Allow building without JEMalloc under MSVC

Summary: This was falling back to a hard depency on JEMalloc, and MSVC doesn't have proper weak linkage, so emulate it with some linker magic to get things building.

Reviewed By: yfeldblum

Differential Revision: D2887695

Pulled By: Orvid

fb-gh-sync-id: 50c812a07c4e4e5bbe71263b86386783bf76ba82

folly/detail/Malloc.h

@@ -36,6 +36,39 @@ int mallctl(const char*, void*, size_t*, void*, size_t)
 int mallctlnametomib(const char*, size_t*, size_t*) __attribute__((__weak__));
 int mallctlbymib(const size_t*, size_t, void*, size_t*, void*, size_t)
       __attribute__((__weak__));
+#elif defined(_MSC_VER)
+// MSVC doesn't have weak symbols, so do some linker magic
+// to emulate them.
+extern void* (*mallocx)(size_t, int);
+extern const char* mallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_mallocx=_mallocxWeak")
+extern void* (*rallocx)(void*, size_t, int);
+extern const char* rallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_rallocx=_rallocxWeak")
+extern size_t(*xallocx)(void*, size_t, size_t, int);
+extern const char* xallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_xallocx=_xallocxWeak")
+extern size_t(*sallocx)(const void*, int);
+extern const char* sallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_sallocx=_sallocxWeak")
+extern void(*dallocx)(void*, int);
+extern const char* dallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_dallocx=_dallocxWeak")
+extern void(*sdallocx)(void*, size_t, int);
+extern const char* sdallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_sdallocx=_sdallocxWeak")
+extern size_t(*nallocx)(size_t, int);
+extern const char* nallocxWeak = nullptr;
+#pragma comment(linker, "/alternatename:_nallocx=_nallocxWeak")
+extern int(*mallctl)(const char*, void*, size_t*, void*, size_t);
+extern const char* mallctlWeak = nullptr;
+#pragma comment(linker, "/alternatename:_mallctl=_mallctlWeak")
+extern int(*mallctlnametomib)(const char*, size_t*, size_t*);
+extern const char* mallctlnametomibWeak = nullptr;
+#pragma comment(linker, "/alternatename:_mallctlnametomib=_mallctlnametomibWeak")
+extern int(*mallctlbymib)(const size_t*, size_t, void*, size_t*, void*, size_t);
+extern const char* mallctlbymibWeak = nullptr;
+#pragma comment(linker, "/alternatename:_mallctlbymib=_mallctlbymibWeak")
 #else
 extern void* (*mallocx)(size_t, int);
 extern void* (*rallocx)(void*, size_t, int);