build: parallelize the qstr build steps #3538
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jepler
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Rather than simply invoking gcc in preprocessor mode with a list of files, use a Python script with the (python3) ThreadPoolExecutor to invoke the preprocessor in parallel. The amount of concurrency is the number of system CPUs, not the makefile "-j" parallelism setting, because there is no simple and correct way for a Python program to correctly work together with make's idea of parallelism. This reduces the build time of stm32f405 feather (a non-LTO build) from 16s to 12s on my 16-thread Ryzen machine.
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This gets a further speedup of about 2s (12s -> 9.5s elapsed build time)
for stm32f405_feather
For what are probably historical reasons, the qstr process involves
preprocessing a large number of source files into a single "qstr.i.last"
file, then reading this and splitting it into one "qstr" file for each
original source ("*.c") file.
By eliminating the step of writing qstr.i.last as well as making the
regular-expression-matching part be parallelized, build speed is further
improved.
Because the step to build QSTR_DEFS_COLLECTED does not access
qstr.i.last, the path is replaced with "-" in the Makefile.
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tannewt
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Oct 12, 2020
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The generation of "qstr defs" involves running the C preprocessor on a selection of source files, extracting pattern matches from them, and writing them out into a single output file. Then, a second sequential step reads all that output to produce individual "qstr" files. Especially with a non-lto build on a powerful desktop computer, these sequential steps actually have a pretty large effect on the overall build time.
The improvements came in two discrete steps; refer to the individual commit messages for a clearer view.
I tested by repeatedly building the feather stm32f405 on my 8 core / 16-thread Ryzen. Before, a build took about 16s. After, a build took about 9.5s. This is elapsed time; CPU time was about 1m12s before and 1m13s after.