[WebAssembly] Fix try placement in fixing unwind mismatches

Summary:
In CFGStackify, `fixUnwindMismatches` function fixes unwind destination
mismatches created by `try` marker placement. For example,
```
try
  ...
  call @qux  ;; This should throw to the caller!
catch
  ...
end
```
When `call @qux` is supposed to throw to the caller, it is possible that
it is wrapped inside a `catch` so in case it throws it ends up unwinding
there incorrectly. (Also it is possible `call @qux` is supposed to
unwind to another `catch` within the same function.)

To fix this, we wrap this inner `call @qux` with a nested
`try`-`catch`-`end` sequence, and within the nested `catch` body, branch
to the right destination:
```
block $l0
  try
    ...
    try                 ;; new nested try
      call @qux
    catch               ;; new nested catch
      local.set n       ;; store exnref to a local
      br $l0
    end
  catch
    ...
  end
end
local.get n             ;; retrieve exnref back
rethrow                 ;; rethrow to the caller
```

The previous algorithm placed the nested `try` right before the `call`.
But it is possible that there are stackified instructions before the
call from which the call takes arguments.
```
try
  ...
  i32.const 5
  call @qux  ;; This should throw to the caller!
catch
  ...
end
```

In this case we have to place `try` before those stackified
instructions.
```
block $l0
  try
    ...
    try                 ;; this should go *before* 'i32.const 5'
      i32.const 5
      call @qux
    catch
      local.set n
      br $l0
    end
  catch
    ...
  end
end
local.get n
rethrow
```

We correctly handle this in the first normal `try` placement phase
(`placeTryMarker` function), but failed to handle this in this
`fixUnwindMismatches`.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77950

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
index 7e867ed..989c3da 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -1109,6 +1109,9 @@ bool WebAssemblyCFGStackify::fixUnwindMismatches(MachineFunction &MF) {
       MachineInstr *RangeBegin = nullptr, *RangeEnd = nullptr;
       std::tie(RangeBegin, RangeEnd) = Range;
       auto *MBB = RangeBegin->getParent();
+      // Store the first function call from this range, because RangeBegin can
+      // be moved to point EH_LABEL before the call
+      MachineInstr *RangeBeginCall = RangeBegin;
 
       // Include possible EH_LABELs in the range
       if (RangeBegin->getIterator() != MBB->begin() &&
@@ -1126,9 +1129,27 @@ bool WebAssemblyCFGStackify::fixUnwindMismatches(MachineFunction &MF) {
         }
       }
 
+      // Local expression tree before the first call of this range should go
+      // after the nested TRY.
+      SmallPtrSet<const MachineInstr *, 4> AfterSet;
+      AfterSet.insert(RangeBegin);
+      AfterSet.insert(RangeBeginCall);
+      for (auto I = MachineBasicBlock::iterator(RangeBeginCall),
+                E = MBB->begin();
+           I != E; --I) {
+        if (std::prev(I)->isDebugInstr() || std::prev(I)->isPosition())
+          continue;
+        if (WebAssembly::isChild(*std::prev(I), MFI))
+          AfterSet.insert(&*std::prev(I));
+        else
+          break;
+      }
+
       // Create the nested try instruction.
+      auto InsertPos = getLatestInsertPos(
+          MBB, SmallPtrSet<const MachineInstr *, 4>(), AfterSet);
       MachineInstr *NestedTry =
-          BuildMI(*MBB, *RangeBegin, RangeBegin->getDebugLoc(),
+          BuildMI(*MBB, InsertPos, RangeBegin->getDebugLoc(),
                   TII.get(WebAssembly::TRY))
               .addImm(int64_t(WebAssembly::BlockType::Void));