During a run of interp, some memory (for example, memory allocated
through runtime.alloc) may not have a known LLVM type. This memory is
alllocated by creating an i8 array.
This does not necessarily work, as i8 has no alignment requirements
while the allocated object may have allocation requirements. Therefore,
the resulting global may have an alignment that is too loose.
This works on some microcontrollers but notably does not work on a
Cortex-M0 or Cortex-M0+, as all load/store operations must be aligned.
This commit fixes this by setting the alignment of untyped memory to the
maximum alignment. The determination of "maximum alignment" is not
great but should get the job done on most architectures.
For a full explanation, see interp/README.md. In short, this rewrite is
a redesign of the partial evaluator which improves it over the previous
partial evaluator. The main functional difference is that when
interpreting a function, the interpretation can be rolled back when an
unsupported instruction is encountered (for example, an actual unknown
instruction or a branch on a value that's only known at runtime). This
also means that it is no longer necessary to scan functions to see
whether they can be interpreted: instead, this package now just tries to
interpret it and reverts when it can't go further.
This new design has several benefits:
* Most errors coming from the interp package are avoided, as it can
simply skip the code it can't handle. This has long been an issue.
* The memory model has been improved, which means some packages now
pass all tests that previously didn't pass them.
* Because of a better design, it is in fact a bit faster than the
previous version.
This means the following packages now pass tests with `tinygo test`:
* hash/adler32: previously it would hang in an infinite loop
* math/cmplx: previously it resulted in errors
This also means that the math/big package can be imported. It would
previously fail with a "interp: branch on a non-constant" error.
