Re: [PATCH] Add .gnu.lto_.meta section.

["Martin Liška" <mliska@suse.cz>]

On 6/21/19 2:34 PM, Richard Biener wrote:
> On Fri, Jun 21, 2019 at 12:20 PM Martin Liška <mliska@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