Re: [Qemu-devel] [PATCH 1/2] Detect and use GCC atomic builtins for locking

[Loïc Minier]

 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