On Thu, Aug 07, 2014 at 09:53:13AM +0200, Richard Biener wrote:
> On Wed, 6 Aug 2014, Mike Stump wrote:
>
> > On Aug 6, 2014, at 12:56 AM, Richard Biener <rguent...@suse.de> wrote:
> > >> My concern is the code we're removing discusses the need to simplify when
> > >> these expressions are in static initializers. What's going to ensure
> > >> that
> > >> we're still simplifying instances which appear in static initializers? I
> > >> don't see anything which tests that. And does it still work for targets
> > >> which utilize PSImode?
> > >
> > > You mean stuff like
> > >
> > > int i[4];
> > > const int p = i - (i + 2);
> >
> > No.
> >
> > long p = i - (i + 2);
> >
> > in C++. It is valid and so it has to work. Further, there is a well known
> > problem that if you get all the constants correct, you get to statically
> > initialize this and have no constructors. Though the above is trivial, in
> > large C++ source bases, there is an important set of things that one must
> > do to hit expected quality. Fold, fold, fold is the name of the game, and
> > if you do it well, you get app startup time that is short, and if you don’t
> > do it well, you get app startup time that sucks. Some folks care about
> > startup time. So for example, there are well known pdf viewers that use
> > gcc to compile and whose startup time would be a order of magnitude or more
> > worse if gcc didn’t optimize it as well as it does. There are large OS
> > vendors that had:
>
> It always works - you eventually get a runtime constructor though.
>
> > -Wglobal-constructors
> > Warn about namespace scope data that requires construction or
> > destruction, or functions that use the constructor
> > attribute or the destructor attribute. Additionally warn if the
> > Objective-C GNU runtime is used to initialize
> > various metadata.
> >
> > and used it to ensure (-Werror) that no constructors would sneak in
> > anyplace, as the corporate guide was no, you don’t get to do that.
> > Exceptions are made and really, the main thing that hurt the most were
> > libraries that users might use, but you get the idea.
> >
> > The right way to think of the cost was page faults proportional to the size
> > of the app, or, 0. Pages faults turn out to be rather expensive as the
> > core speed of the processor is rather fast. I know sounds overly
> > expensive, but that is my experience with Ado.., oops, I mean, with one
> > critical customer.
> >
> > Now, what do you have to do to get it right, evaluate all constant things
> > including constructors and destructors and be able to do this for a class
> > hierarchy. We didn’t do if and while, but it would have been nice to do
> > those. Things like:
> >
> > class B {
> > int k;
> > B(int i) {
> > k = i;
> > }
> >
> > class A :public B {
> > int i;
> > int j;
> > A () :i (1) b(5) {
> > j = 5;
> > }
> > } a;
> >
> > not that demanding… Note, I tried the code at the top, and gcc generate
> > runtime code for it. :-( Ouch. clang does this:
> >
> > _p:
> > .quad -2 ## 0xfffffffffffffffe
> >
> > Ah. I’d like to think that used to work at one point.
>
> Probably not ;)
>
> But seriously it's a matter of a) keeping constructors for each
> variable separate and optimize them in GIMPLE, b) recognize
> resulting code that can be expressed with a initializer
>
> a) can be done with delaying inlining to IPA time and hoping
> that early opts do enough to perform the necessary simplification,
> b) needs a new pass (but I guess it shouldn't be _that_ hard...)
I'm not sure totally skipping inlining in C++ is that great an idea
since most C++ ctors will end up calling things. Isn't it enough to not
inline the initializer for a variable into the file level function that
calls all the initializers, which you could do by internally apply
attribute noinline I guess.
> I think we already have constructor functions marked specially,
> hopefully with a link to the initialized variable.
I'd expect that's what DECL_STATIC_CONSTRUCTOR does, but I'm not sure
there's a link or where we could put one if there isn't.
Trev
>
> Richard.
