAssembly::CATCH_I64:
       case WebAssembly::CATCH_ALL:
+        // Currently the only case there are more than one catch for a try is
+        // for catch terminate pad, in the form of
+        //   try
+        //   catch
+        //     call @__clang_call_terminate
+        //     unreachable
+        //   catch_all
+        //     call @std::terminate
+        //     unreachable
+        //   end
+        // So we shouldn't push the current BB for the second catch_all block
+        // here.
+        if (!WebAssembly::isCatchAllTerminatePad(MBB))
           EHPadStack.push_back(&MBB);
         break;
 
       case WebAssembly::LOOP:
         assert(Stack.back() == &MBB && "Loop top should be balanced");
         Stack.pop_back();
         break;
 
       case WebAssembly::END_BLOCK:

test/CodeGen/WebAssembly/exception.ll

 ; RUN: not llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-keep-registers -exception-model=wasm
 ; RUN: llc < %s -asm-verbose=false -disable-wasm-fallthrough-return-opt -wasm-keep-registers -exception-model=wasm -mattr=+exception-handling | FileCheck -allow-deprecated-dag-overlap %s
+; RUN: llc < %s -disable-wasm-fallthrough-return-opt -wasm-keep-registers -exception-model=wasm -mattr=+exception-handling
 
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32-unknown-unknown"
 
 %struct.Cleanup = type { i8 }
 
 @_ZTIi = external constant i8*