An ARM Backend for PyPyls Tracing JIT - STUPS Group

28 6 A BACKEND FOR PYPY’S JITto be spilled is based on the previously calculated longevity of the variables, spilling thevariable that survives for the longest time. This selection scheme should avoid blockingregisters by linking them to very long lived variables. A possible drawback is althoughif the variable is long-lived and often used, then it could occur that it needs to be readfrequently from memory if spilling occurs between usages. Once a variable is selected forspilling we generate an instruction to move this variable to the spilling area on the stackand store the information about where on the stack the variable is stored in the registerallocator. After moving the variable away we can free the previously bound register andassociate it with a new variable.6.3 Setup to Execute a Compiled LoopApart from compiling a trace, we need a way to actually execute the compiled instructions.This means that we need a mechanism to call into the compiled code once the frontendtries to execute the same loop again. Similar to the approach described in [GFE + 09]we generate code that follows the platform calling conventions (see Section 2.2.3 for howto create a procedure interface [Ear09]) so it can be casted to a function pointer at runtimeand called as a normal function/procedure.The compilation process for a loop is started when the frontend passes an optimized traceto the backend. Before the instructions contained in the trace can be compiled the backendneeds to take some preparing steps to provide an interface to the compiled trace thatcan be executed. This consist of creating a callable interface and generating the instructionsto set up the frame and load the arguments for the loop.6.3.1 Frame LayoutBefore we can execute instruction we need to setup the frame of the procedure. For thiswe generate instructions that create a frame as described below.The frame layout is composed of four parts:• callee saved registers, according to the calling convention• a slot to store the force index, a value used to check if the interpreter level framewas forced (see Section 6.4.6)• an area to store spilled variables• and the stack where push and pop instructions operateThe frame begins with the registers the function has to save according to the calling conventiondescribed in Section 2.2.3, which are on ARM registers r4 up to r11 or FP. Afterthese registers one word on the stack is left to store the force index, see Section 6.4.6.After the location for the force index the frame contains space for spilled register values.The address of the beginning of this area is stored in the FP register and spilled valuesare addressed by their offset from the FP. After this area the classic stack area begins