[PATCH] compiler.h: update definition of unreachable()

[PATCH] compiler.h: update definition of unreachable()

[ndesaulniers]

Fixes the objtool warning seen with Clang:
arch/x86/mm/fault.o: warning: objtool: no_context()+0x220: unreachable
instruction

Fixes commit 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h
mutually exclusive")

Josh noted that the fallback definition was meant to work around a
pre-gcc-4.6 bug. GCC still needs to work around
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365, so compiler-gcc.h
defines its own version of unreachable().  Clang and ICC can use this
shared definition.

Link: https://github.com/ClangBuiltLinux/linux/issues/204
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Miguel, would you mind taking this up in your new compiler attributes
tree?

 include/linux/compiler.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 681d866efb1e..8875fd3243fd 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -124,7 +124,10 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define ASM_UNREACHABLE
 #endif
 #ifndef unreachable
-# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
+# define unreachable() do {		\
+	annotate_unreachable();		\
+	__builtin_unreachable();	\
+} while (0)
 #endif
 
 /*
-- 
2.19.0.605.g01d371f741-goog

Re: [PATCH] compiler.h: update definition of unreachable()

[Miguel Ojeda]

On Mon, Oct 15, 2018 at 7:22 PM <ndesaulniers@google.com> wrote:
>
> Fixes the objtool warning seen with Clang:
> arch/x86/mm/fault.o: warning: objtool: no_context()+0x220: unreachable
> instruction
>
> Fixes commit 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h
> mutually exclusive")
>
> Josh noted that the fallback definition was meant to work around a
> pre-gcc-4.6 bug. GCC still needs to work around
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365, so compiler-gcc.h
> defines its own version of unreachable().  Clang and ICC can use this
> shared definition.

Could we, at the same time, update the comment on compiler-gcc.h as
well? i.e. remove the 4.5 comment, add the link to the GCC PR.

>
> Link: https://github.com/ClangBuiltLinux/linux/issues/204
> Suggested-by: Andy Lutomirski <luto@amacapital.net>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Miguel, would you mind taking this up in your new compiler attributes
> tree?

Sure, will do.

Thanks,

Cheers,
Miguel

Re: [PATCH] compiler.h: update definition of unreachable()

[Nick Desaulniers]

On Mon, Oct 15, 2018 at 11:21 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Mon, Oct 15, 2018 at 7:22 PM <ndesaulniers@google.com> wrote:
> >
> > Fixes the objtool warning seen with Clang:
> > arch/x86/mm/fault.o: warning: objtool: no_context()+0x220: unreachable
> > instruction
> >
> > Fixes commit 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h
> > mutually exclusive")
> >
> > Josh noted that the fallback definition was meant to work around a
> > pre-gcc-4.6 bug. GCC still needs to work around
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365, so compiler-gcc.h
> > defines its own version of unreachable().  Clang and ICC can use this
> > shared definition.
>
> Could we, at the same time, update the comment on compiler-gcc.h as
> well? i.e. remove the 4.5 comment, add the link to the GCC PR.

Looking at commit cb984d101b30 ("compiler-gcc: integrate the various
compiler-gcc[345].h files") it seems that the comment is referring to
gcc 4.4 not supporting __builtin_unreachable().  Looks like it was
added in 4.5 timeframe: https://godbolt.org/z/ugv5QO, so the comment
can be deleted.  Would you prefer a v2 (single patch), or an
additional patch on top?

>
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/204
> > Suggested-by: Andy Lutomirski <luto@amacapital.net>
> > Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> > Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > ---
> > Miguel, would you mind taking this up in your new compiler attributes
> > tree?
>
> Sure, will do.
>
> Thanks,
>
> Cheers,
> Miguel



-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] compiler.h: update definition of unreachable()

[Miguel Ojeda]

On Mon, Oct 15, 2018 at 10:13 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> Looking at commit cb984d101b30 ("compiler-gcc: integrate the various
> compiler-gcc[345].h files") it seems that the comment is referring to
> gcc 4.4 not supporting __builtin_unreachable().  Looks like it was
> added in 4.5 timeframe: https://godbolt.org/z/ugv5QO, so the comment
> can be deleted.  Would you prefer a v2 (single patch), or an
> additional patch on top?

Split would be better, I think, since the changes can be applied
independetly even if related. If you are busy, I can do it myself, so
don't worry too much. :)

Cheers,
Miguel