Issue #100 demonstrated that register allocation for aliased registers is
fundamentally broken. The root of the issue is that currently accesses to the
same virtual register with different masks are treated as different registers.
This PR takes a different approach:
* Liveness analysis is masked: we now properly consider which parts of a register are live
* Register allocation produces a mapping from virtual to physical ID, and aliasing is applied later
In addition, a new pass ZeroExtend32BitOutputs accounts for the fact that 32-bit writes in 64-bit mode should actually be treated as 64-bit writes (the result is zero-extended).
Closes#100
It was pointed out #73 that Bytes() is a poor name for the size of the register in bytes. In idiomatic Go you would probably expect a Bytes() method to return []byte.
This diff changes the Bytes() to Size(). As a result the Size type also needed to be renamed, and Width seemed a reasonable choice.
Fixes#73
Adds methods for referencing sub- or super-registers. For example, for
general purpose registers you can now reference As8(), As16(), ... and
for vector AsX(), AsY(), AsZ().
Closes#1
