Prefer caller-save registers and emit callee-save clobber comments#437
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dop-amin wants to merge 1 commit into
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Prefer caller-save registers and emit callee-save clobber comments#437dop-amin wants to merge 1 commit into
dop-amin wants to merge 1 commit into
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* Add `callee_saved_registers()` to Arm v8.1-M/Helium and RISC-V ISA models * Add `config.constraints.prefer_caller_save_registers`: biases allocation toward input registers and away from callee-saved ones via CP-SAT hints and a secondary minimisation objective (never sacrifices stall counts) * Add `config.emit_clobbered_callee_saves_comment` (default True): prepends a comment listing clobbered callee-saved registers to the output * Accumulate clobber set across preamble, kernel, and postamble in `Heuristics.periodic`; store on `Result` * Add four AArch64 naive tests covering the new features including a forced callee-save allocation case and a SW-pipelining path
mkannwischer
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Jun 24, 2026
mkannwischer
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mkannwischer
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Thanks @dop-amin. Looks like a useful feature! It's not very clear how it interacts with other secondary objectives to me. Have you considered that?
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| Requires the ISA model to define callee_saved_registers(). When the | ||
| ISA model does not provide this method the option has no effect. |
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Can we make it error instead? Sounds easy to misuse otherwise.
| used = set() | ||
| for t in self._get_nodes(): | ||
| used.update(t.inst.args_out) | ||
| used.update(t.inst.args_in) |
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Why are args_in considered clobbered?
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| # Secondary objective: prefer caller-save registers. | ||
| # This runs only in the force_objective (step-2 re-optimization) path, | ||
| # where stalls are already fixed via a hard constraint. | ||
| if force_objective and self._model._callee_saved_used and len(maxlist) == 0: | ||
| callee_vars = list(self._model._callee_saved_used.values()) | ||
| if len(minlist) == 0: | ||
| minlist = callee_vars | ||
| name = "minimize callee-saved register usage" | ||
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What happens here if you combine it with another secondary objective, e.g., maximize_register_lifetimes?
| if not (self.config.emit_clobbered_callee_saves_comment and clobbered): | ||
| return code | ||
| comment_text = "Clobbered callee-saved registers: " + ", ".join( | ||
| sorted(clobbered) |
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sorting lexicographically is probably not what you want to do here. Can we instead use a more natural order?
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callee_saved_registers()to Arm v8.1-M/Helium and RISC-V ISA modelsconfig.constraints.prefer_caller_save_registers: biases allocation toward input registers and away from callee-saved ones via CP-SAT hints and a secondary minimisation objective (never sacrifices stall counts)config.emit_clobbered_callee_saves_comment(default True): prepends a comment listing clobbered callee-saved registers to the outputHeuristics.periodic; store onResult