Documentation and behaviour of the "optimize" attribute

[Richard Sandiford]

Hi,

Sorry in advance if this going over old ground.  And if not, sorry
for the somewhat negative message ;), but ...  I think the current
documentation and/or behaviour of the "optimize" attribute are a little
confusing.

The current behaviour is that, if __attribute__((optimize(...))) does
not specify an optimisation level (-Ox), we act as though the prevailing
-Ox level had been restated.  So:

  __attribute__((optimize("no-gcse")))

behaves like:

  __attribute__((optimize("Ox", "no-gcse")))

where Ox is the current optimisation level.  This means that if you
compile:

  void bar (int);
  void __attribute__((optimize("no-gcse"))) f1 (void)
  {
    bar (1);
    bar (2);
  }
  void f2 (void)
  {
    bar (1);
    bar (2);
  }

with -O2 -fno-omit-frame-pointer, f1 will be implicitly use:

  __attribute__((optimize("O2", "no-gcse")))

and this implicit -O2 will override the explicit -fno-omit-frame-pointer.
So f1 will be compiled without a frame pointer but f2 will be compiled
with one.

Is this really what we want?  If so, I think it's subtle enough to be
worth documenting.  It certainly isn't what I was expecting after
reading the current documentation.

Richard