On Fri, Jan 2, 2009 at 4:05 AM, Richard Guenther <richard.guent...@gmail.com> wrote: > On Fri, Jan 2, 2009 at 12:36 PM, Richard Sandiford > <rdsandif...@googlemail.com> wrote: >> 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. > > IMHO this is a bug. And IMHO this optimize attribute went way over what was > reasonable to implement as a start - leading to all of these > interesting problems. >
Also see: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37565 -- H.J.