4.4 KiB
Synopsis
A sprite compiler that targets 16bit 65816 assembly code on the Apple IIgs computer. The sprite compiler uses informed search techniques to generate optimal code for wholesprite rendering.
Example
The compiler takes a simple masked, sparse byte sequence which are represented by (data, mask, offset) tuples. During the search, it tracks the state of the 65816 CPU registers in order to find an optimal sequence of operations to generated the sprite data. The space of possible actions are defined by the subclasses of the CodeSequence class.
Currently, the compiler can only handle short, unmasked sequences, but it does correctly find optimal code sequences. Here is a sample of the code that the compiler generates
Data = $11
TCS ; 2 cycles
SEP #$10 ; 3 cycles
LDA #$11 ; 2 cycles
STA 00,s ; 4 cycles
REP #$10 ; 3 cycles
; Total Cost = 14 cycles
Data = $11 $22
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
STA 00,s ; 5 cycles
; Total Cost = 10 cycles
Data = $11 $22 $22
TCS ; 2 cycles
LDA #$2222 ; 3 cycles
STA 02,s ; 5 cycles
LDA #$2211 ; 3 cycles
STA 01,s ; 5 cycles
; Total Cost = 18 cycles
Data = $11 $22 $11 $22
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
STA 00,s ; 5 cycles
STA 02,s ; 5 cycles
; Total Cost = 15 cycles
Data = $11 $22 $33 $44 $55 $66
ADC #5 ; 3 cycles
TCS ; 2 cycles
PEA $6655 ; 5 cycles
PEA $4433 ; 5 cycles
PEA $2211 ; 5 cycles
; Total Cost = 20 cycles
Data = $11 $22 $11 $22 $11 $22 $11 $22
ADC #7 ; 3 cycles
TCS ; 2 cycles
LDA #$2211 ; 3 cycles
PHA ; 4 cycles
PHA ; 4 cycles
PHA ; 4 cycles
PHA ; 4 cycles
; Total Cost = 24 cycles
Data = ($11, 0), ($11, 160), ($11, 320)
A simple sprite three lines tall.
TCS ; 2 cycles
SEP #$10 ; 3 cycles
LDA #$11 ; 2 cycles
PHA ; 3 cycles
STA A1,s ; 4 cycles
REP #$10 ; 3 cycles
TSC ; 2 cycles
ADC #321 ; 3 cycles
TCS ; 2 cycles
SEP #$10 ; 3 cycles
LDA #$11 ; 2 cycles
PHA ; 3 cycles
REP #$10 ; 3 cycles
; Total Cost = 35 cycles
Limitations

The search is quite memory intensive and grows too fast to handle multiline sprite data yet. Future versions will incorporate more aggressive heuristic and Iterative Deepening AStar search to mitigate the memory usage.

Carry Bit Tracking. If the stack is moved backwards, the code does not track that the carry is now set, so the next ADC instruction will be off by one. Proper carry bit tracking should remove the need for any CLC/SEC instructions.

High/Low Register States. The state of a register (A/X/Y/S/D) is itentified to be in one of three states: IMMEDIATE, SCREEN_OFFSET, UNINITIALIZED. There are cases where high and low bytes of a register can be in a different state and the compiler does not track this scenario. Ex.
TCS ; A = SCREEN_OFFSET
SEP #$10 ; Endable 8bit accumulator
LDA #$11 ; Low Byte = IMMEDIATE / High Byte = SCREEN_OFFSET
Currently, the accumulator will be marked as having an IMMEDIATE value of $0011, rather than the intederminant value of $11. If there are 16bit data values equal to $0011, then the compiler could emit incorrect code.

Heuristic is too ooptimistic. The Heuristic function does not take into account the cost of masked data stores, stack movement, or switrched between 8bit and 16bit modes. As such, it is too optimistic in many cases and causes the effectrive branching factor of the search to become too large. For example, one test case has a true cost of 37 cycles, but the heuristic estimated a total cost of 19 cycles from the initial state.

Successor function allows nonproductive moves. Specifically, it allows a switch to/from 8/16bit mode at any point, e.g. SEP/REP instructions. As such, the expander will propose long sequences of SEP/REP/SEP/REP/SEP/REP/... Since each instruction is cheaper than any code that actually writes data, many states are expanded that a trivially redundent until their cumulative cost is greater that a store. This can actually be quite high for 16bit masked stores (16 cycles), which is 5 SEP/REP expansions.
License
MIT License