Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

[Nick Desaulniers]

On Tue, Oct 1, 2019 at 2:28 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> Hi Nick,
>
> On Mon, Sep 30, 2019 at 02:50:10PM -0700, Nick Desaulniers wrote:
> > So __attribute__((always_inline)) doesn't guarantee that code will be
> > inlined.  For instance in LLVM's inliner, it asks/answers "should I
> > inline" and "can I inline."  "Should" has to do with a cost model, and
> > is very heuristic-y.  "Can" has more to do with the transforms, and
> > whether they're all implemented and safe.  If you if you say
> > __attribute__((always_inline)), the answer to "can I inline this" can
> > still be *no*.  The only way to guarantee inlining is via the C
> > preprocessor.  The only way to prevent inlining is via
> > __attribute__((no_inline)).  inline and __attribute__((always_inline))
> > are a heuristic laden mess and should not be relied upon.  I would
> > also look closely at code that *requires* inlining or the lack there
> > of to be correct.  That the kernel no longer compiles at -O0 is not a
> > good thing IMO, and hurts developers that want a short
> > compile/execute/debug cycle.
> >
> > In this case, if there's a known codegen bug in a particular compiler
> > or certain versions of it, I recommend the use of either the C
> > preprocessor or __attribute__((no_inline)) to get the desired behavior
> > localized to the function in question, and for us to proceed with
> > Masahiro's cleanup.
>
> Hmm, I don't see how that would help. The problem occurs when things
> are moved out of line by the compiler (see below).

It's being moved out of line because __attribute__((always_inline)) or
just inline provide no guarantees that outlining does not occur.  It
would help to make functions that need to be inlined macros, because
the C preprocessor doesn't have that issue.

>
> > The comment above the use of CONFIG_OPTIMIZE_INLINING in
> > include/linux/compiler_types.h says:
> >   * Force always-inline if the user requests it so via the .config.
> > Which makes me grimace (__attribute__((always_inline)) doesn't *force*
> > anything as per above), and the idea that forcing things marked inline
> > to also be __attribute__((always_inline)) is an "optimization" (re:
> > the name of the config; CONFIG_OPTIMIZE_INLINING) is also highly
> > suspect.  Aggressive inlining leads to image size bloat, instruction
> > cache and register pressure; it is not exclusively an optimization.
>
> Agreed on all of this, but the fact remains that GCC has been shown to
> *miscompile* the arm64 kernel with CONFIG_OPTIMIZE_INLINING=y. Please,
> look at this thread:
>
>         https://www.spinics.net/lists/arm-kernel/msg730329.html
>         https://www.spinics.net/lists/arm-kernel/msg730512.html
>
> GCC decides to pull an atomic operation out-of-line and, in doing so,

If the function is incorrect unless inlined, use a macro.

> gets the register allocations subtly wrong when passing a 'register'
> variable into an inline asm. I would like to avoid this sort of thing
> happening, since it can result in really nasty bugs that manifest at
> runtime and are extremely difficult to debug, which is why I would much
> prefer not to have this option on by default for arm64. I sent a patch
> already:
>
>         https://lkml.kernel.org/r/20190930114540.27498-1-will@xxxxxxxxxx
>
> and I'm happy to spin a v2 which depends on !CC_IS_CLANG as well.

For small things like whether we mark a function always_inline or not,
I think it's simpler to just keep the code consistent between
compilers, even if it's to work around a bug in one compiler.  A
comment in the code would be sufficient.

>
> Reducing the instruction cache footprint is great, but not if the
> resulting code is broken!

You don't have to convince compiler folks about correctness. ;)
Correctness trumps all, especially performance.
-- 
Thanks,
~Nick Desaulniers