[Bug 1932118] Re: Bug in vxWorks epicsAtomicCmpAndSwapIntT

[Dirk Zimoch via Core-talk <core-talk at aps.anl.gov>]

Using the gcc __sync_* buildins, in particular
__sync_val_compare_and_swap, seems to work for vxWorks 6.6 or higher
(using gcc 4.1.2 or higher) on the PPC architecture.

I see two possibilities to modify gcc/epicsAtomicCD.h so that the compiler builtins are used on PPC as well (and not only on x86):
a. Remove the architecture filter and assume gcc 4.1+ will always have __sync_*
b. Add __ppc to the filter.

I would prefer solution a, but cannot test it as I have no other architectures.
It would affect all non x86 architectures (not only vxWorks) with gcc >= 4.1 but not yet defining __GCC_HAVE_SYNC_COMPARE_AND_SWAP_* macros.
The vxWorks 6.9 gcc 4.3.3 already defines those macros, (which explains that this bug is not visible in vxWorks 6.9) thus probably only gcc 4.1 and maybe 4.2 are affected.

Opinions?

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https://bugs.launchpad.net/bugs/1932118

Title:
  Bug in vxWorks epicsAtomicCmpAndSwapIntT

Status in EPICS Base:
  Fix Committed

Bug description:
  The usage of epicsAtomicCmpAndSwapIntT() in callback.c suggests that
  the function should return the previous value. But the VxWorks
  implementation uses vxCas() which is documented as returning "TRUE if
  the write actually occurs, FALSE otherwise."

  vxAtomicLib.html:
  vxCas( )
  NAME
      vxCas( ) - atomically compare-and-swap the contents of a memory location 

  SYNOPSIS
      BOOL vxCas
          (
          atomic_t *  target,    
          atomicVal_t oldValue,  
          atomicVal_t newValue   
          )

  DESCRIPTION
      This routine performs an atomic compare-and-swap; testing that *target contains oldValue, and if it does, setting the value of *target to newValue.

  RETURNS
      TRUE if the swap is actually executed, FALSE otherwise.

  
  Comparing this with the gcc documentation:
  bool __sync_bool_compare_and_swap (type *ptr, type oldval type newval, ...)
  type __sync_val_compare_and_swap (type *ptr, type oldval type newval, ...)

  These builtins perform an atomic compare and swap.  That is, if the
  current value of *ptr is oldval, then write newval into *ptr.

  The “bool” version returns true if the comparison is successful and newval was written.
  The “val” version returns the contents of *ptr before the operation.

  
  That means epicsAtomicCmpAndSwapIntT() behaves behaves like __sync_bool_compare_and_swap(). Simply casting the return value from BOOL to the type of oldVal does not change that.

  But the usage implies an expected bahavior like
  __sync_val_compare_and_swap (), wich is used in the gcc implementation
  of epicsAtomicCmpAndSwapIntT().

  
  This bugs prevents the callback threads from being started in vxWorks 6.7, maybe more or all vxWorks 6 versions.

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