Adds support for lowering clif instructions Fdiv and Fmul
for new vcode backend. Misc adds lowering and test for
sqrtss and removes a redundant to_string() func for the
SseOpcode struct.
Adds support for addss and subss. This is the first lowering for
sse floating point alu and some move operations. The changes here do
some renaming of data structures and adds a couple of new ones
to support sse specific operations. The work done here will likely
evolve as needed to support an efficient, inituative, and consistent
framework.
This patch fixes a subtle bug that occurred in the MachBuffer branch
optimization: in tracking labels at the current buffer tail using a
sorted-by-offset array, the code did not update this array properly when
redirecting labels. As a result, the dead-branch removal was unsafe,
because not every label pointing to a branch is guaranteed to be
redirected properly first.
Discovered while doing performance testing: bz2 silently took a wrong
branch and exited compression early. (Eek!)
To address this problem, this patch adopts a slightly simpler data
structure: we only track the labels *at the current buffer tail*, and
*at the start of each branch*, and we're careful to update these
appropriately to maintain the invariants. I'm pretty confident that this
is correct now, but we should (still) fuzz it a bunch, because wrong
control flow scares me a nonzero amount. I should probably also actually
write out a formal proof that these data-structure updates are correct.
The optimizations are important for performance (removing useless empty
blocks, and taking advantage of any fallthrough opportunities at all),
so I don't think we would want to drop them entirely.
