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{{short description|Subroutines in computer science}}
A '''leaf subroutine''' is a [[subroutine]] which cannot in turn call another subroutine. Some compilers can apply special program optimizations to leaf subroutines to make them more efficient, such as the use of [[link register]]s to avoid having to push the return address on the stack, or not allocating a [[register window]] on CPU architectures descended from [[Berkeley RISC]].<ref>{{cite web|url=https://gcc.gnu.org/onlinedocs/gccint/Leaf-Functions.html|title=Leaf Functions (GCC (GNU Compiler Collection) Internals Manual)}}</ref>
The term "leaf" refers to their position as [[leaf node]]s in the [[call graph]] of the program.
Usually, most non-leaf subroutines call more than one other subroutine. When this is the case, the majority of subroutine calls in the call graph are calls to leaf subroutines, because a [[binary tree]] has more leaf nodes than non-leaf nodes (assuming that all non-leaf nodes have two children). Consequently, the efficiency of calls to leaf subroutine often has a significant effect on the efficiency of the whole program. {{fact}}
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