The register allocator
Registry allocation in FPC
Introduction
Main unit for the registry allocation is rgobj.pas
Main class for the registry allocation is TRgObj located in rgobj.pas
Registry allocator provides imaginary registers for the assembler instructions during the code generation. Then it calculates the real registers to replace the imaginary ones.
The fpc registry allocator uses the Registry coloring for determining the real registers. Also uses registry spilling technique - when there is not enough registers it uses the memory.
How to use the registry allocator
This topic describes how to use the registry allocator during the code generation.
Described like a black box with the public methods you can call to get job done.
Creating the Registry allocator
Creating registry allocator is the first step we make before we can use its functionality.
The low level code generator creates instances of TRgObj class.
They are created when the code generation of specific routine begins.
Code generator works on subroutine level.
The registry allocator also allocates registers for specific method, procedure, function.
One TRgObj instance allocates registers of certain type. For example Integer registers. Thats way we have a few TRgObj instances, one for every type of register that the cpu supports.
Using registers in the code generation
Code generator uses:
TRgObj.getRegister
to get register for some assembler instruction. This way an imaginary register is allocated and can be used after that in specific assembler instruction.
Additionally, the following methods:
getcpuregister ungetcpuregister alloccpuregisters dealloccpuregisters
allocate or free one or multiple 'real' registers (not imaginary ones!) This is usually used for instructions that require a specific register (for example SHL/SHR/SAR on x86, which always uses ECX). This is also used when doing a function call to indicate that certain registers (which depend on the calling convention) may be destroyed by the called function.
There is another method if more than one imaginary register needs to be allocated - TRgObj.allocCpuRegisters
After allocating the register we can use it in some assembler instructions. For every instruction that generates, the code generator notifies the registry allocator. It passes the instruction, also the imaginary register as parameters to the following method.
TRgObj.add_reg_instruction(instr, r, cg.executionweight);
But for MOV instruction there is specific method that is used
TRgObj.add_move_instruction(instr:Taicpu);
If an imaginary register is allocated, but later it becomes useless it can be deallocated by using the following methods
ungetcpuregister - Free register specified. dealloccpuregisters - Free multiple registers specified.
Generating of real registers
At the end when all the assembler instructions are generated we call
do_register_allocation(list: TAsmList; headerTai: TAi)
It calculates real registers for the imaginary ones.
Tips
You can compile your project by using the -sr switch. This will leave the immaginary register names in the generated .s file
Calls hierarchy for the public methods
Public
constructor
destructor
do_register_allocation - level 1 is ordered by calling
insert_regalloc_info_all
generate_interference_graph
add_edges_used(1, 2) get_alias add_edge
add_edge ibitmap.s
prepare_colouring
make_work_list ri_coalesced
sort_simplify_worklist
colour_registers
simplify ri_coalesced
decrement_degree ri_coalesced
coalesce get_alias simplifyworklist.add(v); ibitmap
conservative ri_coalesced
adjacent_ok ri_coalesced ibitmap
add_worklist simplifyworklist.add(u);
combine ibitmap, add_edge
enable_moves
decrement_degree ri_coalesced simplifyworklist.add(m)
add_edge ibitmap.s
freeze
freeze_moves(, 2) get_alias(3)
select_spill
freeze_moves(, 2) get_alias(3)
assign_colours(, 2) get_alias
epilogue_colouring Destroys the objects used during the coloring - worklist_moves, active_moves, frozen_moves, coalesced_moves, constrained_moves, reginfo.movelist
spill_registers
clear_interferences ibitmap.s
instr_spill_register get_alias
getregisterinline
add_edges_used(2, 2) get_alias
add_edge ibitmap.s
ungetregisterinline
get_spill_subreg
do_spill_replace
do_spill_read
do_spill_written
translate_registers
assign_colours(, 2) get_alias
getregister
add_move_instruction
add_to_movelist
combine ri_coalesced(s)
enable_moves
decrement_degree ri_coalesced
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Properties
live_range_direction
set_live_range_direction
live_start
get_live_start
set_live_start
live_end
get_live_end
set_live_end
Class TRgObj
Description
Main class for registry allocation in FPC
Public methods
Protected methods
Private methods
Imaginary registers
Storing imaginary registers
TRgObj stores the imaginary registers in list of TRegInfo structure.
Life of imaginary register
Life of imaginary register starts when it is first used by assembler instruction.
When it first appears in an assembler instruction in the assembler list for the specific routine.
And ends when it is last used in an assembler instruction.
In the TRegInfo structure for the specific imaginary register there are 2 fields for the life of the register.
live_start: TAi; indicates which is the assembler instruction where the register is first used.
live_end: TAi; indicates which is the assembler instruction where the register is last used.