[LLVMdev] LiveIntervals::handlePhysicalRegisterDef, unreachable MBBs

[Vladimir Prus]

I've just spend some time looking at the above function, and while I 
understood what it does, a comment would have helped a lot.

Maybe, something like:

// Determine the end of the live interval for this register
// For physical register, all internvals are within basic blocks so
// we look for the instruction in this basic block which last uses it.

The first loop might have a comment like:

// If the register is dead at instruction which sets it (i.e. not used later)
// the live interval ends at the next instruction

The second loop might have a comment like:

// If the register is not dead at the defining instruction, it must be used
// and killed by some subsequent instruction. Find that instruction now, which
// always exists

Finally, why I've started all this. I forgot to add machine CFG edge and got 
misterious crash. I've applied the attached patch, which checks that all 
basic blocks are reachable from function entry. Any comments on:

- whether this check is reasonable?
- what's the right place for the check? LiveVariables.cpp is the simplest I 
could find, but does not seem right.

- Volodya







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