Bug 1464790 - js/src/wasm/WasmTypes.h: define MaxMemoryAccessSize correctly. r=luke.

Currently we have MaxMemoryAccessSize = sizeof(Val).  That doesn't seem
right, since it includes not only the size of the largest underlying value,
but also the size of the tag (Val::type_) field.  Indeed, printing
MaxMemoryAccessSize produces 24 on x86_64 and 20 on x86.  This patch fixes
it, and adds some sanity checks too.

--HG--
extra : rebase_source : edef632fee092f03b325565cb4d6a8d78e8d8721

js/src/wasm/WasmTypes.cpp

@@ -47,11 +47,19 @@ using mozilla::MakeEnumeratedRange;
 #  endif
 #endif
 
-// Another sanity check.
+// More sanity checks.
 
 static_assert(MaxMemoryInitialPages <= ArrayBufferObject::MaxBufferByteLength / PageSize,
               "Memory sizing constraint");
 
+// All plausible targets must be able to do at least IEEE754 double
+// loads/stores, hence the lower limit of 8.  Some Intel processors support
+// AVX-512 loads/stores, hence the upper limit of 64.
+static_assert(MaxMemoryAccessSize >= 8,  "MaxMemoryAccessSize too low");
+static_assert(MaxMemoryAccessSize <= 64, "MaxMemoryAccessSize too high");
+static_assert((MaxMemoryAccessSize & (MaxMemoryAccessSize-1)) == 0,
+              "MaxMemoryAccessSize is not a power of two");
+
 void
 Val::writePayload(uint8_t* dst) const
 {

js/src/wasm/WasmTypes.h

@@ -653,6 +653,7 @@ class Val
 
     ValType type() const { return type_; }
     bool isSimd() const { return IsSimdType(type()); }
+    static constexpr size_t sizeofLargestValue() { return sizeof(u); }
 
     uint32_t i32() const { MOZ_ASSERT(type_ == ValType::I32); return u.i32_; }
     uint64_t i64() const { MOZ_ASSERT(type_ == ValType::I64); return u.i64_; }
@@ -1993,7 +1994,7 @@ static const unsigned PageSize = 64 * 1024;
 // catch the overflow. MaxMemoryAccessSize is a conservative approximation of
 // the maximum guard space needed to catch all unaligned overflows.
 
-static const unsigned MaxMemoryAccessSize = sizeof(Val);
+static const unsigned MaxMemoryAccessSize = Val::sizeofLargestValue();
 
 #ifdef WASM_HUGE_MEMORY