This new function is like MOVP2R, except it masks out the lower 12 bits,
returning them instead of writing them to the register. These lower
12 bits can then be used as an offset for LDR/STR. This lets us turn
ADRP+ADD+LDR sequences with a zero offset into ADRP+LDR sequences with
a non-zero offset, saving one instruction.
When emulated GBAs were added to Dolphin, it was possible to control them
using the GC TAS input window. (Z was mapped to Select.) Unaware of this,
I broke the functionality in b296248.
To make it possible to control emulated GBAs using TAS input again,
I'm adding a proper TAS input window for GBAs, with a real Select button
and no analog controls.
I recently talked to a homebrew developer who was trying to add exception
handlers at link time but found out that Dolphin was overwriting their
exception handlers. I figure that's not the usual way to do exception
handlers, but... making us load the executable after setting up memory
rather than before is easy, and matches what we do when booting discs,
so I suppose there's no reason not to do it. It also matches the intent
of why Dolphin is writing default exception handlers – we're writing
them because some homebrew relies on exception handlers being left
around from whatever program was running before it (see 3dd777be70).
Let's take advantage of ARM64's input register shifting one last time,
shall we?
Before:
0x1280005b mov w27, #-0x3
0x1b1b7f18 mul w24, w24, w27
After:
0x4b180b18 sub w24, w24, w24, lsl #2
ARM64's flexible shifting of input registers also allows us to calculate
a negative power of two in one instruction; shift the input of a NEG
instruction.
Before:
0x128001f7 mov w23, #-0x10
0x1b1a7efa mul w26, w23, w26
0x93407f58 sxtw x24, w26
After:
0x4b1a13fa neg w26, w26, lsl #4
0x93407f58 sxtw x24, w26
If the destination register doesn't equal the input register, using it
to temporarily hold the immediate value is fair game as it'll be
overwritten with the result of the multiplication anyway. This can
slightly reduce register pressure.
Before:
0x52800659 mov w25, #0x32
0x1b197f5b mul w27, w26, w25
After:
0x5280065b mov w27, #0x32
0x1b1b7f5b mul w27, w26, w27
