__builtin_expect with other compilers

[Jed Brown]

I noticed that GCC mispredicts the conditional in CHKERRQ.  At high
optimization levels it still relocates most of the PetscError call, but
the non-error path is cleaner with

  #define CHKERRQ(n) if (UNLIKELY(n)) {return PetscError(__LINE__,__FUNCT__,__FILE__,__SDIR__,n,0," ");}

where the following definitions are typical

  #define UNLIKELY(c) __builtin_expect(!!(c),0)
  #define LIKELY(c)   __builtin_expect(!!(c),1)

I'm not familiar with the analogues for other compilers.  I don't
suggest cluttering the code with such hints, but just putting it in
CHKERRQ would decrease code size for the non-error path which seems like
a good thing.

Jed
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