AMD x86 manual Use Function Inlining, Overview

22007E/0 — November 1999 AMD Athlon™ Processor x86 Code Optimization

Use Function Inlining

Overview

Make use of the AMD Athlon processor’s large 64-Kbyte instruction cache by inlining small routines to avoid procedure-call overhead. Consider the cost of possible increased register usage, which can increase load/store instructions for register spilling.

Function inlining has the advantage of eliminating function call overhead and allowing better register allocation and instruction scheduling at the site of the function call. The disadvantage is decreasing code locality, which can increase execution time due to instruction cache misses. Therefore, function inlining is an optimization that has to be used judiciously.

In general, due to its very large instruction cache, the AMD Athlon processor is less susceptible than other processors to the negative side effect of function inlining. Function call overhead on the AMD Athlon processor can be low because calls and returns are executed at high speed due to the use of prediction mechanisms. However, there is still overhead due to passing function arguments through memory, which creates STLF (store-to-load-forwarding) dependencies. Some compilers allow for a reduction of this overhead by allowing arguments to be passed in registers in one of their calling conventions, which has the drawback of constraining register allocation in the function and at the site of the function call.

In general, function inlining works best if the compiler can utilize feedback from a profiler to identify the function call sites most frequently executed. If such data is not available, a reasonable heuristic is to concentrate on function calls inside loops. Functions that are directly recursive should not be considered candidates for inlining. However, if they are end-recursive, the compiler should convert them to an iterative equivalent to avoid potential overflow of the AMD Athlon processor return prediction mechanism (return stack) during deep recursion. For best results, a compiler should support function inlining across multiple source files. In addition, a compiler should provide inline templates for commonly used library functions, such as sin(), strcmp(), or memcpy().

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