Bug 256235

rdar://108812140

Currently, the implementation for calls in BBQ JIT first saves all values in caller-saved registers, then moves all argument values into the correct parameter locations per the calling convention. This is correct, but leads to a lot of redundant work. Specifically, I've noticed we tend to see a lot of code like this:

    [  0x61f3] Call
             <53688> 0x13fa791b8:    stur     w0, [fp, #-160]
             <53692> 0x13fa791bc:    stur     w1, [fp, #-176]
             <53696> 0x13fa791c0:    stur     w2, [fp, #-192]
             <53700> 0x13fa791c4:    ldur     w0, [fp, #-160]
             <53704> 0x13fa791c8:    ldur     w1, [fp, #-176]
             <53708> 0x13fa791cc:    ldur     w2, [fp, #-192]
             <53712> 0x13fa791d0:    movz     w3, #0x0
             <53716> 0x13fa791d4:    bl       0x13fa791d4 -> <53716>

Obviously, if we store a register to a stack slot, and then don't modify that register at all, we shouldn't need to load it back from the stack slot immediately. So that should save us maybe half the instructions on its own. But additionally, in BBQ JIT currently, every operand except constants pretty much is a temp. Whenever we use a temp, that corresponds to popping it off the WASM abstract stack, ending its live range - ergo, we shouldn't need to save the value in the first place.

Probably these loads and stores buffer pretty well already, so I don't expect this to be a major performance win (although it probably doesn't hurt!). Maybe more important though is it's kind of a no-brainer code size optimization, since we can eliminate a lot of instructions in a lot of cases without any real downside.