[PATCH] D40683: Make .gnu.hash section smaller.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.
Herald added a subscriber: emaste.

Our on-disk hash table was unnecessarily large. The cost of collision is
not high in the .gnu.hash table because each symbol in the .gnu.hash
table has a hash value with it. So, for each collided symbol, the
dynamic linker just compares an integer, which is pretty cheap.

This patch increases the load factor by about 8. Here's a comparison.

  $ readelf --histogram libclangSema.so.6.0.0svn-new-lld
  Histogram for `.gnu.hash' bucket list length (total of 582 buckets):
   Length  Number     % of total  Coverage
        0  11         (  1.9%)
        1  35         (  6.0%)      1.5%
        2  93         ( 16.0%)      9.5%
        3  108        ( 18.6%)     23.4%
        4  121        ( 20.8%)     44.1%
        5  86         ( 14.8%)     62.6%
        6  63         ( 10.8%)     78.8%
        7  38         (  6.5%)     90.2%
        8  18         (  3.1%)     96.4%
        9  6          (  1.0%)     98.7%
       10  3          (  0.5%)    100.0%
  
  $ readelf --histogram libclangSema.so.6.0.0svn-old-lld
  Histogram for `.gnu.hash' bucket list length (total of 4093 buckets):
   Length  Number     % of total  Coverage
        0  1498       ( 36.6%)
        1  1545       ( 37.7%)     37.7%
        2  712        ( 17.4%)     72.5%
        3  251        (  6.1%)     90.9%
        4  66         (  1.6%)     97.3%
        5  16         (  0.4%)     99.3%
        6  5          (  0.1%)    100.0%
  
  $ readelf --histogram libclangSema.so.6.0.0svn-bfd
  Histogram for `.gnu.hash' bucket list length (total of 1004 buckets):
   Length  Number     % of total  Coverage
      0  92         (  9.2%)
        1  227        ( 22.6%)      9.8%
        2  266        ( 26.5%)     32.6%
        3  222        ( 22.1%)     61.2%
        4  115        ( 11.5%)     81.0%
        5  55         (  5.5%)     92.8%
        6  21         (  2.1%)     98.2%
        7  6          (  0.6%)    100.0%
  
  $ readelf --histogram libclangSema.so.6.0.0svn-gold
  Histogram for `.gnu.hash' bucket list length (total of 2053 buckets):
   Length  Number     % of total  Coverage
        0  671        ( 32.7%)
        1  709        ( 34.5%)     30.4%
        2  470        ( 22.9%)     70.7%
        3  141        (  6.9%)     88.9%
        4  54         (  2.6%)     98.2%
        5  5          (  0.2%)     99.2%
        6  3          (  0.1%)    100.0%


https://reviews.llvm.org/D40683

Files:
  lld/ELF/SyntheticSections.cpp
  lld/test/ELF/gc-sections-shared.s

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