[WebAssembly] Fix a bug in removing unnecessary branches

Summary:
One of the things `removeUnnecessaryInstrs()` in CFGStackify does is to
remove an unnecessary unconditinal branch before an EH pad. 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:
  try
    ...
    br bb2      <- Not necessary
bb1:
  catch
    ...
bb2:
  end
```

This applies when we have a conditional branch followed by an
unconditional one, in which case we should only remove the unconditional
branch. For example:
```
bb0:
  try
    ...
    br_if someplace_else
    br bb2                 <- Not necessary
bb1:
  catch
    ...
bb2:
  end
```

But `TargetInstrInfo::removeBranch` we used removed all existing
branches when there are multiple ones. This patch fixes it by only
deleting the last (= unconditional) branch manually.

Also fixes some `preds` comments in the test file.

Reviewers: dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80572

1 files changed, 20 insertions, 2 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
index 23a5aa6..103fe97 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
@@ -670,9 +670,27 @@ void WebAssemblyCFGStackify::removeUnnecessaryInstrs(MachineFunction &MF) {
     MachineBasicBlock *EHPadLayoutPred = MBB.getPrevNode();
     MachineBasicBlock *Cont = BeginToEnd[EHPadToTry[&MBB]]->getParent();
     bool Analyzable = !TII.analyzeBranch(*EHPadLayoutPred, TBB, FBB, Cond);
+    // This condition means either
+    // 1. This BB ends with a single unconditional branch whose destinaion is
+    //    Cont.
+    // 2. This BB ends with a conditional branch followed by an unconditional
+    //    branch, and the unconditional branch's destination is Cont.
+    // In both cases, we want to remove the last (= unconditional) branch.
     if (Analyzable && ((Cond.empty() && TBB && TBB == Cont) ||
-                       (!Cond.empty() && FBB && FBB == Cont)))
-      TII.removeBranch(*EHPadLayoutPred);
+                       (!Cond.empty() && FBB && FBB == Cont))) {
+      bool ErasedUncondBr = false;
+      for (auto I = EHPadLayoutPred->end(), E = EHPadLayoutPred->begin();
+           I != E; --I) {
+        auto PrevI = std::prev(I);
+        if (PrevI->isTerminator()) {
+          assert(PrevI->getOpcode() == WebAssembly::BR);
+          PrevI->eraseFromParent();
+          ErasedUncondBr = true;
+          break;
+        }
+      }
+      assert(ErasedUncondBr && "Unconditional branch not erased!");
+    }
   }
 
   // When there are block / end_block markers that overlap with try / end_try