ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.20 Compiler decisions on function inlining
When function inlining is enabled, the compiler uses a complex decision tree to decide if a
function is to be inlined.
The following simplified algorithm is used:
1. If the function is qualified with __forceinline, the function is inlined if it is possible to do
so.
2. If the function is qualified with __inline and the option --forceinline is selected, the
function is inlined if it is possible to do so.
If the function is qualified with __inline and the option --forceinline is not selected, the
function is inlined if it is practical to do so.
3. If the optimization level is -O2 or higher, or --autoinline is specified, the compiler
automatically inlines functions if it is practical to do so, even if you do not explicitly give
a hint that function inlining is wanted.
When deciding if it is practical to inline a function, the compiler takes into account several other
criteria, such as:
• the size of the function, and how many times it is called
• the current optimization level
• whether it is optimizing for speed (-Otime) or size (-Ospace)
• whether the function has external or static linkage
• how many parameters the function has
• whether the return value of the function is used.
Ultimately, the compiler can decide not to inline a function, even if the function is qualified with
__forceinline. As a general rule:
• smaller functions stand a better chance of being inlined
• compiling with -Otime increases the likelihood that a function is inlined
• large functions are not normally inlined because this can adversely affect code density and
performance.
A recursive function is inlined into itself only once, even if __forceinline is used.
5.20.1 See also
Concepts
• Inline functions on page 5-29
• Automatic function inlining and static functions on page 5-32
• Automatic function inlining and multifile compilation on page 5-34
• Restriction on overriding compiler decisions about function inlining on page 5-35.
Reference
Compiler Reference:
• --autoinline, --no_autoinline on page 3-14
• __forceinline on page 5-5
• --forceinline on page 3-40
• __inline on page 5-7
• --inline, --no_inline on page 3-53
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