[Bug 1932118] Re: Bug in vxWorks epicsAtomicCmpAndSwapIntT

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

Here are the results of my collection of compilers.
I found the following:
 * gcc 4.1 on 32 bit ppc does not define any __GCC_HAVE_SYNC_COMPARE_AND_SWAP_x macro but supports  __sync_val_compare_and_swap_4 but not data size 1, 2, or 8
 * gcc 4.2 on 32 bit ppc does not define any __GCC_HAVE_SYNC_COMPARE_AND_SWAP_x macro but supports supports data size 1, 2, and 4 but not 8.
 * gcc 4.3 on 32 bit architectures define the macros for data sizes 1, 2, and 4 but not 8 and support __sync_val_compare_and_swap accordingly
 * gcc 4.9 on 64 bit architectures define the macros for data sizes 1, 2, 4, and 8 and support __sync_val_compare_and_swap accordingly
 * all architectures that support any __sync_val_compare_and_swap (gcc 4.1+) so do for at least int, size_t and void*.


** Attachment added: "dirks-atomic-predef.txt"
   https://bugs.launchpad.net/epics-base/+bug/1932118/+attachment/5506973/+files/dirks-atomic-predef.txt

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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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