Change the definition of MALLOC_ALIGN_MASK so it should always be correct on all platforms.

Author: grynspan

stdlib/public/runtime/Heap.cpp

@@ -29,28 +29,27 @@
 using namespace swift;
 
 #if defined(__APPLE__)
-// Apple malloc is always 16-byte aligned.
-#  define MALLOC_ALIGN_MASK 15
-
-#elif defined(__linux__)
-// Linux malloc is 16-byte aligned on 64-bit, and 8-byte aligned on 32-bit.
-#  if defined(__LP64)
-#    define MALLOC_ALIGN_MASK 15
-#  else
-#    define MALLOC_ALIGN_MASK 7
-#  endif
-
-#elif defined(_WIN64)
-// Windows malloc is 16-byte aligned on 64-bit and 8-byte aligned on 32-bit.
-#  define MALLOC_ALIGN_MASK 15
-#elif defined(_WIN32)
-#  define MALLOC_ALIGN_MASK 7
+/// On Apple platforms, \c malloc() is always 16-byte aligned.
+static constexpr size_t MALLOC_ALIGN_MASK = 15;
 
+#elif defined(__linux__) || defined(_WIN32)
+/// On Linux and Windows, \c malloc() returns 16-byte aligned pointers on 64-bit
+/// and 8-byte aligned pointers on 32-bit.
+#if defined(__LP64) || defined(_WIN64)
+static constexpr size_t MALLOC_ALIGN_MASK = 15;
 #else
-// Unknown alignment, but the standard requires alignment suitable for the largest
-// standard types.
-#  define MALLOC_ALIGN_MASK std::max(alignof(void *), alignof(double))
+static constexpr size_t MALLOC_ALIGN_MASK = 7;
+#endif
 
+#else
+/// This platform's \c malloc() constraints are unknown, so fall back to a value
+/// derived from \c std::max_align_t that will be sufficient, but is not
+/// necessarily optimal.
+///
+/// The C and C++ standards defined \c max_align_t as a type whose alignment is
+/// at least that of every scalar type. It is the lower bound for the alignment
+/// of any pointer returned from \c malloc().
+static constexpr size_t MALLOC_ALIGN_MASK = alignof(std::max_align_t) - 1;
 #endif
 
 // This assert ensures that manually allocated memory always uses the