[wasm] Add support and unittests for decoding signed LEB128.

src/wasm/wasm-macro-gen.h

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_WASM_MACRO_GEN_H_
 #define V8_WASM_MACRO_GEN_H_
 
 #include "src/wasm/wasm-opcodes.h"
 
 // Convenience macros for building Wasm bytecode directly into a byte array.
@@ skipping 259 matching lines @@
       static_cast<byte>((v) >> 16), static_cast<byte>((v) >> 24)
 
 #define U16_LE(v) static_cast<byte>(v), static_cast<byte>((v) >> 8)
 
 #define WASM_MODULE_HEADER U32_LE(kWasmMagic), U32_LE(kWasmVersion)
 
 #define SIG_INDEX(v) U16_LE(v)
 #define FUNC_INDEX(v) U16_LE(v)
 #define NAME_OFFSET(v) U32_LE(v)
 
+#define MASK_7 ((1 << 7) - 1)
+#define MASK_14 ((1 << 14) - 1)
+#define MASK_21 ((1 << 21) - 1)
+#define MASK_28 ((1 << 28) - 1)
+
+#define U32V_1(x) static_cast<byte>(x & MASK_7)
+#define U32V_2(x) \
+  static_cast<byte>((x & MASK_7) | 0x80), static_cast<byte>((x >> 7) & MASK_7)
+#define U32V_3(x)                                    \
+  static_cast<byte>((x & MASK_7) | 0x80),            \
+      static_cast<byte>(((x >> 7) & MASK_7) | 0x80), \
+      static_cast<byte>((x >> 14) & MASK_7)
+#define U32V_4(x)                                     \
+  static_cast<byte>((x & MASK_7) | 0x80),             \
+      static_cast<byte>(((x >> 7) & MASK_7) | 0x80),  \
+      static_cast<byte>(((x >> 14) & MASK_7) | 0x80), \
+      static_cast<byte>((x >> 21) & MASK_7)
+#define U32V_5(x)                                     \
+  static_cast<byte>((x & MASK_7) | 0x80),             \
+      static_cast<byte>(((x >> 7) & MASK_7) | 0x80),  \
+      static_cast<byte>(((x >> 14) & MASK_7) | 0x80), \
+      static_cast<byte>(((x >> 21) & MASK_7) | 0x80), \
+      static_cast<byte>((x >> 28) & 0xF)
+
 #endif  // V8_WASM_MACRO_GEN_H_