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.) > > Yes, absolutely, we can, should and probably eventually will do this > when it gets to the top of somebody's todo list. 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. > > $ git grep folio_test.*folio_test > > will find you 82 of them (where they happen to be on the same line) > > if (folio_test_dirty(folio) || folio_test_locked(folio) || > folio_test_writeback(folio)) > break; > > turns into: > > 1f41: 48 8b 29 mov (%rcx),%rbp > 1f44: 48 c1 ed 04 shr $0x4,%rbp > 1f48: 83 e5 01 and $0x1,%ebp > 1f4b: 0f 85 d5 00 00 00 jne 2026 <filemap_range_has_writeback+0x1a6> > 1f51: 48 8b 29 mov (%rcx),%rbp > 1f54: 83 e5 01 and $0x1,%ebp > 1f57: 0f 85 c9 00 00 00 jne 2026 <filemap_range_has_writeback+0x1a6> > 1f5d: 48 8b 29 mov (%rcx),%rbp > 1f60: 48 d1 ed shr $1,%rbp > 1f63: 83 e5 01 and $0x1,%ebp > 1f66: 0f 85 ba 00 00 00 jne 2026 <filemap_range_has_writeback+0x1a6> > > rather than _one_ load from rcx and a test against a mask. Agreed, it would be nice if we could convince the compiler to do this for us, preferably without breaking anything. > > If it turns out that we really do need a not-quite-volatile, what exactly > > does it do? You clearly want it to be able to be optimized so as to merge > > similar accesses. Is there a limit to the number of accesses that can > > be merged or to the region of code over which such merging is permitted? > > Either way, how is the compiler informed of these limits? > > Right, like I said, it's not going to be easy to define exactly what we > want. Or to convince the usual suspects that any definition we might come up with is useful/implementable/teacheable/... :-/ Thanx, Paul
