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: > > > § 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. > > 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
