A machine environment tells the register allocator which registers
are available to allocate and what register may be used as a
scratch register for each class, and some other miscellaneous info
as well.
An Operand encodes everything about a mention of a register in
an instruction: virtual register number, and any constraint that
applies to the register at this program point.
An OperandConstraint specifies where a vreg’s value must be
placed at a particular reference to that vreg via an
Operand. The constraint may be loose – “any register of a given
class”, for example – or very specific, such as “this particular
physical register”. The allocator’s result will always satisfy all
given constraints; however, if the input has a combination of
constraints that are impossible to satisfy, then allocation may
fail or the allocator may panic (providing impossible constraints
is usually a programming error in the client, rather than a
function of bad input).
The “position” of the operand: where it has its read/write
effects. These are positions “in” the instruction, and “early” and
“late” are relative to the instruction’s main effect or
computation. In other words, the allocator assumes that the
instruction (i) performs all reads and writes of “early” operands,
(ii) does its work, and (iii) performs all reads and writes of its
“late” operands.