[wasm] Fix wasm decoder tracing for prefix opcodes.

Using --trace-wasm-decoder prints unknowns for prefix opcodes, example:

  @3      #1:Block               |  env = 0x5547c10, state = R, reason = block:start, control = #0:Start

  @4      v8#14:GetLocal            | i@4:GetLocal[0]
  @6      #e5:Unknown             | s@6:Unknown
  @8      v8#15:SetLocal            | s@8:SetLocal[1]
  @10     v8#14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0]
  @12     v8#14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1]
  @14     #cb:I8Const             | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1] i@14:I8Const
  @16     #e5:Unknown             | s@8:SetLocal[1] i@10:GetLocal[0] i@16:Unknown

Fixed to print:

  @3        #1:Block               |  env = 0x45cac10, state = R, reason = block:start, control = #0:Start

  @4        v8#14:GetLocal            | i@4:GetLocal[0]
  @6    #e5 #1b:I32x4Splat          | s@6:I32x4Splat
  @8        v8#15:SetLocal            | s@8:SetLocal[1]
  @10       v8#14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0]
  @12       v8#14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1]
  @14       #cb:I8Const             | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1] i@14:I8Const
  @16   #e5 #1c:I32x4ExtractLane    | s@8:SetLocal[1] i@10:GetLocal[0] i@16:I32x4ExtractLane

[email protected], [email protected]

Review-Url: https://codereview.chromium.org/2307733002
Cr-Commit-Position: refs/heads/master@{#39142}

src/wasm/ast-decoder.cc

@@ -509,7 +509,7 @@ class WasmFullDecoder : public WasmDecoder {
     }
 
     if (ssa_env_->go()) {
-      TRACE("  @%-6d #xx:%-20s|", startrel(pc_), "ImplicitReturn");
+      TRACE("  @%-8d #xx:%-20s|", startrel(pc_), "ImplicitReturn");
       DoReturn();
       if (failed()) return TraceFailed();
       TRACE("\n");
@@ -707,8 +707,10 @@ class WasmFullDecoder : public WasmDecoder {
     while (true) {  // decoding loop.
       unsigned len = 1;
       WasmOpcode opcode = static_cast<WasmOpcode>(*pc_);
-      TRACE("  @%-6d #%02x:%-20s|", startrel(pc_), opcode,
-            WasmOpcodes::ShortOpcodeName(opcode));
+      if (!WasmOpcodes::IsPrefixOpcode(opcode)) {
+        TRACE("  @%-8d #%02x:%-20s|", startrel(pc_), opcode,
+              WasmOpcodes::ShortOpcodeName(opcode));
+      }
 
       FunctionSig* sig = WasmOpcodes::Signature(opcode);
       if (sig) {
@@ -1271,6 +1273,8 @@ class WasmFullDecoder : public WasmDecoder {
             len++;
             byte simd_index = *(pc_ + 1);
             opcode = static_cast<WasmOpcode>(opcode << 8 | simd_index);
+            TRACE("  @%-4d #%02x #%02x:%-20s|", startrel(pc_), kSimdPrefix,
+                  simd_index, WasmOpcodes::ShortOpcodeName(opcode));
             DecodeSimdOpcode(opcode);
             break;
           }
@@ -1285,6 +1289,9 @@ class WasmFullDecoder : public WasmDecoder {
         for (size_t i = 0; i < stack_.size(); ++i) {
           Value& val = stack_[i];
           WasmOpcode opcode = static_cast<WasmOpcode>(*val.pc);
+          if (WasmOpcodes::IsPrefixOpcode(opcode)) {
+            opcode = static_cast<WasmOpcode>(opcode << 8 | *(val.pc + 1));
+          }
           PrintF(" %c@%d:%s", WasmOpcodes::ShortNameOf(val.type),
                  static_cast<int>(val.pc - start_),
                  WasmOpcodes::ShortOpcodeName(opcode));

src/wasm/wasm-opcodes.cc

@@ -38,6 +38,18 @@ const char* WasmOpcodes::ShortOpcodeName(WasmOpcode opcode) {
   return "Unknown";
 }
 
+bool WasmOpcodes::IsPrefixOpcode(WasmOpcode opcode) {
+  switch (opcode) {
+#define CHECK_PREFIX(name, opcode) \
+  case k##name##Prefix:            \
+    return true;
+    FOREACH_PREFIX(CHECK_PREFIX)
+#undef CHECK_PREFIX
+    default:
+      return false;
+  }
+}
+
 std::ostream& operator<<(std::ostream& os, const FunctionSig& sig) {
   if (sig.return_count() == 0) os << "v";
   for (size_t i = 0; i < sig.return_count(); ++i) {

src/wasm/wasm-opcodes.h

@@ -489,6 +489,7 @@ class WasmOpcodes {
   static const char* OpcodeName(WasmOpcode opcode);
   static const char* ShortOpcodeName(WasmOpcode opcode);
   static FunctionSig* Signature(WasmOpcode opcode);
+  static bool IsPrefixOpcode(WasmOpcode opcode);
 
   static int TrapReasonToMessageId(TrapReason reason);
   static const char* TrapReasonMessage(TrapReason reason);