linux-kernel - Re: [PATCH] tracing: silence GCC 9 array bounds warning

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Message-ID: <CANiq72=hUULnd_oDoGoD2gjE-QvL2Kw2n7hMxke+gkS2_gzCqw@mail.gmail.com>
Date:   Fri, 17 May 2019 21:09:21 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: silence GCC 9 array bounds warning

On Fri, May 17, 2019 at 7:59 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, May 17, 2019 at 2:25 AM Miguel Ojeda
> <miguel.ojeda.sandonis@...il.com> wrote:
> >
> > +       memset((char *)(iter) + offsetof(struct trace_iterator, seq), 0,
> > +              sizeof(struct trace_iterator) -
> > +              offsetof(struct trace_iterator, seq));
>
> Honestly, the above is nasty.
>
> Whenever you have to split an expression or statement over several
> lines, you should ask yourself why it's so complicated.

Will do -- I was trying to keep the code as closely to the original as
possible (I simply replaced the &iter.seq expression :-)

By the way, how do you all feel about moving this as a generic
facility to zero out the suffix/prefix of an structure? In particular,
since we won't have the LAT* stuff according to Steven.

> Also, the while 'offset' is a variable, any compiler will immediately
> see that it's a constant value, so it's not like this will affect the
> generated code at all.

I like C++'s constexpr (for variable defs), maybe one day we will get
it on C; it is useful to cleanly annotate compile-time values like
this.

> Unless you compile with something crazy like
> '-O0', which is not a supported configuration exactly because we
> expect compilers to not be terminally stupid.

Fun fact: it seems clang folds some of these even under -O0. In
godbolt I see it folding the third argument completely. The first one
isn't, but it is computed on the function prologue, leaving the
'offset' variable unused.

Cheers,
Miguel