Fix Bug#24707869 GCC 5 AND 6 MISCOMPILE MACH_PARSE_COMPRESSED

Prevent GCC from moving a mach_read_from_4() before we have checked that
we have 4 bytes to read. The pointer may only point to a 1, 2 or 3
bytes in which case the code should not read 4 bytes. This is a
workaround to a GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77673

Patch submitted by: Laurynas Biveinis <laurynas.biveinis@gmail.com>
RB: 14135
Reviewed by: Pawel Olchawa <pawel.olchawa@oracle.com>

storage/innobase/mach/mach0data.cc

@@ -49,7 +49,22 @@ mach_parse_compressed(
 		/* 0nnnnnnn (7 bits) */
 		++*ptr;
 		return(static_cast<ib_uint32_t>(val));
-	} else if (val < 0xC0) {
+	}
+
+	/* Workaround GCC bug
+	https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77673:
+	the compiler moves mach_read_from_4 right to the beginning of the
+	function, causing and out-of-bounds read if we are reading a short
+	integer close to the end of buffer. */
+#if defined(__GNUC__) && (__GNUC__ >= 5) && !defined(__clang__)
+#define DEPLOY_FENCE
+#endif
+
+#ifdef DEPLOY_FENCE
+	__atomic_thread_fence(__ATOMIC_ACQUIRE);
+#endif
+
+	if (val < 0xC0) {
 		/* 10nnnnnn nnnnnnnn (14 bits) */
 		if (end_ptr >= *ptr + 2) {
 			val = mach_read_from_2(*ptr) & 0x3FFF;
@@ -57,7 +72,15 @@ mach_parse_compressed(
 			*ptr += 2;
 			return(static_cast<ib_uint32_t>(val));
 		}
-	} else if (val < 0xE0) {
+		*ptr = NULL;
+		return(0);
+	}
+
+#ifdef DEPLOY_FENCE
+	__atomic_thread_fence(__ATOMIC_ACQUIRE);
+#endif
+
+	if (val < 0xE0) {
 		/* 110nnnnn nnnnnnnn nnnnnnnn (21 bits) */
 		if (end_ptr >= *ptr + 3) {
 			val = mach_read_from_3(*ptr) & 0x1FFFFF;
@@ -65,7 +88,15 @@ mach_parse_compressed(
 			*ptr += 3;
 			return(static_cast<ib_uint32_t>(val));
 		}
-	} else if (val < 0xF0) {
+		*ptr = NULL;
+		return(0);
+	}
+
+#ifdef DEPLOY_FENCE
+	__atomic_thread_fence(__ATOMIC_ACQUIRE);
+#endif
+
+	if (val < 0xF0) {
 		/* 1110nnnn nnnnnnnn nnnnnnnn nnnnnnnn (28 bits) */
 		if (end_ptr >= *ptr + 4) {
 			val = mach_read_from_4(*ptr) & 0xFFFFFFF;
@@ -73,7 +104,16 @@ mach_parse_compressed(
 			*ptr += 4;
 			return(static_cast<ib_uint32_t>(val));
 		}
-	} else {
+		*ptr = NULL;
+		return(0);
+	}
+
+#ifdef DEPLOY_FENCE
+	__atomic_thread_fence(__ATOMIC_ACQUIRE);
+#endif
+
+#undef DEPLOY_FENCE
+
 	ut_ad(val == 0xF0);
 
 	/* 11110000 nnnnnnnn nnnnnnnn nnnnnnnn nnnnnnnn (32 bits) */
@@ -83,7 +123,6 @@ mach_parse_compressed(
 		*ptr += 5;
 		return(static_cast<ib_uint32_t>(val));
 	}
-	}
 
 	*ptr = NULL;
 	return(0);