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Thu Jul 9 14:32:34 PDT 2020 · https://lists.llvm.org/pipermail/llvm-dev/2020-July/143260.html

I am working on a back end for an architecture whose jump via table instruction
includes the range check. If the index is out of range, the jump table 
instruction just falls through.  I implemented a pass to remove the range check 
generated before the jump table instruction because it is superfluous.
This causes as assertion in MachineBlockPlacement.cpp:
       assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
               !PrevBB->canFallThrough()) &&
              "Unexpected block with un-analyzable fallthrough!");

The method MachineBasicBlock::getFallThrough() uses TII->analyzeBranch().
Using analyze branch there doesn't appear to be anyway to signify that a branch 
is not a simple conditional or a simple indirect but rather a combination of 
the two -- it conditionally branches indirect or falls through.

Is there a way to express this with analyzeBranch?

Can I override the method getFallThough?

Is there a way to annotate a basic block to indicate in can fall through?
It would seem this would save a lot of calls to analyzeBranck.

Any other ideas?

Thanks,
Brian

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Showing content from https://lists.llvm.org/pipermail/llvm-dev/2020-July/143260.html below:

[llvm-dev] question on analyzeBranch and getFallThrough