6d25759c8e
* ensure that all const assignments are placed at the end of the sequence.
This minimises live ranges.
* for the non-const assignments, ignore self-assignments. This can
dramatically reduce the total number of moves generated, because any
self-assignments trigger the overlap-case handling, hence invoking the
double-copy behaviour in cases where it's not necessary.
It's worth pointing out that self-assignments are common, and are not due to
deficiencies in CLIR optimisation. Rather, they occur whenever a loop back
edge doesn't modify *all* loop-carried values. This can easily happen if
the loop has multiple "early" back-edges -- "continues" in C parlance. Eg:
loop_header(a, b, c, d, e, f):
...
a_new = ...
b_new = ...
if (..) goto loop_header(a_new, b_new, c, d, e, f)
...
c_new = ...
d_new = ...
if (..) goto loop_header(a_new, b_new, c_new, d_new, e, f)
etc
For functions with many live values, this can dramatically reduce the number
of spill moves we throw into the register allocator.
In terms of compilation costs, this ranges from neutral for functions which
spill not at all, or minimally (joey_small, joey_med) to a 7.1% reduction in
insn count.
In terms of run costs, for one spill-heavy test (bz2 w/ custom timing harness),
instruction counts are reduced by 4.3%, data reads by 12.3% and data writes
by 18.5%. Note those last two figures include all reads and writes made by the
generated code, not just spills/reloads, so the proportional reduction in
spill/reload traffic must be greater.
This crate contains the core Cranelift code generator. It translates code from an intermediate representation into executable machine code.