docs. on -fno-guess-branch-probability

[Dave Fladebo <dfladebo at starband dot net>]

I recently came across a section of the docs. for gcc that was initially a bit 
confusing to me and would like to propose a change.

Old:

-fno-guess-branch-probability
 Sometimes gcc will opt to guess branch probabilities when none are
 available from either profile directed feedback (`-fprofile-arcs')
 or `__builtin_expect'.  In a hard real-time system, people don't
 want different runs of the compiler to produce code that has different
 behavior; minimizing non-determinism is of paramount import.  This
 switch allows users to reduce non-determinism, possibly at the expense
 of inferior optimization.

New:

-fno-guess-branch-probability
Depending on other optimization switch settings and source code, gcc may use 
built-in branch probability optimization heuristics when information is not 
available from either profile directed feedback (`-fprofile-arcs') or 
(`__built-in_expect'). In a hard real-time system, it may be highly desirable 
that source code with the same logical result [e.g.: if(x <= y) func1(); else 
func2(); changed to if(x > y) func2(); else func1();] cause the generation of 
executable code that performs the same, minimizing the possibility of 
differences being introduce by branch prediction optimizations, even if this 
may result in slower runtime performance. This switch instructs the compiler 
to not use the built-in branch probability optimization heuristics.

Thanks,
Dave Fladebo
dfladebo@starband.net