Am Montag, den 16.11.2020, 12:10 +0100 schrieb Peter Zijlstra: > ( restoring at least linux-toolcha...@vger.kernel.org, since that > seems > to have gone missing ) > > On Mon, Nov 16, 2020 at 10:11:50AM +0100, Richard Biener wrote: > > On Sun, Nov 15, 2020 at 11:53 AM Uecker, Martin > > <martin.uec...@med.uni-goettingen.de> wrote: > > > > On Wed, Nov 04, 2020 at 07:31:42PM +0100, Uros Bizjak wrote: > > > > > Hello! > > > > > > > > > > I was looking at the recent linux patch series [1] where > > > > > segment > > > > > qualifiers (named address spaces) were introduced to handle > > > > > percpu > > > > > variables. In the patch [2], the author mentions that: > > > > > > > > > > --q-- > > > > > Unfortunately, gcc does not provide a way to remove segment > > > > > qualifiers, which is needed to use typeof() to create local > > > > > instances > > > > > of the per-cpu variable. For this reason, do not use the > > > > > segment > > > > > qualifier for per-cpu variables, and do casting using the > > > > > segment > > > > > qualifier instead. > > > > > --/q-- > > > > > > > > C in general does not provide means to strip qualifiers. We > > > > recently had > > > > a _lot_ of 'fun' trying to strip volatile from a type, see > > > > here: > > > > > > > > > > > > https://lore.kernel.org/lkml/875zimp0ay....@mpe.ellerman.id.au > > > > > > > > which resulted in the current __unqual_scalar_typeof() hack. > > > > > > > > If we're going to do compiler extentions here, can we pretty > > > > please have > > > > a sane means of modifying qualifiers in general? > > > > > > Another way to drop qualifiers is using a cast. So you > > > can use typeof twice: > > > > > > typeof((typeof(_var))_var) tmp__; > > > > > > This also works for non-scalars but this is a GCC extension.
(That casts drop qualifiers is standard C. The extensions are 'typeof' and that casts can be used for non-scalar types.) > > > > > > WG14 plans to standardize typeof. I would like to hear opinion > > > whether we should have typeof drop qualifiers or not. > > > > > > Currently, it does not do this on all compilers I tested > > > (except _Atomic on GCC) and there are also use cases for > > > keeping qualifiers. This is an argument for keeping qualifiers > > > should we standardize it, but then we need a way to drop > > > qualifiers. > > > > > > > > > lvalue conversion drops qualifers in C. In GCC, this is not > > > implemented correctly as it is unobvervable in standard C > > > (but it using typeof). > > > > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97702 > > > > > > A have a working patch in preparation to change this. Then you > > > could use > > > > > > typeof( ((void)0, x) ) > > Neat, that actually already works with clang. And I suppose we can > use > the above GCC extention until such time as that GCC is fixed. > > See below.. > > > > to drop qualifiers. But this would then > > > also do array-to-pointer conversion. I am not sure > > > whether this is a problem. > > I don't _think_ so, but.. > > > > Of course, we could also introduce a new feature for > > > dropping qualifiers. Thoughts? > > Just add a new qualifier that un-qualifies? > > > > _Unqual volatile T x; > > > > is T with volatile (evenually) removed. Or just a way to drop > > all using _Unqual? > > > > _Unqual T x; > > > > removing all qualifiers from T. Or add a special _Unqual_all > > to achieve that. I think removing a specific qualification is > > useful. Leaves cases like > > > > _Unqual volatile volatile T x; > > > > to be specified (that is ordering and cancellation of the > > unqual and qual variants of qualifiers). > > I rather like this, however I think I'd prefer the syntax be > something > like: > > _Unqual T x; > > for removing all qualifiers, and: > > _Unqual(volatile) volatile T X; > > for stripping specific qualifiers. The syntax as proposed above seems > very error prone to me. There is also the idea of adding 'auto' to C which would drop qualifiers. (__auto_type does not on GCC but on some other compilers) Best, Martin > > > --- > Subject: compiler: Improve __unqual_typeof() > > Improve our __unqual_scalar_typeof() implementation by relying on C > dropping qualifiers for lvalue convesions. There is one small catch > in > that GCC is currently known broken in this respect, however it > happens > to have a C language extention that achieves the very same, it drops > qualifiers on casts. > > This gets rid of the _Generic() usage and should improve compile > times > (less preprocessor output) as well as increases the capabilities of > the > macros. > > XXX: I've only verified the below actually compiles, I've not > verified > the generated code is actually 'correct'. > > Suggested-by: "Uecker, Martin" <martin.uec...@med.uni-goettingen.de> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > --- > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler- > gcc.h > index 74c6c0486eed..3c5cb52c12f9 100644 > --- a/include/linux/compiler-gcc.h > +++ b/include/linux/compiler-gcc.h > @@ -156,3 +156,11 @@ > #else > #define __diag_GCC_8(s) > #endif > + > +/* > + * GCC has a bug where lvalue conversion doesn't drop qualifiers, > use a GCC > + * extention instead. GCC drops qualifiers on a cast, so use a > double typeof(). > + * > + * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97702 > + */ > +#define __unqual_typeof(type) typeof( (typeof(type))type ) > diff --git a/include/linux/compiler_types.h > b/include/linux/compiler_types.h > index ac3fa37a84f9..4a6e2caab17b 100644 > --- a/include/linux/compiler_types.h > +++ b/include/linux/compiler_types.h > @@ -250,27 +250,14 @@ struct ftrace_likely_data { > /* Are two types/vars the same type (ignoring qualifiers)? */ > #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), > typeof(b)) > > +#ifndef __unqual_typeof > /* > - * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, > leaving > - * non-scalar types unchanged. > + * lvalue conversion drops qualifiers > */ > -/* > - * Prefer C11 _Generic for better compile-times and simpler code. > Note: 'char' > - * is not type-compatible with 'signed char', and we define a > separate case. > - */ > -#define __scalar_type_to_expr_cases(type) > \ > - unsigned type: (unsigned type)0, > \ > - signed type: (signed type)0 > - > -#define __unqual_scalar_typeof(x) typeof( > \ > - _Generic((x), > \ > - char: (char)0, > \ > - __scalar_type_to_expr_cases(char), > \ > - __scalar_type_to_expr_cases(short), > \ > - __scalar_type_to_expr_cases(int), > \ > - __scalar_type_to_expr_cases(long), > \ > - __scalar_type_to_expr_cases(long long), \ > - default: (x))) > +#define __unqual_typeof(type) typeof( ((void)0, type) ) > +#endif > + > +#define __unqual_scalar_typeof(type) __unqual_typeof(type) > > /* Is this type a native word size -- useful for atomic operations > */ > #define __native_word(t) \
