Investigation of TableGen freezes for some Core-V instructions

[PhilippvK]

Affected Instructions

• CV_MACUN (16 x 16 bit multiply + 32bit acc + right shift) -> 33 bit intermediate result
• CV_MULURN (16 x 16 bit multiply + add(1 << (n-1))) + right shift) -> 33 bit intermediate result
• CV_MULSRN -> 33 bit intermediate result
• CV_MACHHSN -> 33 bit intermediate result
• CV_MULHHURN -> 33 bit intermediate result
• CV_MACHHUN -> 33 bit intermediate result
• CV_MACSN -> 33 bit intermediate result
• CV_MULHHSRN -> 33 bit intermediate result
• CV_MACHHSRN -> 34 bit intermediate result
• CV_MACSRN > 34 bit intermediate result
• Most dot products…

Not affected

• CV_ADDN (32 x 32 bit addition + trunc to 32 bits + right shift) -> 32 bit intermediate result
• ...

Observations:

• There is a mismatch between MUL and ALU in RTL of CV32E40P (MUL has 2 carry bits, while ALU just overflows)
• Core-V LLVM compiler uses truncating patterns for MAC/MUL
• CoreDSL (used as Seal5 input) reflects the behavior of the RTL; hence CV_ADDN is not affected by the freeze.
• Needs more experiments

Workarounds

• Currently we just filter out the broken instructions completely:

  seal5/examples/cfg/xcorev/filter.yml

  Line 24 in 848e2ba

  - SEAL5_CV_MACUN

  (Skipping just the pattern gen would also work)
• Adding explicit truncations to the CDSL heavior of the affected instructions would also help, but we would not match the RTL then...

Next steps

• Test on newest version of LLVM
• Build minimal example to reproduce bug (MACUN? ADDN with carry?  Map to pseudo instr to drop dependency on Core-V LLVM)
• Find the reason for the freeze (endless loop?)
• Report bug to LLVM community
• Submit bugfix

[PhilippvK]

Some more context here:

• The patterns explode in size as the intermediate 33/34 bit results are handled as i64. As this type is not natively supported on 32bit targets, the legalizers expands them to a sequence of supported operations.
• TODO @PhilippvK: add example of generated patterns here for affected vs unaffected instructions!