Multiplication operations can currently be folded if:
1. Both arguments are known constant values
2. Either operand is zero (in which case the result is also zero)
3. Either operand is one (in which case the result is the non-one
operand).
By far, one of the most common things to check for is whether or not a
value is zero, as it typically allows folding away unnecesary
operations (other close contenders that can help with eliding operations are 1 and -1).
So instead of requiring a check for an immediate and then actually
retrieving the integral value and checking it, we can wrap it within a
function to make it more convenient.
It's possible to fold cases of exclusive OR operations if they can be
known to be an identity operation, or if both operands happen to be known
immediates, in which case we can just store the result of the
exclusive-OR directly.
enum classes are still considered complete types when forward declared
(as the compiler knows the exact size of the type from the declaration
alone). The only difference in this case being that the members of the
enum class aren't visible. Given we don't use the members within this
header in any way, we can simply forward declare them here and remove
the inclusions.
Currently basic block kind of acts like a 'dumb struct' which makes things
a little more verbose to write (as opposed to keeping it all in one place,
I guess). It's also a little wonky conceptually, considering a block is
composed of instructions (i.e. 'contains' them).
So providing accessors that make it act more like a container can make working
with algorithms a little nicer. It also makes the API a little more
defined.
Ideally, the list would be only available through a function, but
currently, the pool allocator is exposed, which seems somewhat odd,
considering the block itself should manage its overall allocations
(with placement new, and regular new), rather than putting that
sanitizing directly on the IR emitter (it should just care about emission,
not block state). However, recontaining that can be followed up with,
as it's very trivial to do.
Makes the name match the standard library equivalents.
C++17 introduces non-member empty() which allows for nicer handling
in generic contexts. May as well make the data structure compatible with
it.
