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

[Nick Desaulniers]

On Tue, Aug 25, 2020 at 12:10 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> 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/

Sorry, the above link ^ should be this hunk (beyond tired, getting up
in 4.5hrs for plumbers):

diff --git a/Makefile b/Makefile
index c4470a4e131f..6a08cdfa58ae 100644
--- a/Makefile
+++ b/Makefile
@@ -577,6 +577,7 @@ ifneq ($(LLVM_IAS),1)
 CLANG_FLAGS    += -no-integrated-as
 endif
 CLANG_FLAGS    += -Werror=unknown-warning-option
+CLANG_FLAGS    += -fno-builtin-stpcpy
 KBUILD_CFLAGS  += $(CLANG_FLAGS)
 KBUILD_AFLAGS  += $(CLANG_FLAGS)
 export CLANG_FLAGS

>
> 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



-- 
Thanks,
~Nick Desaulniers