This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: -fwhopr and -flto options reorg

> > We have --param lto-partitions parameter for scalability.
> > -flto-partition is used to specify either balanced (the default user wants to
> > keep all the time), 1to1 for testsuite testing and none for compiling everything
> > as one partition (and consequently disabling WHOPR, too)
> Sounds like they should be switched? Internal options --> param, and
> useful scalability parameter --> -f option ?

Hmm, both are bit useless to be tampered with the user.  Our default of 32
seems to be high enough, since WPA stage then grows to be majority of
copmilation time (we stream way too many types and decls and do it way to

But making the parallelizm more accessible doesn't sound like too bad idea,
at least once we fix WPA (that will probably happen after 4.6 is out)

I don't think we have infrastructure for named --param parameters, so it won't be able to accept
> >
> >>
> >> >>
> >> >> Does -flto imply -fwpa by default or not?
> >> > -fwpa and -fltrans are used internaly to pass down the stage of WHOPR and -flto is WHOPR now
> >> > unless you specify -flto-partition=none, so yes.
> >>
> >> Why using partition option to control wpa or not? Is it better to use -fno-wpa?
> >
> > WPA is one of stages of WHOPR and we concluded that user wants to know nothing
> > about that. ?So -fno-wpa to turn off whole WHOPR does not sound any more
> > intuitive to users than -flto-partition=none.
> > -fltrans and -fwpa are not user visible in any way since they are accepted only by
> > lto1 frontend and supplied by plugin or collect2.
> Ideally, WPA should be detected -- not asserted. For instance, if any
> function referenced in the callgraph for the main program does not
> have a IL (byte code) form, and that function is not one of the
> standard library functions,  wpa is then off.  In such cases, there

I think you make wrong assumption that WPA imply whole program assumptions. It does not.
This is reason why I don't think we should make WHOPR to be user visible as it is too
easilly confueed with that.
WPA is just about parallelizm of link time process - it will work in both cases.

-fwhole-program makes whole program assumptions and so does -fuse-linker-plugin
where supported. I plan to make -fuse-linker-plugin default as incremental change.

Users needs to know about both lto and whole-program, unfortunately.
LLVM for example defaults to -fwhole-program when it finds main() function.

> are still some limited full program analysis that can be done with
> linker feedback -- such as global variable address exposure analysis
> -- also applies to shared library build for hidden symbols etc.
> If partition=none means all files are LTOed together in memory, I fail
> to understand why it implies wpa=off.
> Another question, what is the default for parallelism? jobserv/auto?
> or 1? For auto, is it the number of cores available?

1 for the moment.  I plan to switch it to jobserv when environemnt var is found.
Autodetecting number of cores when jobserv is not around seems sane too. We can
call it auto.

> Thanks,
> David
> >
> > Honza
> >>
> >> Thanks,
> >>
> >> David
> >>
> >> >
> >> > Honza
> >> >
> >

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]