# Short-circuit AND/OR

Tag `v3-short-circuit`. Main suite **111/111**.

## Why

V3 evaluated both sides of `AND`/`OR`; gm2 (and the compiler's own
sources) rely on short-circuit — e.g. `SymTab.Resolve`'s
`… AND (tform[t] = FAlias)` dereferences `tform[-1]` when `t` is
invalid. Roughly 25 such guard sites exist in the compiler sources, so
matching gm2 is the right fix rather than rewriting them.

## How

The RHS operand is already parsed (and its code emitted) by the time the
binary-op action runs, so short-circuit needs **deferred code**:

- `QbeGen.DelayBegin` redirects output into a small buffer;
  `DelayEnd` restores the previous sink; `DelayFlush` replays the buffer
  at the branch. `WEmit` routes to the delay buffer while active.
- `SimExpr` (OR) and `Term` (AND) wrap the RHS `Term`/`Fact` in
  `DelayBegin`/`DelayEnd` and emit:
  - `OR`:  `if a then r = 1 else { <RHS>; r = b }`
  - `AND`: `if a then { <RHS>; r = b } else r = 0`
- The merge uses a **4-byte memory slot** (`Slot4`/`StoreW`/`LoadW`)
  rather than a QBE `phi`: a phi must name the right predecessor block,
  but the RHS can span several blocks (e.g. a set-membership trap
  check), which broke the phi's predecessor list. The slot is robust to
  any RHS shape.

## Result

Suite 111/111 (the earlier WIP failed 14 tests: `DelayFlush` popped the
wrong buffer index, then the `CopyOp`-only merge collapsed the two
branches to one compile-time operand).

## Still open (task 2: full self-image)

The self-compiled V3 compiles a single file to byte-identical `.ssa`,
but the **whole compiler source set** still aborts, now in the scanner's
`GetName` (`LexName ← GetIdent ← ProcTypeSection`) — a bounds trap;
next to diagnose.

## Files

`compiler/src/{M2.atg,QbeGen.def,QbeGen.mod,M2.lst}`.