Add MSVC support to MaxAlign

Summary: This adds MSVC support to the detection of `MaxAlign` in `Portability.h`.
Closes #256

Reviewed By: @yfeldblum

Differential Revision: D2283221

Pulled By: @sgolemon

folly/MicroSpinLock.h

@@ -123,11 +123,9 @@ struct FOLLY_ALIGNED_MAX SpinLockArray {
                 "Invalid size of PaddedSpinLock");
 
   // Check if T can theoretically cross a cache line.
-  // NOTE: It should be alignof(std::max_align_t), but max_align_t
-  // isn't supported by gcc 4.6.2.
-  static_assert(alignof(MaxAlign) > 0 &&
-                FOLLY_CACHE_LINE_SIZE % alignof(MaxAlign) == 0 &&
-                sizeof(T) <= alignof(MaxAlign),
+  static_assert(alignof(std::max_align_t) > 0 &&
+                FOLLY_CACHE_LINE_SIZE % alignof(std::max_align_t) == 0 &&
+                sizeof(T) <= alignof(std::max_align_t),
                 "T can cross cache line boundaries");
 
   char padding_[FOLLY_CACHE_LINE_SIZE];

folly/Portability.h

@@ -17,6 +17,8 @@
 #ifndef FOLLY_PORTABILITY_H_
 #define FOLLY_PORTABILITY_H_
 
+#include <cstddef>
+
 #ifndef FOLLY_NO_CONFIG
 #include <folly/folly-config.h>
 #endif
@@ -53,13 +55,6 @@
   #endif
 #endif
 
-// MaxAlign: max_align_t isn't supported by gcc
-#ifdef __GNUC__
-struct MaxAlign { char c; } __attribute__((__aligned__));
-#else /* !__GNUC__ */
-# error Cannot define MaxAlign on this platform
-#endif
-
 // compiler specific attribute translation
 // msvc should come first, so if clang is in msvc mode it gets the right defines
 
@@ -70,7 +65,7 @@ struct MaxAlign { char c; } __attribute__((__aligned__));
 #else
 # error Cannot define FOLLY_ALIGNED on this platform
 #endif
-#define FOLLY_ALIGNED_MAX FOLLY_ALIGNED(alignof(MaxAlign))
+#define FOLLY_ALIGNED_MAX FOLLY_ALIGNED(alignof(std::max_align_t))
 
 // NOTE: this will only do checking in msvc with versions that support /analyze
 #if _MSC_VER
@@ -164,6 +159,12 @@ struct MaxAlign { char c; } __attribute__((__aligned__));
 # endif
 #endif
 
+#if defined(__GNUC__) && !__GNUC_PREREQ(4,9)
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56019
+// gcc 4.8.x incorrectly placed max_align_t in the root namespace
+// Alias it into std (where it's found in 4.9 and later)
+namespace std { typedef ::max_align_t max_align_t; }
+#endif
 
 /* Define macro wrappers for C++11's "final" and "override" keywords, which
  * are supported in gcc 4.7 but not gcc 4.6. */

folly/io/IOBuf.cpp

@@ -112,7 +112,7 @@ struct IOBuf::HeapFullStorage {
 
   HeapStorage hs;
   SharedInfo shared;
-  MaxAlign align;
+  std::max_align_t align;
 };
 
 IOBuf::SharedInfo::SharedInfo()

folly/io/test/IOBufTest.cpp

@@ -24,6 +24,8 @@
 #include <folly/Malloc.h>
 #include <folly/Range.h>
 
+#include <cstddef>
+
 using folly::fbstring;
 using folly::fbvector;
 using folly::IOBuf;
@@ -792,11 +794,7 @@ TEST(IOBuf, takeOwnershipUniquePtr) {
 }
 
 TEST(IOBuf, Alignment) {
-  // max_align_t doesn't exist in gcc 4.6.2
-  struct MaxAlign {
-    char c;
-  } __attribute__((__aligned__));
-  size_t alignment = alignof(MaxAlign);
+  size_t alignment = alignof(std::max_align_t);
 
   std::vector<size_t> sizes {0, 1, 64, 256, 1024, 1 << 10};
   for (size_t size : sizes) {