[PATCH 0/1] blame: remove unnecessary use of get_commit_info()

[Rafael Silva <rafaeloliveira.cs@gmail.com>]

This patch is a code cleanup and consequently a micro optimization in
determine_heat_line().

When `blame`ing with --color-by-age, the determine_line_heat() calls the
get_commit_info() to parse the commit's information.  However, it turns
out this is actually unnecessary because its caller also does the same.
We can instead, pass the already parsed `commit_info` and remove the
internal call to get_commit_info().

The performance optimization made by this patch is relatively small as
get_commit_info() doesn't perform any heavy-load operations. So, the
performance improvements is not that exciting, not at least until you
enter the ms/μs realm.  Nevertheless I thought the code cleanup is
still valid and the optimization is a bonus.


  ... digging into the the performance improvements, for those 
      who are curious ...

Running Git PERF suite in linux.git, I've got a subtle performance
improvement for some runs:

	# git.328c109303 - compiled git from commit 328c109303
	# git.blame-patched - compiled git from commit 328c109303 + this patch
        Test                                          git.328c109303    git.blame-patched
        -------------------------------------------------------------------------------------
        blame --color-by-age kernel/fork.c            1.96(1.81+0.15)   1.95(1.80+0.14) -0.5%
        blame --color-by-age kernel/sys.c             1.67(1.53+0.13)   1.66(1.52+0.14) -0.6%
        blame --color-by-age mm/slab.c                2.16(2.00+0.16)   2.15(1.99+0.15) -0.5%
        blame --color-by-age lib/packing.c            0.20(0.14+0.05)   0.20(0.14+0.05) +0.0%
        blame --color-by-age drivers/cdrom/cdrom.c    1.62(1.46+0.15)   1.62(1.46+0.15) +0.0%
        blame --color-by-age crypto/crypto_engine.c   0.37(0.29+0.06)   0.36(0.28+0.06) -2.7%
        blame --color-by-age net/ipv4/ip_forward.c    1.49(1.35+0.13)   1.48(1.34+0.13) -0.7%

To dig a little deeper, I enabled the Git's trace2 API to record every
call to the determine_line_heat() function:

        ...
+       trace2_region_enter("blame", "determine_line_heat", the_repository);
        determine_line_heat(ent, &default_color);
+       trace2_region_enter("blame", "determine_line_heat", the_repository);
        ...

Then, running `blame` for "kernel/fork.c` and _summing_ all the execution
time for every call (around 1.3k calls) resulted in 2.6x faster execution
(best out 3):

	git built from 328c109303 (The eighth batch, 2021-02-12) = 42ms
	git built from 328c109303 + this patch                   = 16ms

Of course, this is on the milliseconds :).

Lastly, to get a better picture of the performance change, I computed
all the determine_line_heat()'s execution time, for all 3 calls, into
a distribution graph.  The execution time is converted to microseconds
for ease of understanding:

        # git built from 328c109303 (The eighth batch, 2021-02-12)
     	Time in μs     0.0 ~  9.0  |
     	Time in μs    10.0 ~ 19.0  | *
     	Time in μs    20.0 ~ 29.0  | *****************
    	Time in μs    30.0 ~ 39.0  | *****************
     	Time in μs    40.0 ~ 49.0  | **
     	Time in μs    50.0 ~ 59.0  | *
     	Time in μs    60.0 ~ 69.0  | *
     	Time in μs    70.0 ~ 79.0  | *
     	Time in μs    80.0 ~ 89.0  | *
     	Time in μs    90.0 ~ 99.0  | *
     	Time in μs   100.0 ~ 109.0 | *
     	Time in μs   110.0 ~ 119.0 | *
     	Time in μs   120.0 ~ 129.0 |
     	Time in μs   130.0 ~ 139.0 | *
     	[N: 3915 | Median: 30.0 | Min: 19.0 | Max: 137.0]

     	# git built from 328c109303 + this patch
     	Time in μs     0.0 ~  9.0  |
     	Time in μs    10.0 ~ 19.0  | *************************************
     	Time in μs    20.0 ~ 29.0  | *
     	Time in μs    30.0 ~ 39.0  | *
     	Time in μs    40.0 ~ 49.0  | *
     	Time in μs    50.0 ~ 59.0  | *
     	Time in μs    60.0 ~ 69.0  | *
     	Time in μs    70.0 ~ 79.0  | *
     	Time in μs    80.0 ~ 89.0  | *
     	Time in μs    90.0 ~ 99.0  |
	        ... [ zero's omitted ] ...
     	Time in μs   150.0 ~ 159.0 |
     	Time in μs   160.0 ~ 169.0 | *
     	[N: 3915 | Median: 12.0 | Min: 10.0 | Max: 167.0]

	Note: A "*" represent hundreds counter. One "*" is used
	for any count between 1 and 99. (i.e: 23 = "*", 99 = "*",
	100 = "**", 199 = "**", 200 = "***", etc).

With the changes introduced by this patch, the determine_line_heat()
mostly finishes its execution between 10μs and 19μs compared with the
current implementation that mostly finished around 20μs and 39μs.

Rafael Silva (1):
  blame: remove unnecessary use of get_commit_info()

 builtin/blame.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

-- 
2.30.1.842.gf6373b1729