linux-kernel - Re: [PATCH] tracing: Fix clang -Wint-in-bool-context warnings in IF

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Date:   Thu, 26 Sep 2019 01:41:12 +0200
From:   Dávid Bolvanský <david.bolvansky@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] tracing: Fix clang -Wint-in-bool-context warnings in
 IF_ASSIGN macro

GCC C frontend does not warn, GCC C++ FE does. https://godbolt.org/z/_sczyM

So I (we?) think there is something weird in gcc frontends.

>> I can't think of a case that this warning is a bug (maybe David can
explain more),

In this case or generally? General bug example:

if (state == A || B)

(should be if (state == A || state == B))

Since this is just one occurrence and I recommend to just land this small fix.


st 25. 9. 2019 o 19:41 Nick Desaulniers <ndesaulniers@...gle.com> napísal(a):
>
> On Wed, Sep 25, 2019 at 10:29 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > After r372664 in clang, the IF_ASSIGN macro causes a couple hundred
> > warnings along the lines of:
> >
> > kernel/trace/trace_output.c:1331:2: warning: converting the enum
> > constant to a boolean [-Wint-in-bool-context]
> > kernel/trace/trace.h:409:3: note: expanded from macro
> > 'trace_assign_type'
> >                 IF_ASSIGN(var, ent, struct ftrace_graph_ret_entry,
> >                 ^
> > kernel/trace/trace.h:371:14: note: expanded from macro 'IF_ASSIGN'
> >                 WARN_ON(id && (entry)->type != id);     \
> >                            ^
> > 264 warnings generated.
> >
> > Add the implicit '!= 0' to the WARN_ON statement to fix the warnings.
> >
> > Link: https://github.com/llvm/llvm-project/commit/28b38c277a2941e9e891b2db30652cfd962f070b
> > Link: https://github.com/ClangBuiltLinux/linux/issues/686
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
>
> I can't think of a case that this warning is a bug (maybe David can
> explain more), but seems like a small fix that can stop a big spew of
> warnings, and IIUC this is the lone instance we see in the kernel.  In
> that case, I prefer a tiny change to outright disabling the warning in
> case it does find interesting cases later.
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> > ---
> >  kernel/trace/trace.h | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> > index 26b0a08f3c7d..f801d154ff6a 100644
> > --- a/kernel/trace/trace.h
> > +++ b/kernel/trace/trace.h
> > @@ -365,11 +365,11 @@ static inline struct trace_array *top_trace_array(void)
> >         __builtin_types_compatible_p(typeof(var), type *)
> >
> >  #undef IF_ASSIGN
> > -#define IF_ASSIGN(var, entry, etype, id)               \
> > -       if (FTRACE_CMP_TYPE(var, etype)) {              \
> > -               var = (typeof(var))(entry);             \
> > -               WARN_ON(id && (entry)->type != id);     \
> > -               break;                                  \
> > +#define IF_ASSIGN(var, entry, etype, id)                       \
> > +       if (FTRACE_CMP_TYPE(var, etype)) {                      \
> > +               var = (typeof(var))(entry);                     \
> > +               WARN_ON(id != 0 && (entry)->type != id);        \
> > +               break;                                          \
> >         }
> >
> >  /* Will cause compile errors if type is not found. */
> > --
> > 2.23.0
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers