On Mon, Sep 13, 2021 at 1:50 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > On Mon, Sep 13, 2021 at 1:42 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Mon, Sep 13, 2021 at 1:16 PM Nick Desaulniers > > <ndesaulniers@xxxxxxxxxx> wrote: > > > > > > Do we have access to _Generic in GCC 4.9? > > > > We've ended up using it unconditionally since last year, so yes. > > Sorry, grepping would have taken < 1s. I'm very lazy. > http://threevirtues.com/ > > > > > In fact, the compiler version tests got removed when we raised the gcc > > version requirement to 4.9 in commit 6ec4476ac825 ("Raise gcc version > > requirement to 4.9"): > > > > "In particular, raising the minimum to 4.9 means that we can now just > > assume _Generic() exists, which is likely the much better replacement > > for a lot of very convoluted built-time magic with conditionals on > > sizeof and/or __builtin_choose_expr() with same_type() etc" > > > > but we haven't used it much since. > > > > The "seqprop" code for picking the right lock for seqlock is perhaps > > the main example, and staring at that code will make you go blind, so > > look away. > > Looking at my patch: > https://lore.kernel.org/stable/20210913203201.1844253-1-ndesaulniers@xxxxxxxxxx/ > I don't think _Generic helps us in the case of dispatching based on > the result of is_signed_type() (the operands could undergo type > promotion, so we'd need lots of cases that are more concisely covered > by is_signed_type()). It could replace the nested checks in div_64 > with nested _Generics, I think. Not sure it's a huge win for > readability. Maybe cut the number of expansions of the parameters in > half though. Let me give it a try just to see what it looks like. Is this more readable? Same line count. I'm not sure if there's such a thing as "fallthrough" between the "cases" of _Generic to minimize duplication, or whether this could be factored further. Needs lots more () around macro param use and tab'ed out line endings... diff --git a/include/linux/math64.h b/include/linux/math64.h index 8652a8a35d70..8fc4d56a45b9 100644 --- a/include/linux/math64.h +++ b/include/linux/math64.h @@ -162,17 +162,17 @@ static inline s64 div_s64(s64 dividend, s32 divisor) div_u64(dividend, divisor)); \ }) -#define __div_64(dividend) _Generic((divisor), \ - s64: div64_x64, \ - u64: div64_x64, \ - default: div_x64) +#define __div_64(dividend, divisor) _Generic((divisor), \ + s64: div64_x64(dividend, divisor), \ + u64: div64_x64(dividend, divisor), \ + default: div_x64(dividend, divisor)) #define div_64(dividend, divisor) ({ \ BUILD_BUG_ON_MSG(sizeof(dividend) > sizeof(u64), \ "128b div unsupported"); \ _Generic((dividend), \ - s64: __div_64(dividend)(dividend, divisor), \ - u64: __div_64(dividend)(dividend, divisor), \ + s64: __div_64(dividend, divisor), \ + u64: __div_64(dividend, divisor), \ default: dividend / divisor); \ }) -- Thanks, ~Nick Desaulniers
