Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

[Nick Desaulniers]

On Tue, Oct 1, 2019 at 2:26 PM Russell King - ARM Linux admin
<linux@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Oct 01, 2019 at 09:59:38PM +0100, Russell King - ARM Linux admin wrote:
> > On Tue, Oct 01, 2019 at 01:21:44PM -0700, Nick Desaulniers wrote:
> > > On Tue, Oct 1, 2019 at 11:14 AM Russell King - ARM Linux admin
> > > <linux@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > The whole "let's make inline not really mean inline" is nothing more
> > > > than a band-aid to the overuse (and abuse) of "inline".
> > >
> > > Let's triple check the ISO C11 draft spec just to be sure:
> > > Â 6.7.4.6: A function declared with an inline function specifier is an
> > > inline function. Making a
> > > function an inline function suggests that calls to the function be as
> > > fast as possible.
> > > The extent to which such suggestions are effective is
> > > implementation-defined. 139)
> > > 139) For example, an implementation might never perform inline
> > > substitution, or might only perform inline
> > > substitutions to calls in the scope of an inline declaration.
> > > Â J.3.8 [Undefined Behavior] Hints: The extent to which suggestions
> > > made by using the inline function specifier are effective (6.7.4).
> > >
> > > My translation:
> > > "Please don't assume inline means anything."
> > >
> > > For the unspecified GNU C extension __attribute__((always_inline)), it
> > > seems to me like it's meant more for performing inlining (an
> > > optimization) at -O0.  Whether the compiler warns or not seems like a
> > > nice side effect, but provides no strong guarantee otherwise.
> > >
> > > I'm sorry that so much code may have been written with that
> > > assumption, and I'm sorry to be the bearer of bad news, but this isn't
> > > a recent change.  If code was written under false assumptions, it
> > > should be rewritten. Sorry.
> >
> > You may quote C11, but that is not relevent.  The kernel is coded to
> > gnu89 standard - see the -std=gnu89 flag.
>
> There's more to this and why C11 is entirely irrelevant.  The "inline"
> you see in our headers is not the compiler keyword that you find in
> various C standards, it is a macro that gets expanded to either:
>
> #define inline inline __attribute__((__always_inline__)) __gnu_inline \
>         __maybe_unused notrace
>
> or
>
> #define inline inline                                    __gnu_inline \
>         __maybe_unused notrace
>
> __gnu_inline is defined as:
>
> #define __gnu_inline                    __attribute__((__gnu_inline__))
>
> So this attaches the gnu_inline attribute to the function:
>
> `gnu_inline'
>      This attribute should be used with a function that is also declared
>      with the `inline' keyword.  It directs GCC to treat the function
>      as if it were defined in gnu90 mode even when compiling in C99 or
>      gnu99 mode.
> ...
>      Since ISO C99 specifies a different semantics for `inline', this
>      function attribute is provided as a transition measure and as a
>      useful feature in its own right.  This attribute is available in
>      GCC 4.1.3 and later.  It is available if either of the
>      preprocessor macros `__GNUC_GNU_INLINE__' or
>      `__GNUC_STDC_INLINE__' are defined.  *Note An Inline Function is
>      As Fast As a Macro: Inline.
>
> which is quite clear that C99 semantics do not apply to _this_ inline.
> The manual goes on to explain:
>
>  GCC implements three different semantics of declaring a function
> inline.  One is available with `-std=gnu89' or `-fgnu89-inline' or when
> `gnu_inline' attribute is present on all inline declarations, another
> when `-std=c99', `-std=c11', `-std=gnu99' or `-std=gnu11' (without
> `-fgnu89-inline'), and the third is used when compiling C++.

(I wrote the kernel patch for gnu_inline; it only comes into play when
`inline` appears on a function *also defined as `extern`*).

>
> I'd suggest gnu90 mode is pretty similar to gnu89 mode, and as we build
> the kernel in gnu89 mode, that is the inlining definition that is
> appropriate.
>
> When it comes to __always_inline, the GCC manual is the definitive
> reference, since we use the GCC attribute for that:
>
> #define __always_inline                 inline __attribute__((__always_inline__))
>
> and I've already quoted what the GCC manual says for always_inline.
>
> Arguing about what the C11 spec says about inlining when we aren't
> using C11 dialect in the kernel, but are using GCC features, does
> not move the discussion on.
>
> Thanks anyway, maybe it will become relevent in the future if we
> decide to move to C11.

It's not like the semantics of inline are better specified by an older
standard, or changed (The only real semantic change involving `inline`
between ISO C90 and ISO C99 has to do with whether `extern inline`
emits the function with external linkage as you noted).  But that's
irrelevant to the discussion.).  I quoted C11 because ctrl+f doesn't
work for the C90 ISO spec pdf.  C90 spec doesn't even have a section
on Function Specifiers.  From what I can tell, `inline` wasn't
specified until ISO C99.

GNU modes are often modifiers off of ISO C bases; gnu89 corresponds to
ISO C90.  They may permit the use of features from newer ISO C specs
and GNU C extensions without warning under -Wpedantic.  There aren't a
whole lot of semantic differences, at least that I'm aware of.

Please don't assume inline means anything.
-- 
Thanks,
~Nick Desaulniers