Changes to the most accessed headers require recompilation of 500 files avg, which takes about 3 mins on 6 cores. The time tends to grow YoY.
Several of build time improvements may be approached:
Header decomposition to improve files affected per line value.
Use of precompiled headers
Template optimization
This task is about the headers problem investigation.
Take a look how a time changes with precompiled headers enabled.
Work out the guidelines on enabling the precompiled headers generation for used system libraries.
And/or update build scripts to support precompiled headers generation.
If the build will still look too slow, analyze the template usages.
so far precompiling the headers reduces the build time about 5 times:
for ninja minbuild with PFS disabled it dropped from 2:30 to 0:30!
The major disadvantage for this method is that all the headers precompiled are stored in one precompiled file which is forcefully pre-included in every build.
Thus, by using PCH, one may end up with using symbols from the headers that are not actually included.
The alternative is of course c++20 submodules, which does the same, but smart.
The another alternative is GCC which handles .gch files differently: if the precompiled file is available, it is being used, or else the actual header is taken.
So for now I can try to use the alternative way for GCC, but the rest of compilers will remain screwed at either build speed or dependency integrity.
Nikita Malyavin
added a comment - - edited so far precompiling the headers reduces the build time about 5 times:
for ninja minbuild with PFS disabled it dropped from 2:30 to 0:30!
The major disadvantage for this method is that all the headers precompiled are stored in one precompiled file which is forcefully pre-included in every build.
Thus, by using PCH, one may end up with using symbols from the headers that are not actually included.
The alternative is of course c++20 submodules, which does the same, but smart.
The another alternative is GCC which handles .gch files differently: if the precompiled file is available, it is being used, or else the actual header is taken.
So for now I can try to use the alternative way for GCC, but the rest of compilers will remain screwed at either build speed or dependency integrity.
Can it be cmake config option turned off by default?
It would be also good if configuration could choose between gcc/clang for compilation, between ld/lld for linker and whether to ccache or not.
Aleksey Midenkov
added a comment - - edited Can it be cmake config option turned off by default?
It would be also good if configuration could choose between gcc/clang for compilation, between ld/lld for linker and whether to ccache or not.
Only one precompiled header can be used in a particular compilation.
At the same time, c++20 modules support is released both in gcc and clang just not far from now.
GCC:
Module support can be enabled standard-independently, by -fmodules-ts
Moreover, -fmodule-header can help treating header as a module, looking forward to see what it's capable of.
Nikita Malyavin
added a comment - - edited GCC's gch approach has no benefit – it also can only use a single gch per invocation .
Only one precompiled header can be used in a particular compilation.
At the same time, c++20 modules support is released both in gcc and clang just not far from now.
GCC:
Module support can be enabled standard-independently, by -fmodules-ts
Moreover, -fmodule-header can help treating header as a module, looking forward to see what it's capable of.
On integration with cmake: https://www.kitware.com/import-cmake-c20-modules/
Automated message:
----------------------------
Since this issue has not been updated since 6 weeks, it's time to move it back to Stalled.
Julien Fritsch
added a comment - Automated message:
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Since this issue has not been updated since 6 weeks, it's time to move it back to Stalled.
Automated message:
----------------------------
Since this issue has not been updated since 6 weeks, it's time to move it back to Stalled.
JiraAutomate
added a comment - Automated message:
----------------------------
Since this issue has not been updated since 6 weeks, it's time to move it back to Stalled.
Task
Major
so far precompiling the headers reduces the build time about 5 times:
for ninja minbuild with PFS disabled it dropped from 2:30 to 0:30!
The major disadvantage for this method is that all the headers precompiled are stored in one precompiled file which is forcefully pre-included in every build.
Thus, by using PCH, one may end up with using symbols from the headers that are not actually included.
The alternative is of course c++20 submodules, which does the same, but smart.
The another alternative is GCC which handles .gch files differently: if the precompiled file is available, it is being used, or else the actual header is taken.
So for now I can try to use the alternative way for GCC, but the rest of compilers will remain screwed at either build speed or dependency integrity.