Single Compilation Unit

Single Compilation Unit (SCU) is a computer programming technique for the C and C++ languages, which reduces serial compilation time and allows the compiler to perform certain program optimizations even when the compiler itself is lacking support for whole program optimization. The technique can be applied to an entire program or to some subset of source files; when applied to an entire program, it is also known as a unity build.

In the C/C++ compilation model (formally "translation environment"), individual .c/.cpp source files are preprocessed into translation units, which are then translated (compiled) separately by the compiler into multiple object (.o or .obj) files. These object files can then be linked together to create a single executable file or library. However, this leads to multiple passes being performed on common header files, and with C++, multiple template instantiations of the same templates in different translation units.

The Single Compilation Unit technique uses pre-processor directives to "glue" different translation units together at compile time rather than at link time. This reduces the overall build time, due to eliminating the duplication, but increases the incremental build time (the time required after making a change to any single source file that is included in the Single Compilation Unit), due to requiring a full rebuild of the entire unit if any single input file changes. Therefore, this technique is appropriate for a set of infrequently modified source files with significant overlap (many or expensive common headers or templates), or source files that frequently require recompilation together, such as due to all including a common header or template that changes frequently.

Another disadvantage of SCU is that it is serial, compiling all included source files in sequence in one process, and thus cannot be parallelized, as can be done in separate compilation (via distcc or similar programs). Thus SCU requires explicit partitioning (manual partitioning or "sharding" into multiple units) to parallelize compilation.

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