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docs: Add section for handling void instructions

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Signed-off-by: Ronald Caesar <github43132@proton.me>
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Ronald Caesar
2025-12-27 03:02:56 -04:00
parent 1ea02d4ba9
commit c8aaded86f
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src/IR_DESIGN_DOC.md
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@@ -412,3 +412,13 @@ We use Implicit Indexing:
2. We do not "check" if it is created. The act of incrementing `instruction_count` creates it.
This removes the need for an explicit `definition` bitfield in `instructions_t`, creating space to expand `src1`, `src2`, and `src3` to 18 bits.
### What about instructions that don't define anything?
If `instructions[200]` is `STORE v1, [v2]`, it defines no variable for other instructions to use. If we strictly follow implicit indexing, we create `ssa_values[200]`. Is this waste?
**Yes. Its 8 bytes of waste. But this is the best choice we have.**
Its either this or keep track of a `definition` field in `instruction_t` which then shrinks `src1`, `src2`, and `src3` to a tiny 14 bits.
We handle these void instructions by marking the SSA variable as `VOID`: `ssa_versions[200].type = TYPE_VOID`