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meson: Define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 on GNU/Linux if needed

Simon McVittie requested to merge smcv/glib:1576-meson-armv5-atomic into master

armv5 Linux systems implement __sync_bool_compare_and_swap() and friends by calling a function provided by the kernel. This is not technically an atomic intrinsic, so gcc doesn't define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 in this case, but it's good enough for us. Extend the current Android special case to cover GNU/Linux too.

The possibilities are:

  • __sync_foo detected and __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 predefined: calls to __atomic_foo or __sync_foo primitives are inlined into user code by gatomic.h

  • __sync_foo detected but __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 not predefined: user code has an extern reference to g_atomic_foo(), which calls __atomic_foo or __sync_foo because we defined __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 when compiling GLib itself

  • Windows: user code has an extern reference to g_atomic_foo(), which calls InterlockedFoo()

  • !defined(G_ATOMIC_LOCK_FREE): user code has an extern reference to g_atomic_foo(), which emulates atomic operations with a mutex

Closes #1576 (closed)

Testing still needed:

  • Test on Debian armel (armv5 EABI softfloat)
  • Test on Debian armhf (armv7 EABI hardfloat)
Edited by Simon McVittie

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