Improve the explanatory comment

Author: majaha

llvm/lib/Target/WebAssembly/WebAssemblyDebugFixup.cpp

@@ -123,14 +123,14 @@ bool WebAssemblyDebugFixup::runOnMachineFunction(MachineFunction &MF) {
         }
       } else {
 
-        // There is a WebAssembly Peephole optimisation can remove instructions
-        // before and after a wasm unreachable. This is a valid transformation
-        // because unreachable is "stack polymorphic", but stack polymorphism is
-        // not modeled in the llvm wasm backend. In current codegen, unreachable
-        // can only appear at the end of a basic block, and the stack is empty
-        // at the end of every basic block.
-        // So we can assume unreachable has stack-type [t*] -> [], and simply
-        // clear the stack.
+        // There is a WebAssembly peephole optimisation can remove drop
+        // instructions before a wasm unreachable. This is a valid
+        // transformation because unreachable is "stack polymorphic", but stack
+        // polymorphism is not modeled in the llvm wasm backend.
+        // In current codegen, virtual registers can only be stackified within
+        // a basic block, and cannot be stackified across an UNREACHABLE
+        // because it is marked as having unmodeled side effects.
+        // So we can assume the operand stack is empty after an UNREACHABLE.
         if (MI.getOpcode() == WebAssembly::UNREACHABLE) {
           Stack.clear();
           break;