[libdispatch-dev] lib dispatch worker threads may loop depending on compiler optimization

[Kevin Van Vechten]

Great find!

On Sep 20, 2011, at 12:48 AM, Paolo Bonzini wrote:

> On 09/19/2011 06:32 PM, Joakim Johansson wrote:
>> <http://libdispatch.macosforge.org/trac/ticket/35#comment:10>
> 
> Thanks.  It turns out it is _not_ a bug in GCC at all, but in libdispatch.
> 
> Compiling the .i file shows the following:
> 
>        movq    %r13, %rax
>        movq    $1, 0(%r13)
>        movq    %r12, 16(%r13)
> #APP
> # 110 "/tb/builds/thd/sbn/2.4/src/thirdparty/libdispatch/197/src/src/queue_internal.h" 1
>        xchg %rax, 64(%rbx)
> # 0 "" 2
> #NO_APP
>        testq   %rax, %rax
>        movq    %rbp, 24(%r13)
>        movq    $0, 8(%r13)
>        je      .L172
> 
> 
> The APP/NO_APP markers signify that an asm is being used rather than sync
> builtins.  Looking at the source code indeed reveals this...
> 
> // GCC generates suboptimal register pressure
> // LLVM does better, but doesn't support tail calls
> // 6248590 __sync_*() intrinsics force a gratuitous "lea" instruction, with resulting register pressure
> #if 0 && defined(__i386__) || defined(__x86_64__)
> #define dispatch_atomic_xchg(p, n)      ({ typeof(*(p)) _r; asm("xchg %0, %1" : "=r" (_r) : "m" (*(p)), "0" (n)); _r; })
> #else
> #define dispatch_atomic_xchg(p, n)      ((typeof(*(p)))__sync_lock_test_and_set((p), (n)))
> #endif
> 
> 
> ... which is missing parentheses like
> 
> #if 0 && (defined(__i386__) || defined(__x86_64__))
> 
> The asm is wrong, because it doesn't have a clobber for "memory".
> That fixes the testcase.
> 
> Paolo
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