2011/5/26 Jan Hubicka <hubi...@ucw.cz>: >> 2011/5/26 Jan Hubicka <hubi...@ucw.cz>: >> >> On Thu, May 26, 2011 at 12:45 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> >> >> >> >> >> This looks all very hackish with no immediate benefit mostly because >> >> >> of the use of lto_output_string. I think what you should do instead >> >> >> is split up lto_output_string_with_length into the piece that streams >> >> >> the string itself to the string-stream and returns an index into it >> >> >> and the piece streaming the index to the specified stream. Then you >> >> >> can simply bitpack that index and the two int line / column fields. >> >> > >> >> > Hmm, I plan to optimize string streaming (since we always stream one >> >> > uleb to >> >> > set it is non-NULL that can be easilly handled by assigining NULL >> >> > string index >> >> > 0). How precisely you however suggest to bitpack line/column and >> >> > string offset >> >> > together? >> >> >> >> Similar to how you suggested, stream bits for a changed flag but >> >> instead of finishing the bitpack simply stream HOST_BITS_PER_INT >> >> bits for line (if changed), colunn (if changed) and file string index (if >> >> changed and the index is 'int'). >> >> >> >> I mostly want to avoid the split between the changed bits and the >> >> data output, esp. breaking the bitpack. >> > >> > Well, that won't get me for < 1 byte overhead when location is unchanged or >> > unknown (that is true for about half of cases in my stats). >> >> Why not? it would be 3 bits. > > Hmm, I see, so you suggest to move all the data into bitpack in order to > couple > it with the bitpack in tree streaming w/o need to have two functions. (i > originally understood the message as you are objecting to the idea of storing > only the changes). > > I see one can save it into bitpack, though i was under impression that we want > to avoid variably sized bitpacks as then the accessors no longer expands to > simple arithmetics.
Yeah, but we have this issue everywhere when conditionally saving bits, so I wouldn't care that much - the code still is quite nice. >> >> > Additionally >> > HOST_BITS_PER_INT is host sensitive and wasteful compared to ulebs here as >> > the >> > line numbers, file indexes and columns are all usually small numbers, so >> > they >> > ought to fit in 16, 8 and 8 bits most of time. So we would end up in need >> > of >> > inventing something like uleb in bitpack? >> >> Well, we could do that by default for > 8 bit values we pack. I think >> the location CSE using only 3 bits for unchanged locations should >> save the most, not so much the use of ulebs for line/column. >> (you could also encode the number of needed bytes for a changed >> line/column and then only that many number of bytes). > > Well, since almost half of time we save at least one of the 3 indices, I > think the stupid > implementation would burn 3 bytes at average at least. This would be sort of > comparable > with the existing uleb path that is 5 bytes. > > OK, if we want to go for variably sized bitpacks, I guess I can simply store > number of bits > needed to represent the number followed by the number itself for all three > indices. > It will need massaging string i/o together with the patch as currently string > indexes > go into the ulebs. Can do that if you think it is better option. Yes, I think so. > Do we have some limits on maximal bitpack size? No. Richard. > Honza >
