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

[Nick Desaulniers]

On Tue, Aug 18, 2020 at 12:25 PM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Tue, Aug 18, 2020 at 12:19 PM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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.

That's asking for trouble; please don't implement routines with
identifiers from libc but with differing function signatures, and then
proceed to *not* use -ffreestanding.  You can't have it both ways
(optimizations from *not* using -ffreestanding, then breaking all
kinds of assumptions based on conventions used across userspace), at
least not with the tools you currently have.

> >
> > And honestly, a compiler that uses 'bcmp' is just broken. WTH? It's
> > the year 2020, we don't use bcmp. It's that simple. Fix your damn
> > broken compiler and use memcmp. The argument that memcmp is more
> > expensive than bcmp is garbage legacy thinking from four decades ago.
> >
> > It's likely the other way around, where people have actually spent
> > time on memcmp, but not on bcmp.
> >
> > If somebody really *wants* to use bcmp, give them the "Get off my
> > lawn" flag,

I wrote a paper in college on the philosophy and symbolism in "Gran
Torino."  Would recommend (the movie, not the paper).

> > and leave them alone. But never ever should "use bcmp" be
> > any kind of default behavior. That's some batshit crazy stuff.
> >
> >                Linus
>
> You'll have to ask Clement about that.  I'm not sure I ever saw the
> "faster bcmp than memcmp" implementation, but I was told "it exists"
> when I asked for a revert when all of our kernel builds went red.

Also, to Clement's credit, every patch I've ever seen from Clement is
backed up by data; typically fleetwide profiles at Google.  "we spend
a lot of time in memcmp, particularly comparing the result against
zero and no other value; hmm...how do we spend less time in
memcmp...oh, well there's another library function with slightly
different semantics we can call instead."  I don't think anyone would
consider the optimization batshit crazy given the number of cycles
saved across the fleet.  That an embedded project didn't provide an
implementation, is a footnote that can be fixed in the embedded
project, either by using -ffreestanding or -fno-builtin-bcmp, which is
what this series proposes to do.
-- 
Thanks,
~Nick Desaulniers