SCons: Add fast_unsafe option for faster rebuilds

[akien-mga]

This reverts #53828 which had caused a significant drop in incremental
rebuild time for debug builds (from 10s to 23s on my laptop).

The "faster but unsafe" options are re-added, as well as adding
max_drift=60 which we didn't use previously.

These options speed up SCons' own processing of the codebase to decide
what to build/rebuild (i.e. the first step before actually calling the
compiler). This will therefore not make much difference for scratch
builds, and is mostly useful for incremental rebuilds (including "null"
rebuilds with no change).

These options are enabled automatically for debug builds, unless
fast_unsafe=no is passed.
They are disabled by default for release and release_debug builds,
unless fast_unsafe=yes is passed.

See https://github.com/SCons/scons/wiki/GoFastButton and the SCons man page for details.

---

Would be good to have this tested on different hardware and OSes, as well as with different combinations of the three options to see what gives the best results.

"Null rebuild" means issuing a rebuild with the same command after a full build (so no file needs to be rebuilt, nor linked, so it only tests SCons' parsing of our scripts and codebase).

"main.cpp rebuild" is a rebuild with only a cosmetic change done with main.cpp (leading to rebuilding main.cpp and relinking). Note that linker time is now included and can be quite long depending on the OS and linker used. I'm using the mold linker which takes ~2s to link Godot in debug mode for me.

Here's the results on my system:

System:    Host: cauldron Kernel: 5.16.7-desktop-1.mga9 x86_64 bits: 64 Desktop: KDE Plasma 5.23.4 Distro: Mageia 9 mga9 
CPU:       Info: Quad Core model: Intel Core i7-8705G bits: 64 type: MT MCP cache: L2: 8 MiB 
           Speed: 900 MHz min/max: 800/3100 MHz Core speeds (MHz): 1: 900 2: 900 3: 900 4: 900 5: 900 6: 900 7: 900 8: 900 

Python 3.9.8 (CPython)
SCons 4.2.0
Build command:

scons -j7 p=linuxbsd verbose=yes warnings=extra werror=yes tests=yes LINKFLAGS=-B/usr/local/libexec/mold

Combination	Null rebuild speed	main.cpp rebuild speed
None (fast_unsafe=no)	22.7 - 23.8 s	25.0 - 25.4 s
MD5-timestamp	15.3 - 15.5 s	18.3 - 18.7 s
implicit_cache=1	17.2 - 17.9 s	20.1 - 20.3 s
max_drift=60	14.8 - 15.7 s	17.5 - 17.9 s
MD5-timestamp + implicit_cache=1	10.0 - 11.3 s
MD5-timestamp + max_drift=60	15.3 - 15.8 s
implicit_cache=1 + max_drift=60	9.4 - 10.2 s
MD5-timestamp + implicit-cache=1 + max_drift=60	9.8 - 10.3 s	12.6 - 13.2 s

[paddy-exe]

Edited because I was in the wrong repo branch

With an M1 MacBook Air MacOS 12.2:
build command: scons -j7 platform=osx arch=arm64 verbose=yes warnings=extra werror=yes tests=yes --jobs=$(sysctl -n hw.logicalcpu)
Full build time: [Time elapsed: 00:03:52.830]

Null rebuilds with the same commands:

1. [Time elapsed: 00:00:04.494]
2. [Time elapsed: 00:00:04.450]
3. [Time elapsed: 00:00:04.482]
4. [Time elapsed: 00:00:04.480]
5. [Time elapsed: 00:00:04.465]

With added fast_unsafe=no argument:

1. [Time elapsed: 00:00:16.354]
2. [Time elapsed: 00:00:16.160]
3. [Time elapsed: 00:00:16.257]
4. [Time elapsed: 00:00:16.245]
5. [Time elapsed: 00:00:16.209]

[Faless]

Intel Core i7-7700K CPU
Ubuntu 20.04.3 LTS
Samsung SSD 850 EVO 500GB (ext4)

fast_unsafe=no:

