Add a new cursor module and define an EncCursor data type in it. An
EncCursor is a cursor that inserts instructions with a valid encoding
for the ISA. This is useful for passes generating code after
legalization.
Implement a builder interface via the new InstInserterBase trait such
that the EncCursor builders support with_result().
Use EncCursor in coalescing.rs instead of the layout cursor as a proof
of concept.
The Cursor navigation methods all just depend on the cursor's position
and layout reference. Make a CursorBase trait that provides access to
this information with methods and implement the navigation methods on
top of that.
This makes it possible to have multiple types implement the cursor
interface.
Replace the isa::Legalize enumeration with a function pointer. This
allows an ISA to define its own specific legalization actions instead of
relying on the default two.
Generate a LEGALIZE_ACTIONS table for each ISA which contains
legalization function pointers indexed by the legalization codes that
are already in the encoding tables. Include this table in
isa/*/enc_tables.rs.
Give the `Encodings` iterator a reference to the action table and change
its `legalize()` method to return a function pointer instead of an
ISA-specific code.
The Result<> returned from TargetIsa::encode() no longer implements
Debug, so eliminate uses of unwrap and expect on that type.
A function parameter in an incoming_arg stack slot should not be
coalesced into any virtual registers. We don't want to force the whole
virtual register to spill to the incoming_arg slot.
Coalescing means creating virtual registers and transforming the code
into conventional SSA form. This means that every value used as a branch
argument will belong to the same virtual register as the corresponding
EBB argument value.
Conventional SSA form makes it easy to avoid memory-memory copies when
spilling values, and the virtual registers can be used as hints when
picking registers too. This reduces the number of register moves needed
for EBB arguments.
