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Alyssa Rosenzweig
9813553f02
We now have two types of destinations:
* regular destinations. These are SSA. You get exactly 1 per instruction. This
is what almost every instruction should use.
* special destinations, introduced here. These are *not* SSA. They must be
allocated with a special instruction (added later in this PR), and then they
are mutated by the instruction. There are two types, either pure destinations
("out") or read-modify-write source+destinations ("in-out"). The former are
useful for instructions that return multiple destinations, like Memcpy. The
latter are useful for instructions that need a source tied with a special
destination (currently just Pop, introduced later in this series).
Special destinations reuse the mechanism of sources, to get around the
limitations on regular destinations in our current IR. Ops with special
destinations desugar to ops with no destination but extra sources prefixed Out
or Inout.
They further require HasSideEffects so we don't optimize ourselves into corners.
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>