• [Time elapsed: 00:00:17.175]
• [Time elapsed: 00:00:17.065]
• [Time elapsed: 00:00:17.037]

fast_unsafe=yes:

• [Time elapsed: 00:00:07.904]
• [Time elapsed: 00:00:07.861]
• [Time elapsed: 00:00:07.835]

[akien-mga]

@Faless You can add #57820 to it for extra savings and maybe reaching sub-6s null builds :D

[Calinou]

I tested this PR locally:

CPython

❯ hyperfine -w1 "scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes" "scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no"
Benchmark #1: scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes
  Time (mean ± σ):      9.674 s ±  0.566 s    [User: 9.191 s, System: 0.409 s]
  Range (min … max):    8.870 s … 10.413 s    10 runs
 
Benchmark #2: scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no
  Time (mean ± σ):     16.762 s ±  0.588 s    [User: 15.541 s, System: 1.155 s]
  Range (min … max):   16.200 s … 17.888 s    10 runs
 
Summary
  'scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes' ran
    1.73 ± 0.12 times faster than 'scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no'

Pyston

❯ hyperfine -w1 "pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes" "pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no"
Benchmark #1: pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes
  Time (mean ± σ):      6.191 s ±  0.372 s    [User: 5.818 s, System: 0.337 s]
  Range (min … max):    5.859 s …  6.764 s    10 runs
 
Benchmark #2: pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no
  Time (mean ± σ):     13.262 s ±  0.410 s    [User: 12.099 s, System: 1.102 s]
  Range (min … max):   12.605 s … 13.988 s    10 runs
 
Summary
  'pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=yes' ran
    2.14 ± 0.14 times faster than 'pyston-scons platform=linuxbsd use_llvm=yes use_lld=yes -j10 tests=no fast_unsafe=no'

Hardware/software specifications

$ scons --version                                                               
SCons by Steven Knight et al.:
        SCons: v4.3.0.559790274f66fa55251f5754de34820a29c7327a, Tue, 16 Nov 2021 19:09:21 +0000, by bdeegan on octodog
        SCons path: ['/home/hugo/.pyenv/versions/3.9.0/lib/python3.9/site-packages/SCons']
Copyright (c) 2001 - 2021 The SCons Foundation

$ inxi -DCmSP

System:
  Host: fedora Kernel: 5.16.5-100.fc34.x86_64 x86_64 bits: 64
    Desktop: KDE Plasma 5.22.5 Distro: Fedora release 34 (Thirty Four)
Memory:
  RAM: total: 31.3 GiB used: 16.68 GiB (53.3%)
  RAM Report:
    permissions: Unable to run dmidecode. Root privileges required.
CPU:
  Info: quad core model: Intel Core i7-6700K bits: 64 type: MT MCP cache:
    L2: 1024 KiB
  Speed (MHz): avg: 4400 min/max: 800/4400 cores: 1: 4400 2: 4400 3: 4400
    4: 4400 5: 4400 6: 4400 7: 4400 8: 4400
Drives:
  Local Storage: total: 2.27 TiB used: 1.24 TiB (54.3%)
  ID-1: /dev/sda vendor: Samsung model: SSD 860 EVO 1TB size: 931.51 GiB
  ID-2: /dev/sdb vendor: Samsung model: SSD 850 EVO 500GB size: 465.76 GiB
  ID-3: /dev/sdc vendor: Samsung model: SSD 850 EVO 1TB size: 931.51 GiB

[Faless]

You can add #57820 to it for extra savings and maybe reaching sub-6s null builds :D

I'm still on scons 3 , so I always had the compilation DB off 😅 .

[SaracenOne]

Slow build times have been one of the biggest barriers to being able to contribute features which require more iterative development. Any improvements to build speeds are massively appreciated.

[akien-mga]

Cherry-picked for 3.5.

[akien-mga]

Cherry-picked for 3.4.3.