Two layers, the SPIR-V analog of an ISA's mnemonic builders:
- low level: inst_<OpName>(buf, ...) -> Operation, stateless + alloc-free, the
caller owns the operand backing (SPIR-V operands are a slice, so unlike an
ISA's inline [4]Operand they can't be owned by the return value).
- high level: a Builder owning operand storage + a result-<id> allocator;
iadd/load/store/call/variable/ret/ret_value append to the current block and
return the new <id>.
Hand-written here for a representative slice (Id / no-result / variadic / enum
operands) to fix the pattern; tablegen will generate the full per-opcode set.
Validated: a builder-made function body round-trips byte-exact.
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