On Sat, Nov 21, 2020 at 09:29:54PM +0100, Jakub Jelinek wrote: > On Sat, Nov 21, 2020 at 09:23:10PM +0100, Luc Van Oostenryck wrote: > > On Fri, Nov 20, 2020 at 01:51:54PM +0100, Peter Zijlstra wrote: > > > > > > Other option would be to change the const_ilog2 macro, though as the > > > description says it is meant to be used also in C constant expressions, > > > and while GCC will fold it to constant with constant argument even in > > > those, perhaps it is better to avoid using extensions in that case. > > > > Just for info, the description is outdated and Sparse is just fine with > > __builtin_clzll() and friends in constant expressions (since Feb 2017) > > Why is the description outdated? It is still an extension that not every > compiler might fold in constant expressions. And, the large expressions > aren't really a problem in constant expressions, they will be folded there > to constant or error. > The problem the patch was trying to solve is that the large expressions are > a problem at least for GCC in runtime code when guarded by > __builtin_constant_p, because __builtin_constant_p is folded quite late > (intentionally so, so that more constants can be propagated into it, e.g. > after inlining etc.), and the large expressions might confuse inliner > heuristics.
I was only talking about the part "Use this where *sparse* expects a true constant expression, e.g. for array 75 indices." and wanted to say that __builtin_clzll() with a constant argument is now also expanded to a constant, like GCC does (it wasn't the case before 2017 and I think it was the main reason why const_ilog2() is written as it is). -- Luc
