The register allocator is a simple first-fit bitmask with no spilling to stack except for the two predefined spill slots. Only if all 8 registers are in use, it spills the additional registers on the stack. What they call guest registers. No SSA and no BB needed. No crazy mem2reg or graph-coloring. Only once per function.
Only for very big functions one register is spilled, usually just rsi.
Benchmarks:
| Compiler | Compile (ms) | Execute (ms) | Total (ms) |
| :------- | -----------: | -----------: | ---------: |
| RCC | 61 | 754 | 815 |
| TCC | 8 | 628 | 636 |
| SLIMCC | 74 | 642 | 716 |
| KEFIR | 270 | 765 | 1035 |
| GCC0 | 83 | 637 | 720 |
| GCCO2 | 204 | 227 | 431 |
| CLANG0 | 377 | 620 | 997 |
| CLANGO2 | 310 | 221 | 531 |