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How Can GCC's Precompiled Headers Be Effectively Utilized Despite Their Limitations?

Susan Sarandon

Dec 15, 2024 am 03:21 AM

How Can GCC's Precompiled Headers Be Effectively Utilized Despite Their Limitations?

GCC and Precompiled Headers: A Deep Dive

Challenges with Precompiled Headers

Precompiled headers offer performance benefits by storing preprocessed header files in binary format. However, they introduce certain challenges in real-life scenarios:

Defining Macros in .cpp Files: When a macro is defined in a .cpp file that affects how headers are interpreted in the precompiled header, the header needs to be rebuilt.
Including Headers with Preprocessor Directives: If a header is included in a .cpp file that defines preprocessor directives altering the interpretation of an already included header in the precompiled header, rebuilding becomes necessary. This issue can become more complex with recursive header inclusions.
Strict Coding Practices: To avoid these issues, precompiled header usage may necessitate restrictive coding practices, such as limiting the number of headers included in .cpp files and prohibiting macro definitions in .cpp files.

GCC's Limitations

In contrast to Microsoft's compiler, GCC's precompiled header functionality requires manual effort and creativity in the Makefile. Detecting when precompiled headers need to be rebuilt can be challenging, especially when working with multiple libraries.

GCC 4.7 (and earlier) Behavior

For optimal performance in GCC 4.7 and earlier, it is crucial to have a single common header, yourapp.h, that includes all necessary system and application headers. This header should be the first non-comment lexeme in every source file. All preprocessing options (-D, -I, -U) should be consistent across all sources.

Future Developments

The PreParsed Header (pph) branch aimed to improve GCC's precompiled header handling, but it was abandoned in favor of a future standardization of a module mechanism in C . Such a mechanism has not yet been incorporated into the C standard.

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