[WebAssembly] Remap branch dests after fixCatchUnwindMismatches

[aheejin]

Summary

Fixing catch unwind mismatches can sometimes invalidate existing branch
destinations. This CL remaps those destinations after placing
try-delegates.

Fixes https://github.com/emscripten-core/emscripten/issues/13515.

[aheejin]

Fix preds comments in test

[aheejin]

clang-tidy fix

llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp

for (auto &MI : reverse(MBB)) {
       }
     }
   }
 
   assert(EHPadStack.empty());
   if (EHPadToUnwindDest.empty())
     return false;
   NumCatchUnwindMismatches += EHPadToUnwindDest.size();
+  // <current branch dest, future branch dest> map, because fixing catch unwind
+  // mismatches can invalidate branch destinations
+  DenseMap<MachineBasicBlock *, MachineBasicBlock *> BrDestMap;
 
   for (auto &P : EHPadToUnwindDest) {
     MachineBasicBlock *EHPad = P.first;
     MachineBasicBlock *UnwindDest = P.second;
     MachineInstr *Try = EHPadToTry[EHPad];
     MachineInstr *EndTry = BeginToEnd[Try];
     addTryDelegate(Try, EndTry, UnwindDest);
+    BrDestMap[EndTry->getParent()] =
+        EndTry->getParent()->getNextNode()->getNextNode();
+  }
+
+  // Adding a try-delegate wrapping an existing try-catch-end can make existing
+  // branch destination BBs invalid. For example,
+  //
+  // - Before:
+  // bb0:
+  //   block
+  //     br bb3
+  // bb1:
+  //     try
+  //       ...
+  // bb2: (ehpad)
+  //     catch
+  // bb3:
+  //     end_try
+  //   end_block   ;; 'br bb3' targets here
+  //
+  // Suppose this try-catch-end has a catch unwind mismatch, so we need to wrap
+  // this with a try-delegate. Then this becomes:
+  //
+  // - After:
+  // bb0:
+  //   block
+  //     br bb3    ;; invalid destination!
+  // bb1:
+  //     try       ;; (new instruction)
+  //       try
+  //         ...
+  // bb2: (ehpad)
+  //       catch
+  // bb3:
+  //       end_try ;; 'br bb3' still incorrectly targets here!
+  // delegate_bb:  ;; (new BB)
+  //     delegate  ;; (new instruction)
+  // split_bb:     ;; (new BB)
+  //   end_block
+  //
+  // Now 'br bb3' incorrectly branches to an inner scope.
+  //
+  // As we can see in this case, when branches target a BB that has both
+  // 'end_try' and 'end_block' and the BB is split to insert a 'delegate', we
+  // have to remap existing branch destinations so that they target not the
+  // 'end_try' BB but the new 'end_block' BB, which should be the second next BB
+  // of 'end_try' (because there is a 'delegate' BB in between). In this
+  // example, the 'br bb3' instruction should be remapped to 'br split_bb'.
+  for (auto &MBB : MF) {
+    for (auto &MI : MBB) {
+      if (MI.isTerminator()) {
+        for (auto &MO : MI.operands()) {
+          if (MO.isMBB()) {
+            auto It = BrDestMap.find(MO.getMBB());
+            if (It != BrDestMap.end())
+              MO.setMBB(It->second);
+          }
+        }
+      }
+    }
   }
 
   return true;
 }
 
 void WebAssemblyCFGStackify::recalculateScopeTops(MachineFunction &MF) {
   // Renumber BBs and recalculate ScopeTop info because new BBs might have been
   // created and inserted during fixing unwind mismatches.

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

 ehcleanup:                                        ; preds = %entry
   %0 = cleanuppad within none []
   cleanupret from %0 unwind to caller
 
 invoke.cont:                                      ; preds = %entry
   unreachable
 }
 
+; This tests if invalidated branch destinations after fixing catch unwind
+; mismatches are correctly remapped. For example, we have this code and suppose
+; we need to wrap this try-catch-end in this code with a try-delegate to fix a
+; catch unwind mismatch:
+  ; - Before:
+; block
+;   br (a)
+;   try
+;   catch
+;   end_try
+; end_block
+;           <- (a)
+;
+; - After
+; block
+;   br (a)
+;   try
+;     try
+;     catch
+;     end_try
+;           <- (a)
+;   delegate
+; end_block
+;           <- (b)
+; After adding a try-delegate, the 'br's destination BB, where (a) points,
+; becomes invalid because it incorrectly branches into an inner scope. The
+; destination should change to the BB where (b) points.
+
+; NOSORT-LABEL: test20
+; NOSORT: try
+; NOSORT:   br_if   0
+define void @test20(i1 %arg) personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
+entry:
+  br i1 %arg, label %bb0, label %dest
+
+bb0:                                              ; preds = %entry
+  invoke void @foo()
+          to label %bb1 unwind label %catch.dispatch0
+
+bb1:                                              ; preds = %bb0
+  invoke void @bar()
+          to label %try.cont unwind label %catch.dispatch1
+
+catch.dispatch0:                                  ; preds = %bb0
+  %0 = catchswitch within none [label %catch.start0] unwind to caller
+
+catch.start0:                                     ; preds = %catch.dispatch0
+  %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
+
+dest:                                             ; preds = %entry
+  ret void
+
+catch.dispatch1:                                  ; preds = %bb1
+  %4 = catchswitch within none [label %catch.start1] unwind to caller
+
+catch.start1:                                     ; preds = %catch.dispatch1
+  %5 = catchpad within %4 [i8* null]
+  %6 = call i8* @llvm.wasm.get.exception(token %5)
+  %7 = call i32 @llvm.wasm.get.ehselector(token %5)
+  catchret from %5 to label %try.cont
+
+try.cont:                                         ; preds = %catch.start1, %catch.start0, %bb1
+  ret void
+}
+
 ; Check if the unwind destination mismatch stats are correct
-; NOSORT: 19 wasm-cfg-stackify    - Number of call unwind mismatches found
-; NOSORT:  3 wasm-cfg-stackify    - Number of catch unwind mismatches found
+; NOSORT: 20 wasm-cfg-stackify    - Number of call unwind mismatches found
+; NOSORT:  4 wasm-cfg-stackify    - Number of catch unwind mismatches found
 
 declare void @foo()
 declare void @bar()
 declare i32 @baz()
 declare i32 @qux(i32)
 declare void @quux(i32)
 declare void @fun(i32)
 ; Function Attrs: nounwind