This is also https://bugzilla.mozilla.org/show_bug.cgi?id=1552737.
Cranelift currently has a tendency to create redundant copies (self-copies) of
values from a stack slot back to the same stack slot. This generates a
pointless load and store and an unnecessary register use. The copies are
created by `visit_inst` in regalloc/reload.rs. They appear to occur mostly,
but not exclusively, at loop heads. It's unclear why this happens.
This patch adds a special case to `visit_inst` to find such copies. They are
converted into a new instruction, `copy_nop`, which takes and produces the
same SSA names, so as not to break any of the SSA invariants, but which has a
zero-length encoding, hence removing the copy at emission time.
`copy_nop`s source and destination operands must be stack slots and of course
the *same* stack slot. The verifier has been enhanced to check this, since
misuse of `copy_nop` will likely lead to hard-to-find incorrect-code bugs.
Attempts were made to write a standalone .clif test case. But these failed
because it appears the .clif parser accepts but ignores location hints that
are stack slots. So it's impossible to write, in clif, the exact form of
`copy` instruction that triggers the transformation.
This adds test cases to ensure, to a reasonably high degree of certainty, that
the magic-number generators `magic_u32`, `magic_s32`, `magic_u64` and
`magic_s64` work correctly. This is done by iterating through a large number
of `(n, d)` pairs, generating the magic numbers for `d`, interpreting the
magic numbers so as to perform the division, and comparing against the result
produced directly by the hardware. The distribution of numbers is arranged so
that particular emphasis is given to corner cases -- the range ends and
midpoints -- but also so that there is at least some cover for values away
from those areas. In total 50,148,000 tests are performed.
- adds helpers used by other parts of the code
- allows cheap copies by having Instruction be a lightweight ref-cloned
wrapper of the actual instruction's content.
