Inline Expansion
Inline expansion, often referred to as inlining, is a compiler optimization technique where the compiler replaces a function or subroutine call with the actual body of the called routine's code directly at the call site. This eliminates the overhead associated with a function call, such as saving registers, setting up a new stack frame, and branching.
Key Characteristics
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- Compiler Optimization: It is a compile-time optimization performed by the compiler (e.g., IBM Enterprise COBOL, PL/I, C/C++ for z/OS) to improve runtime performance.
- Eliminates Call Overhead: By inserting the called routine's code directly, it removes the instructions required for a traditional call/return sequence, leading to faster execution.
- Potential for Code Size Increase: While improving speed, inline expansion can increase the overall size of the compiled executable module if a routine is inlined at multiple call sites.
- Controlled by Compiler Options: Programmers can typically control inline expansion through specific compiler options (e.g.,
INLINEin COBOL,INLINEpragmas or attributes in C/C++) or by marking functions for inlining. - Scope Limitations: Compilers often have heuristics or limitations on which routines can be inlined (e.g., small routines, routines without complex control flow, or those defined within the same compilation unit).
Use Cases
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- Optimizing Frequently Called Small Routines: Ideal for utility functions or common calculations that are called many times within a loop or performance-critical section, where the call overhead would be significant.
- Performance-Critical Code Paths: Applying inlining to routines within the most frequently executed parts of an application to achieve maximum speed improvements.
- Reducing Branch Penalties: On modern z/Architecture processors, reducing branches can improve instruction pipeline efficiency, and inlining helps by eliminating call/return branches.
- Enhancing Further Optimizations: Once a routine is inlined, the compiler might be able to perform additional optimizations across the expanded code that would not have been possible across a function call boundary.
Related Concepts
Inline expansion is fundamentally linked to compilers and performance tuning. It directly targets the overhead of subroutine and function calls, which are core programming constructs in languages like COBOL and C/C++. While it improves runtime performance, it can indirectly affect the size of load modules and program working storage by increasing the compiled code footprint. It's a specific technique within the broader category of compiler optimizations aimed at generating more efficient machine code for the z/Architecture.
- Profile Before and After: Always profile your application's performance before and after applying inline expansion to ensure it yields the desired performance benefits and doesn't inadvertently increase code size excessively.
- Target Small, Frequently Called Routines: Reserve inline expansion for routines that are genuinely small and called very often. Inlining large or infrequently called routines often results in code bloat without significant performance gains.
- Understand Compiler Heuristics: Be aware of how your specific z/OS compiler (e.g., Enterprise COBOL, C/C++) handles inlining by default and what options are available to control it.
- Monitor Code Size: Keep an eye on the size of your compiled modules. Excessive inlining can lead to larger executables, potentially impacting memory usage or instruction cache efficiency.
- Consider Link-Time Optimization (LTO): For C/C++ on z/OS, explore LTO as it can perform more aggressive inlining across compilation units, offering further optimization opportunities.