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

[Nick Desaulniers]

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.
>
> The two compilers have some peculiarities, which means you really can't
> have functions with the same name that do something else if you want to
> use builtins at all, and can also lead to missed optimizations.
>
> For eg, __builtin_strchr(s,'\0') can be optimized to strlen. gcc will
> optimize it that way even if -ffreestanding is used (so strlen has to
> mean strlen), while clang won't, so it misses a potential optimization.
> This is admittedly a silly example, but you could imagine something like
> strncpy being optimized to memcpy+memset if the source length was
> previously computed.
>
> PS: clang optimizes sprintf, but doesn't provide __builtin_sprintf?

https://bugs.llvm.org/show_bug.cgi?id=47224
-- 
Thanks,
~Nick Desaulniers