Add internal/tsan_assist.h.

Goal here is to facilitate writing "thread-opportunistic" code that
withstands Thread Sanitizer's scrutiny. "Thread-opportunistic" is when
exact result is not required, e.g. some statistics, or execution flow
doesn't have to be unambiguous.
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6786)

include/internal/tsan_assist.h

+/*
+ * Copyright 2018 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the OpenSSL license (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+ 
+/*
+ * Goal here is to facilitate writing "thread-opportunistic" code that
+ * withstands Thread Sanitizer's scrutiny. "Thread-opportunistic" is when
+ * exact result is not required, e.g. some statistics, or execution flow
+ * doesn't have to be unambiguous. Simplest example is lazy "constant"
+ * initialization when one can synchronize on variable itself, e.g.
+ *
+ * if (var == NOT_YET_INITIALIZED)
+ *     var = function_returning_same_value();
+ *
+ * This does work provided that loads and stores are single-instuction
+ * operations (and integer ones are on *all* supported platforms), but
+ * it upsets Thread Sanitizer. Suggested solution is
+ *
+ * if (tsan_load(&var) == NOT_YET_INITIALIZED)
+ *     tsan_store(&var, function_returning_same_value());
+ *
+ * Production machine code would be the same, so one can wonder why
+ * bother. Having Thread Sanitizer accept "thread-opportunistic" code
+ * allows to move on trouble-shooting real bugs.
+ *
+ * We utilize the fact that compilers that implement Thread Sanitizer
+ * implement even atomic operations. Then it's assumed that
+ * ATOMIC_{LONG|INT}_LOCK_FREE are assigned same value as
+ * ATOMIC_POINTER_LOCK_FREE. And check for >= 2 ensures that correspodning
+ * code is inlined. It should be noted that statistics counters become
+ * accurate in such case.
+ */
+
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L \
+    && !defined(__STDC_NO_ATOMICS__)
+# include <stdatomic.h>
+
+# if defined(ATOMIC_POINTER_LOCK_FREE) \
+          && ATOMIC_POINTER_LOCK_FREE >= 2
+#  define TSAN_QUALIFIER _Atomic
+#  define tsan_load(ptr) atomic_load_explicit((ptr), memory_order_relaxed)
+#  define tsan_store(ptr, val) atomic_store_explicit((ptr), (val), memory_order_relaxed)
+#  define tsan_counter(ptr) atomic_fetch_add_explicit((ptr), 1, memory_order_relaxed)
+# endif
+
+#elif defined(__GNUC__) && defined(__ATOMIC_RELAXED)
+
+# if defined(__GCC_ATOMIC_POINTER_LOCK_FREE) \
+          && __GCC_ATOMIC_POINTER_LOCK_FREE >= 2
+#  define TSAN_QUALIFIER volatile
+#  define tsan_load(ptr) __atomic_load_n((ptr), __ATOMIC_RELAXED)
+#  define tsan_store(ptr, val) __atomic_store_n((ptr), (val), __ATOMIC_RELAXED)
+#  define tsan_counter(ptr) __atomic_fetch_add((ptr), 1, __ATOMIC_RELAXED)
+# endif
+
+#elif defined(_MSC_VER) && _MSC_VER>=1200
+
+# define TSAN_QUALIFIER volatile
+# define tsan_load(ptr) (*(ptr))
+# define tsan_store(ptr, val) (*(ptr) = (val))
+# pragma intrinsic(_InterlockedExchangeAdd)
+# ifdef _WIN64
+#  pragma intrinsic(_InterlockedExchangeAdd64)
+#  define tsan_counter(ptr) (sizeof(*ptr) == 8 ? _InterlockedExchangeAdd64((ptr), 1) \
+                                               : _InterlockedExchangeAdd((ptr), 1))
+# else
+#  define tsan_counter(ptr) _InterlockedExchangeAdd((ptr), 1)
+# endif
+
+#endif
+
+#ifndef TSAN_QUALIFIER
+
+# define TSAN_QUALIFIER volatile
+# define tsan_load(ptr) (*(ptr))
+# define tsan_store(ptr, val) (*(ptr) = (val))
+# define tsan_counter(ptr) ((*(ptr))++)
+
+#endif