Hello, Mark H Weaver <m...@netris.org> skribis:
> Ludovic Courtès <l...@gnu.org> writes: [...] >> IIUC, your plan is to have a different tagging on 32-bit platforms, >> without fixflos, right? I’m curious to see how much complexity would >> entail from that. > > Yes, although I'm avoiding the term "fixflos" because IEEE doubles are > also fixed width, and thus the term "fixflos" wouldn't adequately > distinguish them from IEEE doubles. Right! > Anyway, I agree that it's inconvenient to have different tags on > different targets, and I've been working to minimize the differences. > > At present, I'm currently implementing an alternative strategy where > pairs are tagged in their pointers instead of in their CARs, which > enables us to separate the heap tags and immediate tags into two > independent spaces. At first this sounds rather radical :-), but maybe it’s preferable this way. > In this new approach, the heap tags are left unchanged, and the only > tags that vary with target word size are the fixints, fixrats, iflos, > and pair pointers. All other tags will be uniform across targets, > including the non-number immediates. Here's the new version: > > ;; /* with iflos: xxx: iflo (000 < xxx < 110) > ;; (64-bit) 0111: fixrat > ;; 1111: fixnum > ;; 0110: pair > ;; 000: tagged heap object (thob) > ;; tttt1110: other immediate > ;; > ;; without iflos: 1: fixnum > ;; (32-bit) 010: fixrat > ;; 100: pair > ;; 000: tagged heap object (thob) > ;; tttt1110: other immediate > > This new approach brings its own complications, mainly two: > > (1) It breaks the long-standing assumptions in Guile that all > non-immediates have a tag in their first word and that pointers are > always untagged. In my preliminary patch, I introduce a new concept > called a "tagged heap object" or "thob", and most existing checks > for SCM_NIMP or !SCM_IMP must be changed to use SCM_THOB_P. Though an immediate, like a fixnum or an iflo, is still something different from a tagged heap object like a pair, right? So I would expect SCM_THOB_P to be a different test, not a drop-in replacement for SCM_NIMP, is that correct? > (2) Our existing VM instructions almost invariably specify offsets with > a granularity of whole words. To support tagged pair pointers with > good performance, I think we need a few new instructions that > specify byte offsets, to avoid the expensive extra step of removing > the tag before accessing the CAR or CDR of a pair. So instead of a pointer dereference, SCM_CAR becomes mask + dereference, right? I think we disable GC “interior pointer” scanning. With this scheme, an SCM for a pair would actually point in the middle of a pair; could this be an issue for GC? Thank you! Ludo’.