Re: Faster compilation speed

[dewar at gnat dot com (Robert Dewar)]

<<        Saying "do not run any optimization at -O0" shows a tremendous
lack of understanding or investigation.  One wants minimal optimization
even at -O0 to decrease the size of the IL representation of the function
being compiled.  The little bit of computation to perform trivial
optimization more than makes up for itself with the decreased size of the
IL that needs to be processed to generate the output.
>>

There are two reasons to run at -O0

a) make the code as easy to debug as possible
b) speedy compilation

There is also a third reason that is relevant to safety critical code

c) avoid optimization, on the grounds that it inteferes with verification

Now with respect to a), the trouble with GCC is that the code generated
with no optimization is really horrible. Much worse than typical competing
compilers operating in no optimization mode. Now of course we can say
"yes, but gcc is really doing what you want, the other compiler is not"
but the fact remains that you are stuck between two unpleasant choices

  -O0 generates far too much code and giant executables
  -O1 already loses debugging information

I think there is a real need for a mode which would do all possible
optimizations that do NOT intefere with debugging. I would probably
use this as my default development mode all the time.

With respect to b) one has to be careful that sometimes some limited
amount of optimzation (e.g. simple register tracking, and slightly
reasonable register allocation) can cut down the size of the code
enough that compilation time suffers very little, or even is improved.

With respect to c), we find in practice that -O1 mode is manageable for
a lot of certification needs, but probably it is a good idea to retain
the absolutely-no-optimization mode.