[wasm] [cleanup] Construct constexpr arrays even on gcc

Due to a bug in gcc<5, we could not make some arrays constexpr. This CL
fixes this by encapsulating the respective functions in functors.

R=tebbi@chromium.org

Change-Id: I9947e38f7fd9b801f85623663849699c0f8ffd75
Reviewed-on: https://chromium-review.googlesource.com/730303
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48779}

src/wasm/wasm-opcodes.cc

@@ -353,59 +353,51 @@ constexpr const FunctionSig* kSimpleExprSigs[] = {
     nullptr, FOREACH_SIGNATURE(DECLARE_SIG_ENTRY)};
 #undef DECLARE_SIG_ENTRY
 
-// The following constexpr functions are used to initialize the constant arrays
-// defined below. They must have exactly one return statement, and no switch.
-constexpr WasmOpcodeSig GetOpcodeSigIndex(byte opcode) {
-  return
+// gcc 4.7 - 4.9 has a bug which causes the constexpr attribute to get lost when
+// passing functions (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892). Hence
+// encapsulate these constexpr functions in functors.
+// TODO(clemensh): Remove this once we require gcc >= 5.0.
+
+struct GetOpcodeSigIndex {
+  constexpr WasmOpcodeSig operator()(byte opcode) const {
 #define CASE(name, opc, sig) opcode == opc ? kSigEnum_##sig:
-      FOREACH_SIMPLE_OPCODE(CASE)
+    return FOREACH_SIMPLE_OPCODE(CASE) kSigEnum_None;
 #undef CASE
-          kSigEnum_None;
-}
+  }
+};
 
-constexpr WasmOpcodeSig GetAsmJsOpcodeSigIndex(byte opcode) {
-  return
+struct GetAsmJsOpcodeSigIndex {
+  constexpr WasmOpcodeSig operator()(byte opcode) const {
 #define CASE(name, opc, sig) opcode == opc ? kSigEnum_##sig:
-      FOREACH_ASMJS_COMPAT_OPCODE(CASE)
+    return FOREACH_ASMJS_COMPAT_OPCODE(CASE) kSigEnum_None;
 #undef CASE
-          kSigEnum_None;
-}
+  }
+};
 
-constexpr WasmOpcodeSig GetSimdOpcodeSigIndex(byte opcode) {
-  return
+struct GetSimdOpcodeSigIndex {
+  constexpr WasmOpcodeSig operator()(byte opcode) const {
 #define CASE(name, opc, sig) opcode == (opc & 0xff) ? kSigEnum_##sig:
-      FOREACH_SIMD_0_OPERAND_OPCODE(CASE)
+    return FOREACH_SIMD_0_OPERAND_OPCODE(CASE) kSigEnum_None;
 #undef CASE
-          kSigEnum_None;
-}
+  }
+};
 
-constexpr WasmOpcodeSig GetAtomicOpcodeSigIndex(byte opcode) {
-  return
+struct GetAtomicOpcodeSigIndex {
+  constexpr WasmOpcodeSig operator()(byte opcode) const {
 #define CASE(name, opc, sig) opcode == (opc & 0xff) ? kSigEnum_##sig:
-      FOREACH_ATOMIC_OPCODE(CASE)
+    return FOREACH_ATOMIC_OPCODE(CASE) kSigEnum_None;
 #undef CASE
-          kSigEnum_None;
 }
+};
 
-// gcc 4.7 - 4.9 have a bug which prohibits marking the array constexpr
-// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892).
-// TODO(clemensh): Remove this once we require gcc >= 5.0.
-#if defined(__GNUC__) && !defined(__clang__) && __GNUC__ == 4
-#define CONSTEXPR_IF_NOT_GCC_4
-#else
-#define CONSTEXPR_IF_NOT_GCC_4 constexpr
-#endif
-
-CONSTEXPR_IF_NOT_GCC_4 std::array<WasmOpcodeSig, 256> kSimpleExprSigTable =
-    base::make_array<256>(GetOpcodeSigIndex);
-CONSTEXPR_IF_NOT_GCC_4 std::array<WasmOpcodeSig, 256> kSimpleAsmjsExprSigTable =
-    base::make_array<256>(GetAsmJsOpcodeSigIndex);
-CONSTEXPR_IF_NOT_GCC_4 std::array<WasmOpcodeSig, 256> kSimdExprSigTable =
-    base::make_array<256>(GetSimdOpcodeSigIndex);
-CONSTEXPR_IF_NOT_GCC_4 std::array<WasmOpcodeSig, 256> kAtomicExprSigTable =
-    base::make_array<256>(GetAtomicOpcodeSigIndex);
-
-#undef CONSTEXPR_IF_NOT_GCC_4
+constexpr std::array<WasmOpcodeSig, 256> kSimpleExprSigTable =
+    base::make_array<256>(GetOpcodeSigIndex{});
+constexpr std::array<WasmOpcodeSig, 256> kSimpleAsmjsExprSigTable =
+    base::make_array<256>(GetAsmJsOpcodeSigIndex{});
+constexpr std::array<WasmOpcodeSig, 256> kSimdExprSigTable =
+    base::make_array<256>(GetSimdOpcodeSigIndex{});
+constexpr std::array<WasmOpcodeSig, 256> kAtomicExprSigTable =
+    base::make_array<256>(GetAtomicOpcodeSigIndex{});
 
 }  // namespace