On Mon, Apr 08, 2024 at 06:03:11PM +0100, Matthew Wilcox wrote: > On Mon, Apr 08, 2024 at 09:55:23AM -0700, Paul E. McKenney wrote: > > On Mon, Apr 08, 2024 at 05:02:37PM +0100, Matthew Wilcox wrote: > > > In my ideal world, the compiler would turn this into: > > > > > > newfolio->flags |= folio->flags & MIGRATE_MASK; > > > > Why not accumulate the changes in a mask, and then apply the mask the > > one time? (In situations where __folio_set_foo() need not apply.) > > But it irks me that we can't tell the compiler this is a safe > transformation for it to make. There are a number of places where > similar things happen. Same thing comes up with bignum code - you really want to be able to tell the compiler "you can apply x/y/z optimizations for these functions", e.g. replace add(mul(a, b), c) with fma(a, b, c). Compiler optimizations are just algebraic transformations, we just need a way to tell the compiler what the algebraic properties of our functions are.
