ccache is mostly beneficial for frequent builds where -DNO_CLEAN is not
used to achieve a safe pseudo-incremental build.  This is explained in
more detail upstream [1] [2].  It incurs about a 20%-28% hit to populate the
cache, but with a full cache saves 30-50% in build times.  When combined with
the WITH_FAST_DEPEND feature it saves up to 65% since ccache does cache the
resulting dependency file, which it does not do when using mkdep(1)/'CC
-E'.  Stats are provided at the end of this message.

This removes the need to modify /etc/make.conf with the CC:= and CXX:=
lines which conflicted with external compiler support [3] (causing the
bootstrap compiler to not be built which lead to obscure failures [4]),
incorrectly invoked ccache in various stages, required CCACHE_CPP2 to avoid
Clang errors with parenthesis, and did not work with META_MODE.

The option name was picked to match the existing option in ports.  This
feature is available for both in-src and out-of-src builds that use
/usr/share/mk.

Linking, assembly compiles, and pre-processing avoid using ccache since it is
only overhead.  ccache does nothing special in these modes, although there is
no harm in calling it for them.

CCACHE_COMPILERCHECK is set to 'content' when using the in-tree bootstrap
compiler to hash the content of the compiler binary to determine if it
should be a cache miss.  For external compilers the 'mtime' option is used
as it is more efficient and likely to be correct.  Future work may optimize the
'content' check using the same checks as whether a bootstrap compiler is needed
to be built.

The CCACHE_CPP2 pessimization is currently default in our devel/ccache
port due to Clang requiring it.  Clang's -Wparentheses-equality,
-Wtautological-compare, and -Wself-assign warnings do not mix well with
compiling already-pre-processed code that may have expanded macros that
trigger the warnings.  GCC has so far not had this issue so it is allowed to
disable the CCACHE_CPP2 default in our port.

Sharing a cache between multiple checkouts, or systems, is explained in
the ccache manual.  Sharing a cache over NFS would likely not be worth
it, but syncing cache directories between systems may be useful for an
organization.  There is also a memcached backend available [5].  Due to using
an object directory outside of the source directory though you will need to
ensure that both are in the same prefix and all users use the same layout.  A
possible working layout is as follows:
  Source: /some/prefix/src1
  Source: /some/prefix/src2
  Source: /some/prefix/src3
  Objdir: /some/prefix/obj
  Environment: CCACHE_BASEDIR='${SRCTOP:H}' MAKEOBJDIRPREFIX='${SRCTOP:H}/obj'
This will use src*/../obj as the MAKEOBJDIRPREFIX and tells ccache to replace
all absolute paths to be relative.  Using something like this is required due
to -I and -o flags containing both SRC and OBJDIR absolute paths that ccache
adds into its hash for the object without CCACHE_BASEDIR.

distcc can be hooked into by setting CCACHE_PREFIX=/usr/local/bin/distcc.
I have not personally tested this and assume it will not mix well with
using the bootstrap compiler.

The cache from buildworld can be reused in a subdir by first running
'make buildenv' (from r290424).

Note that the cache is currently different depending on whether -j is
used or not due to ccache enabling -fdiagnostics-color automatically if
stderr is a TTY, which bmake only does if not using -j.

The system I used for testing was:
  WITNESS
  Build options: -j20 WITH_LLDB=yes WITH_DEBUG_FILES=yes WITH_CCACHE_BUILD=yes
  DISK: ZFS 3-way mirror with very slow disks using SSD l2arc/log.
        The arc was fully populated with src tree files and ccache objects.
  RAM: 76GiB
  CPU: Intel(R) Xeon(R) CPU L5520 @2.27GHz
       2 package(s) x 4 core(s) x 2 SMT threads = hw.ncpu=16

The WITH_FAST_DEPEND feature was used for comparison here as well to show
the dramatic time savings with a full cache.

