Bug#20045167 UT_DELAY MISSING COMPILER BARRIER

UT_RELAX_CPU(): Use a compiler barrier.

ut_delay(): Remove the dummy global variable ut_always_false.

RB: 11399
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>

Backported from MySQL-5.7 - patch https://github.com/mysql/mysql-server/commit/5e3efb03962838a13afbf1579abbb96aee48ad64

Suggestion by Stewart Smith

storage/innobase/include/ut0ut.h

@@ -88,13 +88,8 @@ struct ut_when_dtor {
    the YieldProcessor macro defined in WinNT.h. It is a CPU architecture-
    independent way by using YieldProcessor. */
 #  define UT_RELAX_CPU() YieldProcessor()
-# elif defined(HAVE_ATOMIC_BUILTINS)
-#  define UT_RELAX_CPU() do { \
-     volatile lint	volatile_var; \
-     os_compare_and_swap_lint(&volatile_var, 0, 1); \
-   } while (0)
 # else
-#  define UT_RELAX_CPU() ((void)0) /* avoid warning for an empty statement */
+#  define UT_RELAX_CPU() __asm__ __volatile__ ("":::"memory")
 # endif
 
 /*********************************************************************//**

storage/innobase/ut/ut0ut.cc

@@ -45,9 +45,6 @@ Created 5/11/1994 Heikki Tuuri
 # include <string>
 #endif /* UNIV_HOTBACKUP */
 
-/** A constant to prevent the compiler from optimizing ut_delay() away. */
-UNIV_INTERN ibool	ut_always_false	= FALSE;
-
 #ifdef __WIN__
 /*****************************************************************//**
 NOTE: The Windows epoch starts from 1601/01/01 whereas the Unix
@@ -411,10 +408,6 @@ ut_delay(
 		UT_RELAX_CPU();
 	}
 
-	if (ut_always_false) {
-		ut_always_false = (ibool) j;
-	}
-
 	return(j);
 }
 #endif /* !UNIV_HOTBACKUP */

storage/xtradb/include/ut0ut.h

@@ -80,18 +80,13 @@ struct ut_when_dtor {
 
 # elif defined(HAVE_FAKE_PAUSE_INSTRUCTION)
 #  define UT_RELAX_CPU() __asm__ __volatile__ ("rep; nop")
-# elif defined(HAVE_ATOMIC_BUILTINS)
-#  define UT_RELAX_CPU() do { \
-     volatile lint	volatile_var; \
-     os_compare_and_swap_lint(&volatile_var, 0, 1); \
-   } while (0)
 # elif defined(HAVE_WINDOWS_ATOMICS)
    /* In the Win32 API, the x86 PAUSE instruction is executed by calling
    the YieldProcessor macro defined in WinNT.h. It is a CPU architecture-
    independent way by using YieldProcessor. */
 #  define UT_RELAX_CPU() YieldProcessor()
 # else
-#  define UT_RELAX_CPU() ((void)0) /* avoid warning for an empty statement */
+#  define UT_RELAX_CPU() __asm__ __volatile__ ("":::"memory")
 # endif
 
 /*********************************************************************//**

storage/xtradb/ut/ut0ut.cc

@@ -46,9 +46,6 @@ Created 5/11/1994 Heikki Tuuri
 # include <string>
 #endif /* UNIV_HOTBACKUP */
 
-/** A constant to prevent the compiler from optimizing ut_delay() away. */
-UNIV_INTERN ibool	ut_always_false	= FALSE;
-
 #ifdef __WIN__
 /*****************************************************************//**
 NOTE: The Windows epoch starts from 1601/01/01 whereas the Unix
@@ -412,10 +409,6 @@ ut_delay(
 		UT_RELAX_CPU();
 	}
 
-	if (ut_always_false) {
-		ut_always_false = (ibool) j;
-	}
-
 	return(j);
 }
 #endif /* !UNIV_HOTBACKUP */