* Replace a single-character string literal with a character literal.
* Use is_some() instead of comparing with Some(_).
* Add code-quotes around type names in comments.
* Use !...is_empty() instead of len() != 0.
* Tidy up redundant returns.
* Remove redundant .clone() calls.
* Remove unnecessary explicit lifetime parameters.
* Tidy up unnecessary '&'s.
* Add parens to make operator precedence explicit.
* Use debug_assert_eq instead of debug_assert with ==.
* Replace a &Vec argument with a &[...].
* Replace `a = a op b` with `a op= b`.
* Avoid unnecessary closures.
* Avoid .iter() and .iter_mut() for iterating over containers.
* Remove unneeded qualification.
Add a spilling pass which lowers register pressure by assigning SSA
values to the stack. Important missing features:
- Resolve conflicts where an instruction uses the same value more than
once in incompatible ways.
- Deal with EBB arguments.
Fix bugs in the reload pass exposed by the first test case:
- Create live ranges for temporary registers.
- Set encodings on created spill and fill instructions.
The register pressure tracker now has to separate register counts: base
and transient.
The transient counts are used to track spikes of register pressure, such
as dead defs or temporary registers needed to satisfy instruction
constraints.
The base counts represent long-lived variables.
The spilling and reload passes need to ensure that the set of live
ranges with register affinity can always be assigned registers. The
register pressure tracker can count how many registers are in use for
each top-level register class and give guidance on the type of
registers that need to be spilled when limits are exceeded.
Pressure tracking is extra complicated for the arm32 floating point
register bank because there are multiple top-level register classes (S,
D, Q) competing for the same register units.
