[llvm-dev] How to get Greedy RA to not spill results of trivially rematerializable instructions

Nemanja Ivanovic via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 20 16:12:16 PST 2019

I have encountered a rather odd situation with Greedy where it will end up
spilling a register that was populated with a zero (with a trivially
rematerializable load-immediate instruction).
In fact, it spills 3 such values (LICM moves stuff out of a loop, register
coalescer replaces copies with load-immediates and then Greedy spills them).

I personally can't think of a situation where a spill (with a reload later
presumably) is better than simply rematerializing the value where it would
have otherwise been reloaded. To that end, would it be possible for Greedy
to simply duplicate the trivially rematerializable instruction at every
reload site? Perhaps this is something it would need to query the target
for? Perhaps Greedy would be able to call something like
TargetInstrInfo::rematerializeValue(MachineInstr &RematMI,
MachineBasicBlock::iterator InsertAt) or something along those lines?
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