buildworld:
  x buildworld-before
  + buildworld-ccache-empty
  * buildworld-ccache-full
  % buildworld-ccache-full-fastdep
  # buildworld-fastdep
  +-------------------------------------------------------------------------------+
  |%            *                               #                                +|
  |%            *                               #                                +|
  |%            *                               #             xxx                +|
  |                                                           |A                  |
  |                                                                              A|
  |             A                                                                 |
  |A                                                                              |
  |                                             A                                 |
  +-------------------------------------------------------------------------------+
      N           Min           Max        Median           Avg        Stddev
  x   3       3744.13       3794.31       3752.25     3763.5633     26.935139
  +   3          4519       4525.04       4520.73       4521.59     3.1104823
  Difference at 95.0% confidence
          758.027 +/- 43.4565
          20.1412% +/- 1.15466%
          (Student's t, pooled s = 19.1726)
  *   3       1823.08        1827.2       1825.62        1825.3     2.0785572
  Difference at 95.0% confidence
          -1938.26 +/- 43.298
          -51.5007% +/- 1.15045%
          (Student's t, pooled s = 19.1026)
  %   3       1266.96       1279.37       1270.47     1272.2667     6.3971113
  Difference at 95.0% confidence
          -2491.3 +/- 44.3704
          -66.1952% +/- 1.17895%
          (Student's t, pooled s = 19.5758)
  #   3       3153.34       3155.16        3154.2     3154.2333    0.91045776
  Difference at 95.0% confidence
          -609.33 +/- 43.1943
          -16.1902% +/- 1.1477%
          (Student's t, pooled s = 19.0569)

buildkernel:
  x buildkernel-before
  + buildkernel-ccache-empty
  * buildkernel-ccache-empty-fastdep
  % buildkernel-ccache-full
  # buildkernel-ccache-full-fastdep
  @ buildkernel-fastdep
  +-------------------------------------------------------------------------------+
  |#                        @   %                  *                              |
  |#                        @   %                  *     x                      + |
  |#                        @   %                  *     xx                     ++|
  |                                                      MA                       |
  |                                                                             MA|
  |                                                A                              |
  |                             A                                                 |
  |A                                                                              |
  |                         A                                                     |
  +-------------------------------------------------------------------------------+
      N           Min           Max        Median           Avg        Stddev
  x   3        571.57        573.94        571.79     572.43333     1.3094401
  +   3        727.97        731.91        728.06     729.31333     2.2492295
  Difference at 95.0% confidence
          156.88 +/- 4.17129
          27.4058% +/- 0.728695%
          (Student's t, pooled s = 1.84034)
  *   3         527.1        528.29        528.08     527.82333    0.63516402
  Difference at 95.0% confidence
          -44.61 +/- 2.33254
          -7.79305% +/- 0.407478%
          (Student's t, pooled s = 1.02909)
  %   3         400.4        401.05        400.62        400.69     0.3306055
  Difference at 95.0% confidence
          -171.743 +/- 2.16453
          -30.0023% +/- 0.378128%
          (Student's t, pooled s = 0.954969)
  #   3        201.94        203.34        202.28        202.52    0.73020545
  Difference at 95.0% confidence
          -369.913 +/- 2.40293
          -64.6212% +/- 0.419774%
          (Student's t, pooled s = 1.06015)
  @   3        369.12        370.57         369.3     369.66333    0.79033748
  Difference at 95.0% confidence
          -202.77 +/- 2.45131
          -35.4225% +/- 0.428227%
          (Student's t, pooled s = 1.0815)

[1] https://ccache.samba.org/performance.html
[2] http://www.mail-archive.com/ccache@lists.samba.org/msg00576.html
[3] https://reviews.freebsd.org/D3484
[5] https://github.com/jrosdahl/ccache/pull/30

PR:		182944 [4]
MFC after:	3 weeks
Sponsored by:	EMC / Isilon Storage Division
Relnotes:	yes