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...cause their register operands are not always available. This makes MachineLICM::IsProfitableToHoist() return false, preventing the hoist even though MachineLICM::IsLoopInvariantInst() returns true. The comment in IsProfitableToHoist() is: // If the instruction is cheap, only hoist if it is re-materilizable [sic]. LICM // will increase register pressure. It's probably not worth it if the // instruction is cheap. The function then proceeds to actually *estimate* register pressure for non-cheap instructions to determine whether or not to hoist them. This heuristic seems reasonable, but doesn'...

Yes machine-licm can and should hoist constant materialization instructions out of the loop. If it's not doing that, it's probably because the target is not modeling the instruction correctly. I would walk through MachineLICM::IsLoopInvariantInst() in the debugger to figure it out. You can also try compiling the same bitcode for a target like ARM or X86 as a comparison. Evan On Mar 7,

Hi All, I work on a backend for a target similar to Mips, where large immediates are loaded into registers with 2 instructions, 1 to load the MSBits and 1 to load the LSBits. I've noticed a recurring pattern where, despite low register pressure, these constants will be rematerialized in every iteration of a loop, rather than being hoisted. Here's an example using the