• Austin Clements's avatar
    runtime: reclaim scan/dead bit in first word · a20fd1f6
    Austin Clements authored
    With the switch to separate mark bitmaps, the scan/dead bit for the
    first word of each object is now unused. Reclaim this bit and use it
    as a scan/dead bit, just like words three and on. The second word is
    still used for checkmark.
    
    This dramatically simplifies heapBitsSetTypeNoScan and hasPointers,
    since they no longer need different cases for 1, 2, and 3+ word
    objects. They can instead just manipulate the heap bitmap for the
    first word and be done with it.
    
    In order to enable this, we change heapBitsSetType and runGCProg to
    always set the scan/dead bit to scan for the first word on every code
    path. Since these functions only apply to types that have pointers,
    there's no need to do this conditionally: it's *always* necessary to
    set the scan bit in the first word.
    
    We also change every place that scans an object and checks if there
    are more pointers. Rather than only checking morePointers if the word
    is >= 2, we now check morePointers if word != 1 (since that's the
    checkmark word).
    
    Looking forward, we should probably reclaim the checkmark bit, too,
    but that's going to be quite a bit more work.
    
    Tested by setting doubleCheck in heapBitsSetType and running all.bash
    on both linux/amd64 and linux/386, and by running GOGC=10 all.bash.
    
    This particularly improves the FmtFprintf* go1 benchmarks, since they
    do a large amount of noscan allocation.
    
    name                      old time/op    new time/op    delta
    BinaryTree17-12              2.34s ± 1%     2.38s ± 1%  +1.70%  (p=0.000 n=17+19)
    Fannkuch11-12                2.09s ± 0%     2.09s ± 1%    ~     (p=0.276 n=17+16)
    FmtFprintfEmpty-12          44.9ns ± 2%    44.8ns ± 2%    ~     (p=0.340 n=19+18)
    FmtFprintfString-12          127ns ± 0%     125ns ± 0%  -1.57%  (p=0.000 n=16+15)
    FmtFprintfInt-12             128ns ± 0%     122ns ± 1%  -4.45%  (p=0.000 n=15+20)
    FmtFprintfIntInt-12          207ns ± 1%     193ns ± 0%  -6.55%  (p=0.000 n=19+14)
    FmtFprintfPrefixedInt-12     197ns ± 1%     191ns ± 0%  -2.93%  (p=0.000 n=17+18)
    FmtFprintfFloat-12           263ns ± 0%     248ns ± 1%  -5.88%  (p=0.000 n=15+19)
    FmtManyArgs-12               794ns ± 0%     779ns ± 1%  -1.90%  (p=0.000 n=18+18)
    GobDecode-12                7.14ms ± 2%    7.11ms ± 1%    ~     (p=0.072 n=20+20)
    GobEncode-12                5.85ms ± 1%    5.82ms ± 1%  -0.49%  (p=0.000 n=20+20)
    Gzip-12                      218ms ± 1%     215ms ± 1%  -1.22%  (p=0.000 n=19+19)
    Gunzip-12                   36.8ms ± 0%    36.7ms ± 0%  -0.18%  (p=0.006 n=18+20)
    HTTPClientServer-12         77.1µs ± 4%    77.1µs ± 3%    ~     (p=0.945 n=19+20)
    JSONEncode-12               15.6ms ± 1%    15.9ms ± 1%  +1.68%  (p=0.000 n=18+20)
    JSONDecode-12               55.2ms ± 1%    53.6ms ± 1%  -2.93%  (p=0.000 n=17+19)
    Mandelbrot200-12            4.05ms ± 1%    4.05ms ± 0%    ~     (p=0.306 n=17+17)
    GoParse-12                  3.14ms ± 1%    3.10ms ± 1%  -1.31%  (p=0.000 n=19+18)
    RegexpMatchEasy0_32-12      69.3ns ± 1%    70.0ns ± 0%  +0.89%  (p=0.000 n=19+17)
    RegexpMatchEasy0_1K-12       237ns ± 1%     236ns ± 0%  -0.62%  (p=0.000 n=19+16)
    RegexpMatchEasy1_32-12      69.5ns ± 1%    70.3ns ± 1%  +1.14%  (p=0.000 n=18+17)
    RegexpMatchEasy1_1K-12       377ns ± 1%     366ns ± 1%  -3.03%  (p=0.000 n=15+19)
    RegexpMatchMedium_32-12      107ns ± 1%     107ns ± 2%    ~     (p=0.318 n=20+19)
    RegexpMatchMedium_1K-12     33.8µs ± 3%    33.5µs ± 1%  -1.04%  (p=0.001 n=20+19)
    RegexpMatchHard_32-12       1.68µs ± 1%    1.73µs ± 0%  +2.50%  (p=0.000 n=20+18)
    RegexpMatchHard_1K-12       50.8µs ± 1%    52.0µs ± 1%  +2.50%  (p=0.000 n=19+18)
    Revcomp-12                   381ms ± 1%     385ms ± 1%  +1.00%  (p=0.000 n=17+18)
    Template-12                 64.9ms ± 3%    62.6ms ± 1%  -3.55%  (p=0.000 n=19+18)
    TimeParse-12                 324ns ± 0%     328ns ± 1%  +1.25%  (p=0.000 n=18+18)
    TimeFormat-12                345ns ± 0%     334ns ± 0%  -3.31%  (p=0.000 n=15+17)
    [Geo mean]                  52.1µs         51.5µs       -1.00%
    
    Change-Id: I13e74da3193a7f80794c654f944d1f0d60817049
    Reviewed-on: https://go-review.googlesource.com/22632Reviewed-by: 's avatarRick Hudson <rlh@golang.org>
    Run-TryBot: Austin Clements <austin@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    a20fd1f6
gcinfo_test.go 6.24 KB