Re: toolchain/54431: Missing 64-bit atomics on 32-bit platforms

[Joerg Sonnenberger <joerg%bec.de@localhost>]

The following reply was made to PR toolchain/54431; it has been noted by GNATS.

From: Joerg Sonnenberger <joerg%bec.de@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: toolchain/54431: Missing 64-bit atomics on 32-bit platforms
Date: Fri, 2 Aug 2019 14:27:41 +0200

 On Fri, Aug 02, 2019 at 11:35:00AM +0000, he%NetBSD.org@localhost wrote:
 > 	It appears that at least our powerpc and i386 ports do not
 > 	support any sort of 64-bit atomic operations.
 
 I do not consider this a bug. The reality is that there is no good way
 to provide fall back implementations for larger-than-pointer atomics on
 most UP platforms or atomic emulation in general on MP platforms.
 We have some hacks on SPARC using hashed locks, but they have major
 subtile error cases. Most noticable, they don't work on shared memory.
 There is a secondary issue for larger-than-pointer atomics because most
 architectures don't have atomic read or write instructions for that
 size. At least for the old __sync_* interface that is a major deal
 breaker as the load/store is typically written as fence + plain C
 volatile load/store.
 
 Any correct implementation will involve a system and hashing of the
 physical address of the access memory for larger-than-pointer access for
 any operation. I'm pretty sure that is going to kill the performance of
 a lot of those users. It doesn't help that there is no way for the user
 to promise that a certain memory address is not shared...
 
 Joerg