On Mon, Nov 09, 2020 at 11:50:15AM -0800, Nick Desaulniers wrote: > On Mon, Nov 9, 2020 at 11:46 AM Segher Boessenkool > <seg...@kernel.crashing.org> wrote: > > > > On Mon, Nov 09, 2020 at 01:47:13PM +0100, Peter Zijlstra wrote: > > > > > > + lots of people and linux-toolchains > > > > > > 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. > > > > Most ways you can try to use the result are undefined behaviour, even. > > Yes, removing `const` from a `const` declared variable (via cast) then > expecting to use the result is a great way to have clang omit the use > from the final program. This has bitten us in the past getting MIPS > support up and running, and one of the MTK gfx drivers.
Stripping const to delcare another variable is useful though. Sure C has sharp edges, esp. if you cast stuff, but since when did that stop anyone ;-) The point is, C++ has these very nice template helpers that can strip qualifiers, I want that too, for much of the same reasons. We might not have templates :-(, but we've become very creative with our pre-processor. Surely our __unqual_scalar_typeof() cries for a better solution.
