Building QET is dominated by re-parsing Qt's headers. A 214-line source
file expands to roughly 198,000 preprocessed lines, and compiling one
translation unit costs ~4.1 s, of which only ~0.35 s is optimisation --
switching -O3 to -O0 saves just 8%, so the usual "build Debug for faster
compiles" advice does not help here. A precompiled header caches the
parsed header state, which is the part that actually costs.
Measured on a 24-thread Xeon E5-2650 v4 with Qt 5.15.18 and GCC 15.2,
same build tree, only the option differing:
compile one translation unit 4.12 s -> 1.21 s
edit one .cpp -> linked binary 5.22 s -> 1.65 s
Deliberately OFF by default. A PCH satisfies includes that a source file
neglected to make for itself, so code written with it enabled can fail to
compile for everyone else. Leaving the default off keeps CI and
contributors on the strict behaviour; only developers who opt in trade
that away for the speed.
Two details in the implementation are load-bearing:
- The generator expressions are not decoration. This target also compiles
the 18 C files of the bundled LZMA decoder, and an unguarded header list
applies to every language in the target, so the Qt headers would be fed
to the C compiler and fail with "unknown type name 'namespace'".
$<ANGLE-R> is needed because a literal '>' would end the generator
expression.
- target_precompile_headers() requires CMake 3.16 while the project still
declares a 3.5 minimum, so the block warns and skips rather than raising
the project-wide requirement for an opt-in developer feature.
Verified both ways: with the option off no PCH artefacts are generated and
the build is byte-for-byte the previous behaviour; with it on, all 18 C
files still compile, the generated PCH is C++-only (cmake_pch.hxx, with no
cmake_pch.h), the C compile commands carry no PCH, and the resulting
binary runs.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
ispyisail