This commit changes many things:
* Most interface-related operations are moved into an optimization
pass for more modularity. IR construction creates pseudo-calls which
are lowered in this pass.
* Type codes are assigned in this interface lowering pass, after DCE.
* Type codes are sorted by usage: types more often used in type
asserts are assigned lower numbers to ease jump table construction
during machine code generation.
* Interface assertions are optimized: they are replaced by constant
false, comparison against a constant, or a typeswitch with only
concrete types in the general case.
* Interface calls are replaced with unreachable, direct calls, or a
concrete type switch with direct calls depending on the number of
implementing types. This hopefully makes some interface patterns
zero-cost.
These changes lead to a ~0.5K reduction in code size on Cortex-M for
testdata/interface.go. It appears that a major cause for this is the
replacement of function pointers with direct calls, which are far more
susceptible to optimization. Also, not having a fixed global array of
function pointers greatly helps dead code elimination.
This change also makes future optimizations easier, like optimizations
on interface value comparisons.
* Loading from a dirty global must be done at runtime (!). For some
reason this wasn't already the case.
* Global variables somehow had IsConstant() the wrong way round,
returning the inverse from what they should.
* Do binary and logical operations at runtime if necessary, relying on
const propagation in the IR builder.
* Don't try to interpret functions that take a dirty parameter. Call
them at runtime.
Cast operations will still be evaluated at compile time in all cases
they did before because of the built-in constant propagation of the
IRBuilder, but when one of the parameters is not a constant it will
transparently be evaluated at runtime.
This avoids some errors in the partial evaluator. It is not yet complete
as all binops will need a similar treatment.
This interpreter currently complements the Go SSA level interpreter. It
may stay complementary or may be the only interpreter in the future.
This interpreter is experimental and not yet finished (there are known
bugs!) so it is disabled by default. It can be enabled by passing the
-initinterp flag.
The goal is to be able to run all initializations at compile time except
for the ones having side effects. This mostly works except perhaps for a
few edge cases.
In the future, this interpeter may be used to actually run regular Go
code, perhaps in a shell.
