Jit64: Use farcode for exception exit in twX.

[AdmiralCurtiss]

See https://forums.dolphin-emu.org/Thread-new-5-0-15445-causes-jump-target-too-far-away-needs-force5bytes-true

I didn't do it yet, but it might be worth bisecting what change exactly caused the distance to become too long?

[JosJuice]

I added the following extra instruction when fixing True Crime. Maybe that managed to push it over the edge...

dolphin/Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp

Line 2574 in f4e2d36

MOV(32, PPCSTATE_SRR1, Imm32(static_cast<u32>(ProgramExceptionCause::Trap)));

Do you think it would make sense to make twX use farcode? Or is the exception path commonly taken?

[AdmiralCurtiss]

I have not even the slightest idea how often this path is taken. Would be nice if we had metrics like that... But yes, if it's somewhat rare that's probably not a bad idea.

[JosJuice]

tw and twi are the only instructions that generate ProgramExceptionCause::Trap, so one way to measure how common it is could be to add an if statement here that outputs a message to the log if SRR1 == ProgramExceptionCause::Trap:

dolphin/Source/Core/Core/PowerPC/PowerPC.cpp

Line 486 in b997048

SRR0 = PC;

[saulfabregwiivc]

@AdmiralCurtiss @JosJuice

I tested this build with this pull request on Windows x64 [pr-10275-win-x64] (i saw that is based in Dolphin 5.0-15605) x64 and the problem of "Jump target too far away, needs force5Bytes = true" is now gone, now the Not64 emulator works again as a charm, as i specified in my thread (https://forums.dolphin-emu.org/Thread-new-5-0-15445-causes-jump-target-too-far-away-needs-force5bytes-true) with old 5.0-15262.

Will you bring this fix for a new development or beta release?

[Sintendo]

Do we know of any other titles using tw/twi? Issue 7253 mentions Monopoly, but I couldn't hit this instruction for any title that I happened to have at hand.

[JMC47]

Does forcing 5 bytes mean that we lose performance? Does this cost performance in general?

[JosJuice]

It will make the code size a little larger. In some cases, this can cause a small performance drop due to less code being able to fit in the icache.

[AdmiralCurtiss]

The difference should be minuscule in practice though, especially since apparently this instruction is barely used by games.

[AdmiralCurtiss]

I hacked myself some metrics and didn't encounter a single exceptional exit while playing around, so yes moving this to farcode is probably a good idea.