Inline Caching in HotSpot VM

The class CompiledIC represents a call-site with an inline cache.

cpu/x86/vm/LIR_Assembler_x86.cpp: emit_call() emits the machine code for a call.

ic_call() generates the native code for a virtual call with inline cache.

Clean state

This is the initial state for a compiled call-site.

The code generated is
mov imm32, eax
call _resolve_virtual_call_C

Initially this imm32 value is a no-op. When the site becomes monomorphic this is set to the expected class.

Monomorphic state

share/vm/runtime/SharedRuntime.cpp:1268
The _resolve_virtual_call_C function gets the receiver (this) and the method handle by inspecting the stack and relevant bytecodes. This code can be found in SharedRuntime:: resolve_sub_helper(). The state of the IC is then made monomorphic by CompiledIC:: set_to_monomorphic(). set_to_monomorphic() patches the mov and call instruction. The imm32 value is a pointer to class handle and the target of the call instruction is the address of the method implementation for that class.

Megamorphic state

When a cache miss occurs the inline cache enters megamorphic state where a full vtable lookup is performed.

The code which checks for a cache miss is generated by check_icache(). This code is present in all non-static methods. This code loads this->class and compares it with value in eax. If they are not equal, it jumps to handle_wrong_method_ic_miss() which calls handle_ic_miss_helper() which performs the vtable lookup, patches the call-site to make it megamorphic and call the actual method.

For a megamorphic call-site, the eax contains method handle and the call target is a vtable lookup routine. The vtable lookup routine can use the "this" argument and the method handle to jump to the correct method.

Notes on Implementation

The remaining discussion sketches the call-site code at various times.

Clean state

mov #method, eax
call setup_ic

Monomorphic state

mov #class, eax
call method

Megamorphic state

mov #method, eax
call vtable_lookup

setup_ic:
Lookup vtable
if method not yet compiled {
Jump to trampoline
/* Leave the call-site untouched */
} else {
Patch the call-site
Jump to the method
}

vtable_lookup:
jump to this->class->vtable->native_ptr[i]

inline_cache_check:
cmp eax, this->class
jne inline_cache_miss

inline_cache_miss:
Patch call-site to megamorphic
Lookup vtable and jump to it