Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

[Arvind Sankar]

On Tue, Aug 18, 2020 at 01:58:51PM -0700, Nick Desaulniers wrote:
> On Tue, Aug 18, 2020 at 1:27 PM Nick Desaulniers
> <ndesaulniers@xxxxxxxxxx> wrote:
> >
> > On Tue, Aug 18, 2020 at 1:24 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Aug 18, 2020 at 12:13:22PM -0700, Linus Torvalds wrote:
> > > > On Tue, Aug 18, 2020 at 12:03 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> > > > >
> > > > > I'm not saying "change the semantics", nor am I saying that playing
> > > > > whack-a-mole *for a limited time* is unreasonable. But I would like to go back
> > > > > to the compiler authors and get them to implement such a #pragma: "this
> > > > > freestanding implementation *does* support *this specific library function*,
> > > > > and you are free to call it."
> > > >
> > > > I'd much rather just see the library functions as builtins that always
> > > > do the right thing (with the fallback being "just call the standard
> > > > function").
> > > >
> > > > IOW, there's nothing wrong with -ffreestanding if you then also have
> > > > __builtin_memcpy() etc, and they do the sane compiler optimizations
> > > > for memcpy().
> > > >
> > > > What we want to avoid is the compiler making *assumptions* based on
> > > > standard names, because we may implement some of those things
> > > > differently.
> > > >
> > >
> > > -ffreestanding as it stands today does have __builtin_memcpy and
> > > friends. But you need to then use #define memcpy __builtin_memcpy etc,
> > > which is messy and also doesn't fully express what you want. #pragma, or
> > > even just allowing -fbuiltin-foo options would be useful.
> 
> I do really like the idea of -fbuiltin-foo.  For example, you'd specify:
> 
> -ffreestanding -fbuiltin-bcmp
> 
> as an example. `-ffreestanding` would opt you out of ALL libcall
> optimizations, `-fbuiltin-bcmp` would then opt you back in to
> transforms that produce bcmp.  That way you're informing the compiler
> more precisely about the environment you'd be targeting.  It feels
> symmetric to existing `-fno-` flags (clang makes -f vs -fno- pretty
> easy when there is such symmetry).  And it's already convention that
> if you specify multiple conflicting compiler flags, then the latter
> one specified "wins."  In that sense, turning back on specific
> libcalls after disabling the rest looks more ergonomic to me.
> 
> Maybe Eli or David have thoughts on why that may or may not be as
> ergonomic or possible to implement as I imagine?
> 

Note that -fno-builtin-foo seems to mean slightly different things in
clang and gcc. From experimentation, clang will neither optimize a call
to foo, nor perform an optimization that introduces a call to foo. gcc
will avoid optimizing calls to foo, but it can still generate new calls
to foo while optimizing something else. Which means that
-fno-builtin-{bcmp,stpcpy} only solves things for clang, not gcc. It's
just that gcc doesn't seem to have implemented those optimizations.