[PATCH] D115040: [gn build] Build with Fission on non-mac non-win when using lld

[Nico Weber via Phabricator via llvm-commits]

thakis added a comment.

Full numbers:

is_debug = true

  $ ministat dbg_before.txt dbg_after.txt
  x dbg_before.txt
  + dbg_after.txt
  +----------------------------------------------------------------------------------------------------------------------------+
  |+       +         +  +                            +                             x   xx               x                     x|
  | |________________M_A__________________|                                     |_______M_________A________________|           |
  +----------------------------------------------------------------------------------------------------------------------------+
      N           Min           Max        Median           Avg        Stddev
  x   5     5.2263865     5.7603414     5.2823768     5.4064289    0.22182468
  +   5     4.2161133     4.8450658     4.4456208     4.4625463    0.23867625
  Difference at 95.0% confidence
  -0.943883 +/- 0.336032
  -17.4585% +/- 6.21541%
  (Student's t, pooled s = 0.230405)

release/sym

  thakis at thakis:~/src/llvm-project/out/gn$ ministat rel_before.txt rel_after.txt
  x rel_before.txt
  + rel_after.txt
  +----------------------------------------------------------------------------------------------------------------------------+
  |+  +  +   + +                                                                                               xx    x     x  x|
  | |____A____|                                                                                                |_____MA_____|  |
  +----------------------------------------------------------------------------------------------------------------------------+
      N           Min           Max        Median           Avg        Stddev
  x   5      2.204227      2.335144     2.2594814     2.2635095   0.057581912
  +   5     1.2554414     1.3601334     1.3091564     1.3093105   0.042406506
  Difference at 95.0% confidence
  -0.954199 +/- 0.0737487
  -42.1557% +/- 3.25816%
  (Student's t, pooled s = 0.0505667)

For fun, I also tried with gold ld and bfd ld, but only in a release+sym build since it was fairly slow there already.

gold used -Wl,--gdb-index, except in the _noindex runs. bfd ld doesn't have --gdb-index, so I think fission doesn't work well with it (?). (I also haven't tried actually running the binary under gdb in these builds). (after vs after2 is doing the same 12 hours later, so there's some long-term noise here apparently. I trust the after2 numbers less since they claim gold is faster if it has to compute an index in those numbers, and that seems like nonsense).

I think the summary here is that fission helps a _lot_ more for gold and bfd ld (but even with fission, they're still way slower than lld). The very fastest version here is 1/3rd the speed of lld, and the slowest takes over 45x (!) as long as lld. gold without fission takes way over 10x as long as lld without fission.

  $ ~/src/ministat/ministat gcc*
  x gcc_bfd_after2.txt
  + gcc_bfd_after.txt
  * gcc_bfd_before.txt
  % gcc_gold_after2_noindex.txt
  # gcc_gold_after2.txt
  @ gcc_gold_after.txt
  O gcc_gold_before_noindex.txt
  ~ gcc_gold_before.txt
  +----------------------------------------------------------------------------------------------------------------------------+
  |#   %       x  @@                                                        O               ~                                * |
  |##  % %     x x@@@ ++                                                  OOO             ~~~~                    *   *     ** |
  |A|  MA      MA |A|A_|                                                   |A             |_A|                      |____A__M_||
  +----------------------------------------------------------------------------------------------------------------------------+
      N           Min           Max        Median           Avg        Stddev
  x   5     10.555829     11.590927     10.691926     10.836577    0.42719901
  +   5     12.727631     15.063868     13.113123     13.631338     1.0142038
  Difference at 95.0% confidence
  2.79476 +/- 1.13492
  25.7901% +/- 10.4731%
  (Student's t, pooled s = 0.778174)
  *   5     64.289209     70.342028     69.866205     68.286443     2.6778043
  Difference at 95.0% confidence
  57.4499 +/- 2.79647
  530.148% +/- 25.8059%
  (Student's t, pooled s = 1.91744)
  %   5     6.3529992     7.0316789     6.3846138     6.5051929    0.29492724
  Difference at 95.0% confidence
  -4.33138 +/- 0.535351
  -39.97% +/- 4.94022%
  (Student's t, pooled s = 0.36707)
  #   5     4.0398211     4.4484537     4.1234727     4.1840817    0.16065823
  Difference at 95.0% confidence
  -6.6525 +/- 0.470684
  -61.3893% +/- 4.34347%
  (Student's t, pooled s = 0.322731)
  @   5     12.177341     13.516762     12.589737     12.712317    0.50764556
  Difference at 95.0% confidence
  1.87574 +/- 0.684228
  17.3093% +/- 6.31406%
  (Student's t, pooled s = 0.46915)
  O   5     42.854285     43.832344     43.634122     43.500009    0.38393929
  Difference at 95.0% confidence
  32.6634 +/- 0.59234
  301.418% +/- 5.46612%
  (Student's t, pooled s = 0.406146)
  ~   5      51.27234      53.16056     52.245277     52.270382    0.69728535
  Difference at 95.0% confidence
  41.4338 +/- 0.843319
  382.351% +/- 7.78215%
  (Student's t, pooled s = 0.578233)


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https://reviews.llvm.org/D115040