This is just the basic 'imul' the M instruction set also has mulh/mulhu which
yield the high bits of a multiplication, and there are div/rem instructions to
be implemented.
These instructions are gated by the use_m predicate, but ISA predicates are not
completely implemented yet.
Three predicates affect each extension:
- supports_m determines whether the target CPU supports the instruction set.
- enable_m determines if the instructions should be used, assuming they're
available.
- use_m is the predicate used to actually use the instructions.
Add new instruction predicates to support the 'I' encoding recipe: IsSignedInt,
IsUnsignedInt used to test that an immediate operand is in the allowed range.
Predcates are boolean functions. There will be ISA predicates and instruction
predicates.
The ISA predicates will be turned into member functions on the generated Flags
structs.
Clarify terminology by always referring to a 'Target ISA' instead of just
'Target'. Use 'isa' as a module name instead of 'target' both in Rust and Python
code.
This is only to clarify terminology and not at all because Cargo insists on
using the 'target' sub-directory for build products. Oh, no. Not at all.
