xdiff: delete chastore from xdfile_t

xdfile_t currently uses chastore_t which is an arena allocator. I
think that xrecord_t used to be a linked list and recs didn't exist
originally. When recs was added I think they forgot to remove
xdfile_t.next, but was overlooked. This dual data structure setup
makes the code somewhat confusing.

Additionally the C type chastore_t isn't FFI friendly, and provides
little to no performance benefit over using realloc to grow an array.

Performance impact of deleting fields from xdfile_t:
Deleting ha is about 5% slower.
Deleting cha is about 5% faster.

Delete ha, but keep cha
  time hyperfine --warmup 3 -L exe build_v2.51.0/git,build_delete_ha/git '{exe} log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null'
  Benchmark 1: build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.269 s ±  0.017 s    [User: 1.135 s, System: 0.128 s]
    Range (min … max):    1.249 s …  1.286 s    10 runs

  Benchmark 2: build_delete_ha/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.339 s ±  0.017 s    [User: 1.234 s, System: 0.099 s]
    Range (min … max):    1.320 s …  1.358 s    10 runs

  Summary
    build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null ran
      1.06 ± 0.02 times faster than build_delete_ha/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null

Delete cha, but keep ha
  time hyperfine --warmup 3 -L exe build_v2.51.0/git,build_delete_chastore/git '{exe} log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null'
  Benchmark 1: build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.290 s ±  0.001 s    [User: 1.154 s, System: 0.130 s]
    Range (min … max):    1.288 s …  1.292 s    10 runs

  Benchmark 2: build_delete_chastore/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.232 s ±  0.017 s    [User: 1.105 s, System: 0.121 s]
    Range (min … max):    1.205 s …  1.249 s    10 runs

  Summary
    build_delete_chastore/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null ran
      1.05 ± 0.01 times faster than build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null

Delete ha AND chastore
  time hyperfine --warmup 3 -L exe build_v2.51.0/git,build_delete_ha_and_chastore/git '{exe} log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null'
  Benchmark 1: build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.291 s ±  0.002 s    [User: 1.156 s, System: 0.129 s]
    Range (min … max):    1.287 s …  1.295 s    10 runs

  Benchmark 2: build_delete_ha_and_chastore/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null
    Time (mean ± σ):      1.306 s ±  0.001 s    [User: 1.195 s, System: 0.105 s]
    Range (min … max):    1.305 s …  1.308 s    10 runs

  Summary
    build_v2.51.0/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null ran
      1.01 ± 0.00 times faster than build_delete_ha_and_chastore/git log --oneline --shortstat --diff-algorithm=myers -3000 v2.39.1 >/dev/null

Best-viewed-with: --color-words
Signed-off-by: Ezekiel Newren <ezekielnewren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions