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Require compiler barrier support.

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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
This commit is contained in:
Thomas Munro 2024-07-30 22:12:42 +12:00
parent 8138526136
commit a011dc399c
3 changed files with 3 additions and 23 deletions
src
backend/port
atomics.c
include/port
atomics.h
atomics
fallback.h

8
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

3
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

15
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)