Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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@armlinux.org.uk> wrote:
> >
> > On Tue, Oct 01, 2019 at 11:00:11AM -0700, Nick Desaulniers wrote:
> > > On Tue, Oct 1, 2019 at 10:55 AM Russell King - ARM Linux admin
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > On Tue, Oct 01, 2019 at 10:44:43AM -0700, Nick Desaulniers wrote:
> > > > > I apologize; I don't mean to be difficult.  I would just like to avoid
> > > > > surprises when code written with the assumption that it will be
> > > > > inlined is not.  It sounds like we found one issue in arm32 and one in
> > > > > arm64 related to outlining.  If we fix those two cases, I think we're
> > > > > close to proceeding with Masahiro's cleanup, which I view as a good
> > > > > thing for the health of the Linux kernel codebase.
> > > >
> > > > Except, using the C preprocessor for this turns the arm32 code into
> > > > yuck:
> > > >
> > > > 1. We'd need to turn get_domain() and set_domain() into multi-line
> > > >    preprocessor macro definitions, using the GCC ({ }) extension
> > > >    so that get_domain() can return a value.
> > > >
> > > > 2. uaccess_save_and_enable() and uaccess_restore() also need to
> > > >    become preprocessor macro definitions too.
> > > >
> > > > So, we end up with multiple levels of nested preprocessor macros.
> > > > When something goes wrong, the compiler warning/error message is
> > > > going to be utterly _horrid_.
> > >
> > > That's why I preferred V1 of Masahiro's patch, that fixed the inline
> > > asm not to make use of caller saved registers before calling a
> > > function that might not be inlined.
> >
> > ... which I objected to based on the fact that this uaccess stuff is
> > supposed to add protection against the kernel being fooled into
> > accessing userspace when it shouldn't.  The whole intention there is
> > that [sg]et_domain(), and uaccess_*() are _always_ inlined as close
> > as possible to the call site of the accessor touching userspace.
> 
> Then use the C preprocessor to force the inlining.  I'm sorry it's not
> as pretty as static inline functions.
> 
> >
> > Moving it before the assignments mean that the compiler is then free
> > to issue memory loads/stores to load up those registers, which is
> > exactly what we want to avoid.
> >
> >
> > In any case, I violently disagree with the idea that stuff we have
> > in header files should be permitted not to be inlined because we
> > have soo much that is marked inline.
> 
> So there's a very important subtly here.  There's:
> 1. code that adds `inline` cause "oh maybe it would be nice to inline
> this, but if it isn't no big deal"
> 2. code that if not inlined is somehow not correct.
> 3. avoid ODR violations via `static inline`
> 
> I'll posit that "we have soo much that is marked inline [is
> predominantly case 1 or 3, not case 2]."  Case 2 is a code smell, and
> requires extra scrutiny.
> 
> > Having it moved out of line,
> > and essentially the same function code appearing in multiple C files
> > is really not an improvement over the current situation with excessive
> > use of inlining.  Anyone who has looked at the code resulting from
> > dma_map_single() will know exactly what I'm talking about, which is
> > way in excess of the few instructions we have for the uaccess_* stuff
> > here.
> >
> > The right approach is to move stuff out of line - and by that, I
> > mean _actually_ move the damn code, so that different compilation
> > units can use the same instructions, and thereby gain from the
> > whole point of an instruction cache.
> 
> And be marked __attribute__((noinline)), otherwise might be inlined via LTO.
> 
> >
> > 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:
> § 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.
> § 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.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
According to speedtest.net: 11.9Mbps down 500kbps up