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Loop Unrolling Transformation: Difference between revisions
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→Loop unroll pragmas
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== Loop Unrolling in C/C++ Compilers == | == Loop Unrolling in C/C++ Compilers == | ||
Most modern C/C++ compilers apply automatic loop unrolling when aggressive optimization levels (such as | Most modern C/C++ compilers apply automatic loop unrolling when aggressive optimization levels (such as -O2 or -O3) are enabled. The decision to unroll a loop depends on factors such as loop trip count, loop body complexity, and potential performance gains. Compilers analyze loops to identify cases where unrolling reduces overhead or exposes further optimization opportunities, such as vectorization. | ||
In addition to automatic unrolling, developers can explicitly influence unrolling behavior through compiler-specific pragmas. These pragmas allow developers to: | In addition to automatic unrolling, developers can explicitly influence unrolling behavior through compiler-specific pragmas. These pragmas allow developers to: | ||
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Pragmas are compiler directives that influence how a compiler processes specific sections of code, such as loops. They offer direct control over transformations like loop unrolling, bypassing the compiler's default heuristics. This allows developers to optimize for performance or code size, depending on the application requirements. Below are pragma options available for different compilers. | Pragmas are compiler directives that influence how a compiler processes specific sections of code, such as loops. They offer direct control over transformations like loop unrolling, bypassing the compiler's default heuristics. This allows developers to optimize for performance or code size, depending on the application requirements. Below are pragma options available for different compilers. | ||
==== '''Generic Pragmas (Applicable across multiple compilers)''' ==== | |||
* <code>#pragma unroll(n)</code> — Requests the compiler to unroll the loop by a factor of n. | |||
* <code>#pragma nounroll</code> — Explicitly disables unrolling for the annotated loop. | |||
* | ==== Clang Pragmas ==== | ||
* | * <code>#pragma clang loop unroll(enable)</code> — Enables loop unrolling. | ||
* <code>#pragma clang loop unroll(disable)</code> — Disables loop unrolling. | |||
* <code>#pragma clang loop unroll(full)</code> — Requests full unrolling of the loop. | |||
* <code>#pragma clang loop unroll_count(4)</code> — Specifies an unroll factor of 4. | |||
==== GCC Pragmas ==== | |||
* <code>#pragma GCC unroll</code> — Enables automatic unrolling. | |||
* <code>#pragma GCC nounroll</code> — Prevents any unrolling. | |||
* <code>#pragma GCC unroll(UNROLLCOUNT)</code> — Requests a specific unroll factor. | |||
* | |||
* | |||
* | |||
==== OpenMP Pragmas ==== | |||
OpenMP 5.0 introduced loop unrolling pragmas to allow explicit unrolling in parallel programs: | OpenMP 5.0 introduced loop unrolling pragmas to allow explicit unrolling in parallel programs: | ||
* | * <code>#pragma omp unroll</code> — Enables unrolling with default heuristics. | ||
* | * <code>#pragma omp unroll full</code> — Requests full unrolling. | ||
* | * <code>#pragma omp unroll partial</code> — Enables partial unrolling. | ||
* | * <code>#pragma omp unroll partial(3)</code> — Specifies an unroll factor of 3. | ||
These pragmas give developers flexibility to tailor loop transformations based on hardware characteristics (e.g., cache size, vector register width) or software constraints (e.g., real-time requirements or binary size limits). | These pragmas give developers flexibility to tailor loop transformations based on hardware characteristics (e.g., cache size, vector register width) or software constraints (e.g., real-time requirements or binary size limits). | ||