This change adds a software implementation of the `math.ctlz` operation
and includes it in `--convert-math-to-funcs`.
This is my first change to MLIR, so please bear with me as I'm still learning
the idioms of the codebase.
The context for this change is that I have some larger scale project in which
I'd like to lower from a mix of MLIR dialects to CIRCT, but many of the CIRCT
passes don't support the `math` dialect.
I noticed the content of `convert-math-to-funcs` was limited entirely to
the `pow` functions, but otherwise provided the needed structure to implement
this feature with minimal changes.
Highlight of the changes:
- Add a dependence on the SCF dialect for this lower. I could have lowered
directly to cf, following the pow lowerings in the same pass, but I felt it
was not necessary given the existing support for lowering scf to cf.
- Generalize the DenseMap storing op implementations: modify the callback
function hashmap to be keyed by both OperationType (for me this effectively
means the name of the op being implemented in software) and the type
signature of the resulting function.
- Implement the ctlz function as a loop. I had researched a variety of
implementations that claimed to be more efficient (such as those based on a
de Bruijn sequence), but it seems to me that the simplest approach would make
it easier for later compiler optimizations to do a better (platform-aware)
job optimizing this than I could do by hand.