[Runtime] Have swift_slowAlloc malloc directly if the alignMask is sufficiently small.

rdar://problem/22975669

Author: mikeash

stdlib/public/runtime/Heap.cpp

@@ -18,16 +18,53 @@
 #include "swift/Runtime/Heap.h"
 #include "Private.h"
 #include "swift/Runtime/Debug.h"
+#include <algorithm>
 #include <stdlib.h>
 
 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
+
+#else
+// Unknown alignment, but the standard requires alignment suitable for the largest
+// standard types.
+#  define MALLOC_ALIGN_MASK std::max(alignof(void *), alignof(double))
+
+#endif
+
+
+
 void *swift::swift_slowAlloc(size_t size, size_t alignMask) {
-  void *p = AlignedAlloc(size, alignMask + 1);
+  void *p;
+  if (alignMask <= MALLOC_ALIGN_MASK) {
+    p = malloc(size);
+  } else {
+    p = AlignedAlloc(size, alignMask + 1);
+  }
   if (!p) swift::crash("Could not allocate memory.");
   return p;
 }
 
 void swift::swift_slowDealloc(void *ptr, size_t bytes, size_t alignMask) {
-  AlignedFree(ptr);
+  if (alignMask <= MALLOC_ALIGN_MASK) {
+    free(ptr);
+  } else {
+    AlignedFree(ptr);
+  }
 }