failing to move index addition to constant table pointer

[mikebenfield]

Two LLVM IR functions

Here are two IR functions implementing this pseudocode:

def somefunction(x):
    return switch.table[x - 15]

When somefunction2 is compiled the -15 is moved to the constant table pointer, rather than doing an operation at runtime. Great.

This doesn't happen with somefunction1. Presumably because the compiler wants to maintain the same 8-bit wraparound behavior, so wants to do the arithmetic in 8 bits. But that behavior is irrelevant, as can be determined by either the llvm.assume or the size of the table.