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

[Nick Desaulniers]

On Mon, Aug 24, 2020 at 10:34 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> On Tue, Aug 25, 2020 at 12:57:22AM +0900, Masahiro Yamada wrote:
> >
> >
> > To prevent transformation from foo() into bar(),
> > there are two ways in Clang to do that;
> > -fno-builtin-foo, and -fno-builtin-bar.
> > There is only one in GCC; -fno-buitin-foo.
> >
> > Is this correct?
> >
>
> It looked that way from previous experimentation, but...
>
> >
> >
> > I just played the optimization
> > from printf("helloworld\n") to puts("helloworld").
> >
> > https://godbolt.org/z/5s4ded
> >
> >
> > -fno-builtin-puts cannot prevent clang
> > from emitting puts.
> > Is it because clang does not support
> > -fno-builtin-puts?
>
> Ugh. clang doesn't have __builtin_puts() but it optimizes printf() into
> puts(). It doesn't have __builtin_putchar() but will optimize
> printf("c") into putchar('c').

Bah, merely a <strikethrough>flesh
wound</strikethrough><strikethrough>compiler bug</strikethrough>rather
long TODO in the compiler.
https://github.com/llvm/llvm-project/blob/be2bc7d4cef2edd66c7fb74b70adf62fc68754db/clang/include/clang/Basic/Builtins.def#L943

Anyways, give me a week and I'll hack through the rest of them
https://reviews.llvm.org/D86508.  Certainly made HPA's point hit home,
that's a lot of functionality to implement or disable in an
environment.

Masahiro, are you implying that we shouldn't take the
-fno-builtin-stpcpy patch, because Clang is inconsistent? (That can be
fixed.) Even though -fno-builtin-stpcpy works here as intended?
https://lore.kernel.org/lkml/20200817220212.338670-2-ndesaulniers@xxxxxxxxxx/

Otherwise we need to provide an implementation of this symbol in the kernel.
https://lore.kernel.org/lkml/20200815020946.1538085-1-ndesaulniers@xxxxxxxxxx/

Please, pick your poison.
-- 
Thanks,
~Nick Desaulniers