On 6/21/19 2:34 PM, Richard Biener wrote: > On Fri, Jun 21, 2019 at 12:20 PM Martin Liška <mli...@suse.cz> wrote: >> >> Hi. >> >> The patch is about a new ELF section that will contain information >> about LTO version. And for the future, used compression will be stored >> here. The patch removes streaming of the version into each section. > > I'd like each section to have a header containing a compression method > (compressed or not, and now zlib vs. zstd). We currently have a mix > and knowledge is hidden.
That would be possible, good idea. > > I also remember I had old patches to make the data streamer compress > the stream on-the-fly, not requiring the full uncompressed stream in > memory and then re-access it for compression (needing more temporary > memory). But IIRC the speedup was marginal. > > My thought is also that we should have exactly _one_ ELF section > for the LTO bytecode and our own container inside (much like > simple-object does for non-ELF). So having another one is, well, ugly ;) Having N sections for all symbols (functions and variables) is handy because we read some of the during WPA (by IPA ICF). So having all in a single section would make decompression more complicated. I'm going to prepare a patch that will pull out a LTO section header from compressed stream. Martin > >> Disadvantage is a format change that will lead to following errors when >> LTO bytecode is used from a different version: >> >> $ gcc x.o >> lto1: fatal error: bytecode stream in file ‘x.o’ generated with GCC compiler >> older than 10.0 >> >> $ gcc-9 main.o >> lto1: fatal error: bytecode stream in file ‘main.o’ generated with LTO >> version 850.0 instead of the expected 8.0 >> >> I've been testing the patch. >> Thoughts? >> Martin
