How does escape analysis work in the Graal compiler?

Escape analysis determines whether an object allocated inside a method can ever be referenced ("escape") outside that method's scope - by being returned, stored in a field, or passed to another thread.

If Graal proves an object never escapes, it can perform scalar replacement: instead of allocating the object on the heap at all, its fields are broken out into separate local variables (registers or stack slots), completely eliminating the allocation and the GC pressure that would have come with it.

Graal goes further than a simple all-or-nothing analysis with partial escape analysis: an object can be scalar-replaced along the branches of a method where it provably doesn't escape, while still being materialized (allocated normally) only on the less common branches where it does - so the optimization applies even when escape behavior differs by code path.

This matters most for code with lots of small, short-lived helper objects, like builder patterns or boxed values in tight loops, where removing the allocation removes both allocation cost and downstream GC work.