fix(common): set consistent timestamps on template-generated files

[matejcik]

this helps the build system keep track of whether anything has actually changed

the important part is lines 154-156, the rest is fluff

cc @cepetr, I suspect that the bad rebuild times with scons could be helped a lot by finding & fixing bugs like this one

[matejcik]

comment from 795d14c:

some pretty safe & reasonable defaults to speed up SCons:

• look at timestamps to figure out whether a file has changed
• but only if the timestamp is older than 10 secs (which should avoid any problems with 1s precision of mtime, as well as clock drifts etc)
• implicit cache is something to do with parsing C files for dependencies

see https://github.com/SCons/scons/wiki/GoFastButton

[prusnak]

This is not portable, nproc does not exist on macOS and most probably other non-GNU systems

Quick fix would be:

Suggested change

	JOBS = $(shell nproc)
	JOBS = $(shell nproc || echo 4)

[matejcik]

would this work? sysctl -n hw.ncpu
i'm happy to do a switch for darwin for this

[prusnak]

Yes, it works on Darwin, but it is not ideal for Apple Silicon machines. Apple Silicon machines are ARM big.LITTLE and nproc reports the number of all physical cores (both Performance and Efficiency), but the compilation performs better if only number of only Performance cores is used.

What I've seen in other projects was they use the following (in order of priority):

• hw.perflevel0.physicalcpu
• hw.physicalcpu
• hw.ncpu

which translates to

sysctl -n hw.perflevel0.physicalcpu || sysctl -n hw.physicalcpu || sysctl -n hw.ncpu

If we want to be 100% perfect, we could use this beast:

sysctl -n hw.perflevel0.physicalcpu 2>/dev/null || sysctl -n hw.physicalcpu 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || nproc 2>/dev/null || echo 2

[matejcik]

5aba843

[cepetr]

I've done some evaluation (on my machine), and here are results:

build type	before	after
build_unix / all files	25.5s	19s
build_unix / no changes	2.5s	2.5s
build_unix / 1 .c file changed	11.5s	2.5s
build_unix / 1 .rs file changed	3s	3 s
build_firmware / all files	38s	35s
build_firmware / no changes	10.5s	2.5s
build_firmware / 1 .c file changed	19.5s	4s
build_firmware / 1 .rs file changed	11s	10s

It looks great.

changes implemented