What is the size of methods that JIT automatically inlines?

Question

I've heard that JIT automatically inlines small methods, like getters (they have about 5 bytes). What is the boundary? Is there any JVM flag?

Answer 1

HotSpot JIT inlining policy is rather complicated. It involves many heuristics like caller method size, callee method size, IR node count, inlining depth, invocation count, call site count, throw count, method signatures etc.

Some limits are skipped for accessor methods (getters/setters) and for trivial methods (bytecode count less than 6).

The related source code is mostly in bytecodeInfo.cpp.
See InlineTree::try_to_inline, should_inline, should_not_inline functions.

The main JVM flags to control inlining are

-XX:MaxInlineLevel (maximum number of nested calls that are inlined)
-XX:MaxInlineSize (maximum bytecode size of a method to be inlined)
-XX:FreqInlineSize (maximum bytecode size of a frequent method to be inlined)
-XX:MaxTrivialSize (maximum bytecode size of a trivial method to be inlined)
-XX:MinInliningThreshold (min. invocation count a method needs to have to be inlined)
-XX:LiveNodeCountInliningCutoff (max number of live nodes in a method)

Answer 2

https://docs.oracle.com/javase/8/embedded/develop-apps-platforms/codecache.htm#BABGGHJE

MaxInlineSize

Default: 35

Maximum bytecode size of a method to be inlined

See the document Java HotSpot VM Options for JDK 7 and earlier releases.