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Try harder to not recompile sketch without modifications. #2961

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@cmaglie cmaglie commented Jul 29, 2025

Please check if the PR fulfills these requirements

See how to contribute

  • The PR has no duplicates (please search among the Pull Requests
    before creating one)
  • The PR follows
    our contributing guidelines
  • Tests for the changes have been added (for bug fixes / features)
  • Docs have been added / updated (for bug fixes / features)
  • UPGRADING.md has been updated with a migration guide (for breaking changes)
  • configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Improves the checks made to the sketch to avoid useless recompilation and preprocessing.

What is the current behavior?

The sketch source code, before being compiled, undergoes a two-phase processing:

  1. If the sketch has multiple source files, these files are merged, forming a single .cpp file.
  2. The merged .cpp file is then "preprocessed" to automatically obtain the forward declarations of the functions defined in the sketch.

Previously, these two operations were always performed, touching the final sketch.cpp. This resulted in sketch.cpp being recompiled every time. Moreover, the library discovery cache could not be exploited because of the sketch.cpp timestamp change.

What is the new behavior?

The merged sketch file is saved in a separate sketch.cpp.merged file, and it is compared to the newly generated content. If they match, the already preprocessed sketch.cpp and sketch.cpp.merged are left untouched.
This is enough for the current build system to recognize that the file is already up-to-date and skip both compilation and library discovery, which greatly improves compile times.

Does this PR introduce a breaking change, and is titled accordingly?

Unless I'm missing some convoluted case, it shouldn't.

Other information

Ivoz reacted with thumbs up emoji
@cmaglie cmaglie self-assigned this Jul 29, 2025
@cmaglie cmaglie added type: enhancement Proposed improvement topic: build-process Related to the sketch build process labels Jul 29, 2025
@cmaglie cmaglie force-pushed the do-not-recompile-sketch branch 2 times, most recently from 18335b6 to aa33373 Compare July 29, 2025 15:33
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codecov bot commented Jul 29, 2025 •
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Codecov Report

❌ Patch coverage is 75.67568% with 54 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@3eecf20). Learn more about missing BASE report.
⚠️ Report is 3 commits behind head on master.

Files with missing lines Patch % Lines
...l/arduino/builder/internal/detector/source_file.go 54.54% 19 Missing and 6 partials ⚠️
internal/arduino/builder/internal/utils/utils.go 56.52% 7 Missing and 3 partials ⚠️
...nternal/arduino/builder/internal/detector/cache.go 40.00% 4 Missing and 2 partials ⚠️
...rnal/arduino/builder/internal/detector/detector.go 81.48% 3 Missing and 2 partials ⚠️
...nal/arduino/builder/internal/preprocessor/ctags.go 71.42% 1 Missing and 3 partials ⚠️
internal/arduino/builder/sketch.go 66.66% 2 Missing and 1 partial ⚠️
...lder/internal/preprocessor/arduino_preprocessor.go 0.00% 1 Missing ⚠️
Additional details and impacted files 
@@ Coverage Diff @@
## master #2961 +/- ##
=========================================
 Coverage ? 68.31% 
=========================================
 Files ? 242 
 Lines ? 22831 
 Branches ? 0 
=========================================
 Hits ? 15596 
 Misses ? 6029 
 Partials ? 1206
Flag Coverage Δ
unit 68.31% <75.67%> (?)

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@per1234 per1234 added the topic: code Related to content of the project itself label Aug 17, 2025
@cmaglie cmaglie force-pushed the do-not-recompile-sketch branch 2 times, most recently from aa41179 to 1ca362c Compare August 25, 2025 14:16
cmaglie added 10 commits September 2, 2025 17:42 
During the library detection phase, we do not have objects files yet,
the only thing that we should check is if the dependency file is
up-to-date.
In this commit we let GCC create a dep file during the library discovery
phase.
We must ensure that the build-path folder exists otherwise the compiler
will not be able to create the dep file.
@cmaglie cmaglie force-pushed the do-not-recompile-sketch branch from 1ca362c to f32315f Compare September 2, 2025 17:49
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Successfully merging this pull request may close these issues.

Do not recompile the sketch if there are no changes and libraries have not changed Sketch re-compiled when unnecessary
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