Jan Voung
2012-Sep-26 21:08 UTC
[LLVMdev] [PATCH / PROPOSAL] bitcode encoding that is ~15% smaller for large bitcode files...
On Wed, Sep 26, 2012 at 2:07 AM, Renato Golin <rengolin at systemcall.org>wrote:> On 26 September 2012 01:08, Jan Voung <jvoung at chromium.org> wrote: > > - Forward references will create negative-valued ids (which end up being > > written out as large 32-bit integers, as far as I could tell). > > Can you use an SLEB-like representation? > > It's probably not going to be backwards compatible, but if there isn't > an SLEB/ULEB representation in bitcode, I'd say it's a good > improvement. ;) > >I think most of the instructions operands are encoded as VBR(N) for some N number of bits, and it seems to me that VBR(8) is like ULEB? I think that the default N is 6 bits, but I haven't tried tweaking these parameters.> -- > cheers, > --renato > > http://systemcall.org/ >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120926/07294d3c/attachment.html>
Renato Golin
2012-Sep-27 09:57 UTC
[LLVMdev] [PATCH / PROPOSAL] bitcode encoding that is ~15% smaller for large bitcode files...
On 26 September 2012 22:08, Jan Voung <jvoung at chromium.org> wrote:> I think most of the instructions operands are encoded as VBR(N) > for some N number of bits, and it seems to me that VBR(8) is like ULEB? > I think that the default N is 6 bits, but I haven't tried tweaking these > parameters.It has a similar purpose, but slightly different. SLEB (the signed version) wouldn't use the whole word for -1, since the sign expansion happens after the number is encoded to 7 bits package, so any negative number that can be represented using 7 bits would still be stored in a byte. -- cheers, --renato http://systemcall.org/
Jan Voung
2012-Sep-27 23:07 UTC
[LLVMdev] [PATCH / PROPOSAL] bitcode encoding that is ~15% smaller for large bitcode files...
On Thu, Sep 27, 2012 at 2:57 AM, Renato Golin <rengolin at systemcall.org>wrote:> On 26 September 2012 22:08, Jan Voung <jvoung at chromium.org> wrote: > > I think most of the instructions operands are encoded as VBR(N) > > for some N number of bits, and it seems to me that VBR(8) is like ULEB? > > I think that the default N is 6 bits, but I haven't tried tweaking these > > parameters. > > It has a similar purpose, but slightly different. SLEB (the signed > version) wouldn't use the whole word for -1, since the sign expansion > happens after the number is encoded to 7 bits package, so any negative > number that can be represented using 7 bits would still be stored in a > byte. > >Ah I see. It looks like LLVM's bitcode writer does have a signed version of the VBRs, which is used for integer constants. I tried using that for phis and added the data to the *second sheet* in the original spreadsheet. See the tab "With SLEB-like Signed VBR for phi", at the bottom. It's certainly a bit better (1.3% uncompressed and 0.7% with gzip too), but it's not always better, and I think it can still be done in a backward compatible way. I added numbers for what would happen if we didn't touch phis at all, too. Finally, checked the average size of individual bc files from a build of the llvm-test-suite (vs llvm-linked bitcode files), to make sure this isn't really regressing sizes of small files. The average size of such small bitcode object files is ~3.5% better. Didn't check compression for that case though...> > -- > cheers, > --renato > > http://systemcall.org/ >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20120927/c3ab8cd6/attachment.html>
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