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

[Arvind Sankar <nivedita@alum.mit.edu>]

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@zytor.com> 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?