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cmd/link: use BestSpeed for DWARF compression

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DWARF compression accounts for roughly 30% of the linker's time. This
CL switches from DefaultCompression to BestSpeed, which virtually
eliminates this time. This roughly halves the overhead of handling
DWARF in the linker:

name \ time/op         nodwarf     dwarf        dwarf-speed
BuildCmdGoAll          10.0s ±11%   10.6s ± 5%   10.8s ± 5%
             nodwarf                  +6.41%       +8.03%
               dwarf                                 ~
LinkCmdGo              626ms ± 5%  1096ms ± 2%   860ms ± 2%
             nodwarf                 +75.17%      +37.36%
               dwarf                              -21.59%

Previously, enabling DWARF had a 75% overhead in link time for cmd/go.
This change reduces this overhead to 37% (a 22% reduction).

The effect on binary size is minimal compared to DefaultCompression,
and still substantially better than no compression:

             cmd/go bytes
nodwarf      10106953
dwarf        12159049 nodwarf+20%
dwarf-speed  12408905 nodwarf+23%
dwarf-nozlib 17766473 nodwarf+76%

Updates #26318.

Change-Id: I33bb7caa038a2753c29104501663daf4839e7054
Reviewed-on: https://go-review.googlesource.com/123356
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
This commit is contained in:
Austin Clements 2018-07-11 16:13:04 -04:00
parent 9300d223a3
commit 12ed0ddec1
1 changed files with 8 additions and 1 deletions
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9
src/cmd/link/internal/ld/data.go
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@ -2177,7 +2177,14 @@ func compressSyms(ctxt *Link, syms []*sym.Symbol) []byte {
binary.BigEndian.PutUint64(sizeBytes[:], uint64(total))
buf.Write(sizeBytes[:])
z := zlib.NewWriter(&buf)
// Using zlib.BestSpeed achieves very nearly the same
// compression levels of zlib.DefaultCompression, but takes
// substantially less time. This is important because DWARF
// compression can be a significant fraction of link time.
z, err := zlib.NewWriterLevel(&buf, zlib.BestSpeed)
if err != nil {
log.Fatalf("NewWriterLevel failed: %s", err)
}
for _, sym := range syms {
if _, err := z.Write(sym.P); err != nil {
log.Fatalf("compression failed: %s", err)