[WebAssembly] Fix ScopeTops info in CFGStackify for EH pads

[aheejin]

Summary

When creating ScopeTops info for try ~ catch ~ end_try, we
should create not only end_try -> try mapping but also catch ->
try mapping as well. If this is not created, block and end_block
markers later added may span across an existing catch, resulting in
the incorrect code like:

try
  block     --|  (X)
catch         |
  end_block --|
end_try

lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

#endif
 
   // Mark the end of the TRY.
   InsertPos = getEarliestInsertPos(Cont, BeforeSet, AfterSet);
   MachineInstr *End =
       BuildMI(*Cont, InsertPos, Bottom->findBranchDebugLoc(),
               TII.get(WebAssembly::END_TRY));
   registerTryScope(Begin, End, &MBB);
 
-  // Track the farthest-spanning scope that ends at this point.
-  int Number = Cont->getNumber();
+  // Track the farthest-spanning scope that ends at this point. We create two
+  // mappings: (BB with 'end_try' -> BB with 'try') and (BB with 'catch' -> BB
+  // with 'try'). We need to create 'catch' -> 'try' mapping here too because
+  // markers should not span across 'catch'. For example, this should not
+  // happen:
+  //
+  // try
+  //   block     --|  (X)
+  // catch         |
+  //   end_block --|
+  // end_try
+  for (int Number : {Cont->getNumber(), MBB.getNumber()}) {
     if (!ScopeTops[Number] ||
         ScopeTops[Number]->getNumber() > Header->getNumber())
       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:

test/CodeGen/WebAssembly/cfg-stackify-eh.ll

 ;       foo();
 ;     } catch (int) {
 ;       foo();
 ;     }
 ;   }
 ; }
 
 ; CHECK-LABEL: test1
-; TODO Fill in check lines
+; CHECK: try
+; CHECK:   call      foo
+; CHECK: catch
+; CHECK:   block
+; CHECK:     block
+; CHECK:       br_if     0, {{.*}}                     # 0: down to label7
+; CHECK:       i32.call  $drop=, __cxa_begin_catch
+; CHECK:       try
+; CHECK:         call      foo
+; CHECK:         br        2                           # 2: down to label6
+; CHECK:       catch
+; CHECK:         try
+; CHECK:           block
+; CHECK:             br_if     0, {{.*}}               # 0: down to label11
+; CHECK:             i32.call  $drop=, __cxa_begin_catch
+; CHECK:             try
+; CHECK:               call      foo
+; CHECK:               br        2                     # 2: down to label10
+; CHECK:             catch
+; CHECK:               call      __cxa_end_catch
+; CHECK:               rethrow                         # down to catch3
+; CHECK:             end_try
+; CHECK:           end_block                           # label11:
+; CHECK:           call      __cxa_rethrow
+; CHECK:           unreachable
+; CHECK:         catch     {{.*}}                      # catch3:
+; CHECK:           call      __cxa_end_catch
+; CHECK:           rethrow                             # to caller
+; CHECK:         end_try                               # label10:
+; CHECK:         call      __cxa_end_catch
+; CHECK:         br        2                           # 2: down to label6
+; CHECK:       end_try
+; CHECK:     end_block                                 # label7:
+; CHECK:     call      __cxa_rethrow
+; CHECK:     unreachable
+; CHECK:   end_block                                   # label6:
+; CHECK:   call      __cxa_end_catch
+; CHECK: end_try
 define void @test1() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
 entry:
   invoke void @foo()
           to label %try.cont11 unwind label %catch.dispatch
 
 catch.dispatch:                                   ; preds = %entry
   %0 = catchswitch within none [label %catch.start] unwind to caller