[wasm] Introduce {BranchOnExceptionImmediate} helper.

This introduces a helper class for the immediates to a "br_on_exn"
instruction. It is basically a wrapper around a pair of existing
immediates to simplify PC offset calculations. This is similar to what
is being done for e.g. {MemoryCopyImmediate} already.

R=clemensh@chromium.org
BUG=v8:8091

Change-Id: I85a552baa6bfa78304abc6cf9509aef365d47057
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547865Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60586}

src/wasm/function-body-decoder-impl.h

@@ -296,6 +296,18 @@ struct BranchDepthImmediate {
   }
 };
 
+template <Decoder::ValidateFlag validate>
+struct BranchOnExceptionImmediate {
+  BranchDepthImmediate<validate> depth;
+  ExceptionIndexImmediate<validate> index;
+  uint32_t length = 0;
+  inline BranchOnExceptionImmediate(Decoder* decoder, const byte* pc)
+      : depth(BranchDepthImmediate<validate>(decoder, pc)),
+        index(ExceptionIndexImmediate<validate>(decoder, pc + depth.length)) {
+    length = depth.length + index.length;
+  }
+};
+
 template <Decoder::ValidateFlag validate>
 struct CallIndirectImmediate {
   uint32_t table_index;
@@ -1203,9 +1215,8 @@ class WasmDecoder : public Decoder {
       }
 
       case kExprBrOnExn: {
-        BranchDepthImmediate<validate> imm_br(decoder, pc);
-        ExceptionIndexImmediate<validate> imm_idx(decoder, pc + imm_br.length);
-        return 1 + imm_br.length + imm_idx.length;
+        BranchOnExceptionImmediate<validate> imm(decoder, pc);
+        return 1 + imm.length;
       }
 
       case kExprSetLocal:
@@ -1736,14 +1747,12 @@ class WasmFullDecoder : public WasmDecoder<validate> {
         }
         case kExprBrOnExn: {
           CHECK_PROTOTYPE_OPCODE(eh);
-          BranchDepthImmediate<validate> imm_br(this, this->pc_);
-          if (!this->Validate(this->pc_, imm_br, control_.size())) break;
-          ExceptionIndexImmediate<validate> imm_idx(this,
-                                                    this->pc_ + imm_br.length);
-          if (!this->Validate(this->pc_ + imm_br.length, imm_idx)) break;
-          Control* c = control_at(imm_br.depth);
+          BranchOnExceptionImmediate<validate> imm(this, this->pc_);
+          if (!this->Validate(this->pc_, imm.depth, control_.size())) break;
+          if (!this->Validate(this->pc_ + imm.depth.length, imm.index)) break;
+          Control* c = control_at(imm.depth.depth);
           auto exception = Pop(0, kWasmExceptRef);
-          const WasmExceptionSig* sig = imm_idx.exception->sig;
+          const WasmExceptionSig* sig = imm.index.exception->sig;
           size_t value_count = sig->parameter_count();
           // TODO(mstarzinger): This operand stack mutation is an ugly hack to
           // make both type checking here as well as environment merging in the
@@ -1753,11 +1762,11 @@ class WasmFullDecoder : public WasmDecoder<validate> {
           Vector<Value> values(stack_.data() + c->stack_depth, value_count);
           if (!TypeCheckBranch(c)) break;
           if (control_.back().reachable()) {
-            CALL_INTERFACE(BrOnException, exception, imm_idx, imm_br.depth,
+            CALL_INTERFACE(BrOnException, exception, imm.index, imm.depth.depth,
                            values);
             c->br_merge()->reached = true;
           }
-          len = 1 + imm_br.length + imm_idx.length;
+          len = 1 + imm.length;
           for (size_t i = 0; i < value_count; ++i) Pop();
           auto* pexception = Push(kWasmExceptRef);
           *pexception = exception;

src/wasm/wasm-text.cc

@@ -120,11 +120,9 @@ void PrintWasmText(const WasmModule* module, const ModuleWireBytes& wire_bytes,
         break;
       }
       case kExprBrOnExn: {
-        BranchDepthImmediate<Decoder::kNoValidate> imm_br(&i, i.pc());
-        ExceptionIndexImmediate<Decoder::kNoValidate> imm_idx(
-            &i, i.pc() + imm_br.length);
-        os << WasmOpcodes::OpcodeName(opcode) << ' ' << imm_br.depth << ' '
-           << imm_idx.index;
+        BranchOnExceptionImmediate<Decoder::kNoValidate> imm(&i, i.pc());
+        os << WasmOpcodes::OpcodeName(opcode) << ' ' << imm.depth.depth << ' '
+           << imm.index.index;
         break;
       }
       case kExprElse: