Re: git: 2508372b7b46 - main - cdefs.h: Assume the compiler supports at least GNU C 3.0 extensions

Hey John,

On Mon, Jul 1, 2024 at 3:49 PM John Baldwin <jhb@freebsd.org> wrote:

> On 6/20/24 7:41 PM, Warner Losh wrote:
> > The branch main has been updated by imp:
> >
> > URL:
> https://cgit.FreeBSD.org/src/commit/?id=2508372b7b46117a9fb801b50624265d30888442
> >
> > commit 2508372b7b46117a9fb801b50624265d30888442
>
...

> > -/*
> > - * GNU C version 2.96 adds explicit branch prediction so that
> > - * the CPU back-end can hint the processor and also so that
> > - * code blocks can be reordered such that the predicted path
> > - * sees a more linear flow, thus improving cache behavior, etc.
> > - *
> > - * The following two macros provide us with a way to utilize this
> > - * compiler feature.  Use __predict_true() if you expect the expression
> > - * to evaluate to true, and __predict_false() if you expect the
> > - * expression to evaluate to false.
> > - *
> > - * A few notes about usage:
> > - *
> > - *   * Generally, __predict_false() error condition checks (unless
> > - *     you have some _strong_ reason to do otherwise, in which case
> > - *     document it), and/or __predict_true() `no-error' condition
> > - *     checks, assuming you want to optimize for the no-error case.
> > - *
> > - *   * Other than that, if you don't know the likelihood of a test
> > - *     succeeding from empirical or other `hard' evidence, don't
> > - *     make predictions.
> > - *
> > - *   * These are meant to be used in places that are run `a lot'.
> > - *     It is wasteful to make predictions in code that is run
> > - *     seldomly (e.g. at subsystem initialization time) as the
> > - *     basic block reordering that this affects can often generate
> > - *     larger code.
> > - */
> > -#if __GNUC_PREREQ__(2, 96)
> >   #define     __predict_true(exp)     __builtin_expect((exp), 1)
> >   #define     __predict_false(exp)    __builtin_expect((exp), 0)
> > -#else
> > -#define      __predict_true(exp)     (exp)
> > -#define      __predict_false(exp)    (exp)
> > -#endif
>
> I think the comment was worth keeping around.  You just need to modify
> the start of it to "Modern compilers include explicit branch prediction..."
> In particular, I think our current practice is still to apply prediction
> hints rather conservatively.
>

That's a fair point I hadn't considered. https://reviews.freebsd.org/D45837
adds it back with some tiny tweaks to the language, justification, etc.

Warner