Require compiler barrier support.

Previously we had a fallback implementation of pg_compiler_barrier()
that called an empty function across a translation unit boundary so the
compiler couldn't see what it did.  That shouldn't be needed on any
current systems, and might not even work with a link time optimizer.
Since we now require compiler-specific knowledge of how to implement
atomics, we should also know how to implement compiler barriers on a
hypothetical new system.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Suggested-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/721bf39a-ed8a-44b0-8b8e-be3bd81db748%40technowledgy.de
Discussion: https://postgr.es/m/3351991.1697728588%40sss.pgh.pa.us

src/backend/port/atomics.c

@@ -40,14 +40,6 @@ pg_spinlock_barrier(void)
 }
 #endif
 
-#ifdef PG_HAVE_COMPILER_BARRIER_EMULATION
-void
-pg_extern_compiler_barrier(void)
-{
-	/* do nothing */
-}
-#endif
-
 
 #ifdef PG_HAVE_ATOMIC_U64_SIMULATION
 

src/include/port/atomics.h

@@ -98,6 +98,9 @@
 #if !defined(PG_HAVE_ATOMIC_U32_SUPPORT)
 #error "could not find an implementation of pg_atomic_uint32"
 #endif
+#if !defined(pg_compiler_barrier_impl)
+#error "could not find an implementation of pg_compiler_barrier"
+#endif
 
 /*
  * Provide a spinlock-based implementation of the 64 bit variants, if

src/include/port/atomics/fallback.h

@@ -33,21 +33,6 @@ extern void pg_spinlock_barrier(void);
 #define pg_memory_barrier_impl pg_spinlock_barrier
 #endif
 
-#ifndef pg_compiler_barrier_impl
-/*
- * If the compiler/arch combination does not provide compiler barriers,
- * provide a fallback.  The fallback simply consists of a function call into
- * an externally defined function.  That should guarantee compiler barrier
- * semantics except for compilers that do inter translation unit/global
- * optimization - those better provide an actual compiler barrier.
- *
- * A native compiler barrier for sure is a lot faster than this...
- */
-#define PG_HAVE_COMPILER_BARRIER_EMULATION
-extern void pg_extern_compiler_barrier(void);
-#define pg_compiler_barrier_impl pg_extern_compiler_barrier
-#endif
-
 
 #if !defined(PG_HAVE_ATOMIC_U64_SUPPORT)