asm: implement write-only operands #10759
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This change adds support to the new assembler for write-only operands. This implementation appeared first in [bytecodealliance#10754] but is split out here to unblock implementation of instructions that require it: multiplication, conversions, moves, etc. This starts roughly the same as what was implemented for write-only XMMs in [bytecodealliance#10754] but includes support for write-only GPRs as well and generates the temporary registers which are needed. [bytecodealliance#10754]: bytecodealliance#10754 Co-authored-by: Johnnie Birch <[email protected]>
alexcrichton
approved these changes
May 9, 2025
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| Write => match one.location.kind() { | ||
| // One write-only register output? Output the | ||
| // instruction and that register. | ||
| OperandKind::Reg(r) => { | ||
| let ty = r.reg_class().unwrap().to_string(); | ||
| let var = ty.to_lowercase(); | ||
| fmtln!(f, "let {var} = {r}.as_ref().clone();"); | ||
| fmtln!(f, "AssemblerOutputs::Ret{ty} {{ inst, {var} }}"); | ||
| } | ||
| _ => unimplemented!(), | ||
| }, |
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Should this get collapsed into ReadWrite as well below? This has the same handling for Reg and all the other cases look applicable for one-operand-written instructions too (e.g. if something stores in memory or outputs to a fixed register)
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Unfortunately, there's slightly different behavior for retrieving the underlying Cranelift type. The ReadWrite variant needs to access the write field; this one should actually be doing a as_ref().to_reg() (let me fix that). I think in the fullness of time we will want to merge some things like I did over in the assembler generator but let's leave this as-is until we have a few more write-only instructions implemented.
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This change adds support to the new assembler for write-only operands. This implementation appeared first in #10754 but is split out here to unblock implementation of instructions that require it: multiplication, conversions, moves, etc. This starts roughly the same as what was implemented for write-only XMMs in #10754 but includes support for write-only GPRs as well and generates the temporary registers which are needed.