frontend: support "casting" between MLIR types

[AlexanderViand]

Tries to address the use case presented in #1789 though it does so not via type hints but via adding support for casts:

def sub_bytes(x: Secret[I8]):
    x0 = I1((x >> 7) & 1)
    x1 = I1((x >> 6) & 1)
    x2 = I1((x >> 5) & 1)
    x3 = I1((x >> 4) & 1)
    ...

Still WIP:

• There are some open questions around what the expected behavior should be for arithmetic FHE (see discussion in the issue)
• If we want to only support it for CGGI for now, this might depend on frontend: add cggi scheme support up to cggi dialect #1783 (not sure if that's 100% necessary, though)
• There should probably be some tests once we determined what exactly it should be doing/not doing in the C++ side of things.
• For practical reasons, this also depends on frontend: various cleanups and minor fixes #1824

[AlexanderViand]

@j2kun / @asraa: I see the cggi-frontend tests fail when doing bazel test //frontend:cggi_test but they work fine when running directly (python frontend/cggi_test.py). I'd guess this isn't related to this PR but to running CGGI from python under bazel more generally, since it happens even with the standard CGGI test (which wasn't enabled in bazel before)...

The error message I get when running with bazel is

HEIR Error at /home/sdp/.cache/bazel/_bazel_sdp/e1a6590193554bff6e6c8ff665423111/sandbox/processwrapper-sandbox/10673/execroot/_main/bazel-out/k8-dbg/bin/frontend/cggi_test.runfiles/_main/frontend/cggi_test.py (11): HEIR compilation for function foo failed:
running `heir-opt --canonicalize --mlir-to-cggi --mlir-print-debuginfo --view-op-graph` produced these errors:
ERROR: Can't open input file `/home/sdp/.cache/bazel/_bazel_sdp/e1a6590193554bff6e6c8ff665423111/sandbox/processwrapper-sandbox/10673/execroot/_main/bazel-out/k8-dbg/bin/frontend/cggi_test.runfiles/_main/techmap.v' for reading: No such file or directory

[j2kun]

@j2kun / @asraa: I see the cggi-frontend tests fail when doing bazel test //frontend:cggi_test but they work fine when running directly (python frontend/cggi_test.py). I'd guess this isn't related to this PR but to running CGGI from python under bazel more generally, since it happens even with the standard CGGI test (which wasn't enabled in bazel before)...

The error message I get when running with bazel is

HEIR Error at /home/sdp/.cache/bazel/_bazel_sdp/e1a6590193554bff6e6c8ff665423111/sandbox/processwrapper-sandbox/10673/execroot/_main/bazel-out/k8-dbg/bin/frontend/cggi_test.runfiles/_main/frontend/cggi_test.py (11): HEIR compilation for function foo failed:
running `heir-opt --canonicalize --mlir-to-cggi --mlir-print-debuginfo --view-op-graph` produced these errors:
ERROR: Can't open input file `/home/sdp/.cache/bazel/_bazel_sdp/e1a6590193554bff6e6c8ff665423111/sandbox/processwrapper-sandbox/10673/execroot/_main/bazel-out/k8-dbg/bin/frontend/cggi_test.runfiles/_main/techmap.v' for reading: No such file or directory

The techmap error looks like it's an issue with your build file. You'd need something like data = ["@heir//lib/Transforms/YosysOptimizer/yosys:techmap.v"], but even then it should be looking for this file in HEIR_YOSYS_SCRIPTS_DIR which may need to be configured properly (it's in heir_cli and tools/BUILD...).

Ignoring that, it is true that we can't enable CGGI tests right now because we don't have a frontend-compatible backend for it. HEIR doesn't have a CGGI backend via OpenFHE's binfhe yet, and the frontend doesn't support rust for tfhe-rs yet (nor Python for jaxite). @raghav198 has a draft from his paper in https://github.com/raghav198/heir/tree/cggi-to-openfhe-draft, but I doubt we need full CGGI support in the frontend for this PR. Can it wait?

[AlexanderViand]

Ignoring that, it is true that we can't enable CGGI tests right now because we don't have a frontend-compatible backend for it.

Yep, but this is using the CleartextBackend (so just testing down to cggi dialect) and works fine when being called outside the bazel environment, so these tests should work even w/o CGGI backend support.

I'll look into the BUILD file thing you mentioned!
EDIT: Looks like it was indeed just missing the HEIR_YOSYS_SCRIPTS_DIR (and HEIR_ABC_BINARY) paths in testing.bzl, thanks for the hint!

[AlexanderViand]

Tests pass w/ bazel now, so should be good to review :)

[j2kun]

LGTM, a few nits