NB: Addition of these builtins was prompted by qemu failing to build on armel in Ubuntu; this is because we default to Thumb 2 mode which doesn't have the assembly instructions in question. http://launchpadlibrarian.net/38837077/buildlog_ubuntu-lucid-armel.qemu-kvm_0.12.2-0ubuntu6_FAILEDTOBUILD.txt.gz [...] CC arm-softmmu/syborg_virtio.o CC arm-softmmu/exec.o /tmp/cc24C9yx.s: Assembler messages: /tmp/cc24C9yx.s:5392: Error: selected processor does not support `swp r4,r4,[r3]' /tmp/cc24C9yx.s:6599: Error: selected processor does not support `swp r6,r6,[r3]' make[2]: *** [exec.o] Error 1 make[1]: *** [subdir-arm-softmmu] Error 2 [...]
On Sat, Feb 20, 2010, malc wrote: > Please look up "gcc 4.1 implements compiler builtins for atomic ops" > thread for reasons why this might not be the best idea. I found a very old thred (2005) on libc-alpha with this subject; is this the one you mean? People participating in the libc-alpha were not really constructive and were presenting some bogus arguments; let me try to go over the various arguments (long): - some people wanted atomic builtins to be inline for performance and others wanted them to be library calls to allow changing their behavior later (e.g. to support a new CPU); the implementation actually uses both: inline assembly when supported on the architecture, or calls into libgcc which will call into the kernel otherwise (or direct calls into the kernel when building statically obviously), such as when the ISA doesn't offer sufficient primitives. Because *any* instruction might be gotten wrong in hardware, I don't see a need to special case locking inline assembly. - userspace apps abusing atomic builtins for locking; this is actually the case of qemu, but I think using gcc primitives will actually make it easier to get it right and will allow coverage of more architectures; in my opinion, there's no need to maintain hand-written assembly for locks in 2010. - someone claimed that modern architectures can do these operations in assembly without calling into a library; that's what the atomic builtins do, and actually some modern architectures can't do all operations in assembly. - there were arguments over where such functions belong and the semantics of each call; these are in my eyes purely political and don't relate to qemu. Which are your concerns with atomic builtins and are these in the above list? -- Loïc Minier