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cmd/link: compress DWARF sections in ELF binaries
Forked from CL 111895. The trickiest part of this is that the binary layout code (blk, elfshbits, and various other things) assumes a constant offset between symbols' and sections' file locations and their virtual addresses. Compression, of course, breaks this constant offset. But we need to assign virtual addresses to everything before compression in order to resolve relocations before compression. As a result, compression needs to re-compute the "address" of the DWARF sections and symbols based on their compressed size. Luckily, these are at the end of the file, so this doesn't perturb any other sections or symbols. (And there is, of course, a surprising amount of code that assumes the DWARF segment comes last, so what's one more place?) Relevant benchmarks: name old time/op new time/op delta StdCmd 10.3s ± 2% 10.8s ± 1% +5.43% (p=0.000 n=30+30) name old text-bytes new text-bytes delta HelloSize 746kB ± 0% 746kB ± 0% ~ (all equal) CmdGoSize 8.41MB ± 0% 8.41MB ± 0% ~ (all equal) [Geo mean] 2.50MB 2.50MB +0.00% name old data-bytes new data-bytes delta HelloSize 10.6kB ± 0% 10.6kB ± 0% ~ (all equal) CmdGoSize 252kB ± 0% 252kB ± 0% ~ (all equal) [Geo mean] 51.5kB 51.5kB +0.00% name old bss-bytes new bss-bytes delta HelloSize 125kB ± 0% 125kB ± 0% ~ (all equal) CmdGoSize 145kB ± 0% 145kB ± 0% ~ (all equal) [Geo mean] 135kB 135kB +0.00% name old exe-bytes new exe-bytes delta HelloSize 1.60MB ± 0% 1.05MB ± 0% -34.39% (p=0.000 n=30+30) CmdGoSize 16.5MB ± 0% 11.3MB ± 0% -31.76% (p=0.000 n=30+30) [Geo mean] 5.14MB 3.44MB -33.08% Fixes #11799. Updates #6853. Change-Id: I64197afe4c01a237523a943088051ee056331c6f Reviewed-on: https://go-review.googlesource.com/118276 Run-TryBot: Heschi Kreinick <firstname.lastname@example.org> TryBot-Result: Gobot Gobot <email@example.com> Reviewed-by: Austin Clements <firstname.lastname@example.org> Reviewed-by: Ian Lance Taylor <email@example.com>
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