From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: "Jose E. Marchesi" <jose.marchesi@oracle.com>
Cc: bpf <bpf@vger.kernel.org>, David Faust <david.faust@oracle.com>,
Cupertino Miranda <cupertino.miranda@oracle.com>
Subject: Re: [PATCH bpf-next] bpf: make list_for_each_entry portable
Date: Fri, 10 May 2024 10:03:51 -0700 [thread overview]
Message-ID: <CAADnVQ+-w1cD83mE0u0QhCP4cCvSSB1-GNoqErYRRhtxcwkTmg@mail.gmail.com> (raw)
In-Reply-To: <874jb62ht9.fsf@oracle.com>
On Fri, May 10, 2024 at 1:27 AM Jose E. Marchesi
<jose.marchesi@oracle.com> wrote:
>
>
> > On Thu, May 9, 2024 at 1:47 AM Jose E. Marchesi
> > <jose.marchesi@oracle.com> wrote:
> >> +/* A `break' executed in the head of a `for' loop statement is bound
> >> + to the current loop in clang, but it is bound to the enclosing loop
> >> + in GCC. Note both compilers optimize the outer loop out with -O1
> >> + and higher. This macro shall be used to annotate any loop that
> >> + uses cond_break within its header. */
> >> +#ifdef __clang__
> >> +#define __compat_break
> >> +#else
> >> +#define __compat_break for (int __control = 1; __control; --__control)
> >> +#endif
> > ..
> >> + __compat_break
> >> for (i = zero; i < cnt; cond_break, i++) {
> >> struct elem __arena *n = bpf_alloc(sizeof(*n));
> >
> > This is too ugly. It ruins the readability of the code.
> > Let's introduce can_loop macro similar to cond_break
> > that returns 0 or 1 instead of break/continue and use it as:
> >
> > for (i = zero; i < cnt && can_loop; i++) {
> >
> > pw-bot: cr
>
> I went with the ugliness because I was trying to avoid rewriting the
> loops in the tests, assuming the tests were actually testing using
> cond_break in these particular locations would result in a particular
> number of iterations.
>
> The loops
>
> for (i = zero; i < cnt; cond_break, i++) BODY
>
> and
>
> for (i = zero; i < cnt && can_loop; i++) BODY
>
> are not equivalent if can_loop implements the same logic than
> cond_break.
It's off by one and it's fine.
The loops don't and shouldn't expect the precise number allowed
by may_goto.
btw there are tests that use cond_break inside {}.
They don't need to change.
>
> The may_goto instructions are somehow patched at run-time, and in a
> predictable way since the tests are checking for explicit iteration
> counts, right?
They're patched by the verifier, but they're unpredictable.
Right now it's a simpler counter, but sooner or later
it will be time based.
next prev parent reply other threads:[~2024-05-10 17:04 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-09 8:46 [PATCH bpf-next] bpf: make list_for_each_entry portable Jose E. Marchesi
2024-05-09 21:48 ` Alexei Starovoitov
2024-05-10 8:26 ` Jose E. Marchesi
2024-05-10 17:03 ` Alexei Starovoitov [this message]
2024-05-10 17:16 ` Jose E. Marchesi
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