FinalizeCarryOverflow didn't maintain XER[OV/SO] properly due to an
oversight. Here's the code it would generate:
0: 9c pushf
1: 80 65 3b fe and BYTE PTR [rbp+0x3b],0xfe
5: 71 04 jno b <jno>
7: c6 45 3b 03 mov BYTE PTR [rbp+0x3b],0x3
000000000000000b <jno>:
b: 9d popf
At first glance it seems reasonable. The host flags are carefully
preserved with PUSHF. The AND instruction clears XER[OV]. Next, an
conditional branch checks the host's overflow flag and, if needed, skips
over a MOV that sets XER[OV/SO]. Finally, host flags are restored with
POPF.
However, the AND instruction also clears the host's overflow flag. As a
result, the branch that follows it is always taken and the MOV is always
skipped. The end result is that XER[OV] is always cleared while XER[SO]
is left unchanged.
Putting POPF immediately after the AND would fix this, but we already
have GenerateOverflow doing it correctly (and without the PUSHF/POPF
shenanigans too). So let's just use that instead.
Happens in Super Mario Sunshine. You could probably do something similar
for b == -1 (like we do for subfic), but I couldn't find any titles that
do this.
- Case 1: d == a
Before:
41 8B C7 mov eax,r15d
41 BF 00 00 00 00 mov r15d,0
44 2B F8 sub r15d,eax
After:
41 F7 DF neg r15d
- Case 2: d != a
Before:
BF 00 00 00 00 mov edi,0
41 2B FD sub edi,r13d
After:
41 8B FD mov edi,r13d
F7 DF neg edi
Consider the case where d and a refer to the same PowerPC register,
which is known to hold an immediate value by the RegCache. We place a
ReadWrite constraint on this register and bind it to an x86 register.
The RegCache then allocates a new register, initializes it with the
immediate, and returns a RCX64Reg for both d and a.
At this point information about the immediate value becomes unreachable.
In the case of subfx, this generates suboptimal code:
Before 1:
BF 1E 00 00 00 mov edi,1Eh <- done by RegCache
8B C7 mov eax,edi
8B FE mov edi,esi
2B F8 sub edi,eax
Before 2:
BE 00 AC 3F 80 mov esi,803FAC00h <- done by RegCache
8B C6 mov eax,esi
8B 75 EC mov esi,dword ptr [rbp-14h]
2B F0 sub esi,eax
The solution is to explicitly handle the constant a case before having
the RegCache allocate registers for us.
After 1:
8D 7E E2 lea edi,[rsi-1Eh]
After 2:
8B 75 EC mov esi,dword ptr [rbp-14h]
81 EE 00 AC 3F 80 sub esi,803FAC00h
The special case doesn't appear to make a significant difference in any games, and the current implementation has a (minor, fixable) issue that breaks Super Mario Sunshine (both with a failed assertion (https://bugs.dolphin-emu.org/issues/11742) and a rendering issue (https://bugs.dolphin-emu.org/issues/7476)). Hardware testing wasn't able to reproduce the special case, either, so it may just not exist.
PR #9315 contains a fixed implementation of the special case on all video backends, and can serve as a basis for it being reintroduced if it is found to exist under more specific circumstances. For now, I don't see a reason to keep it present.
-If adding 2 devices with the same name, they their unique id wouldn't be increased, causing a conflict.
-Removing a device wouldn't actually remove it from the internal devices list because the list of devices had already been updated when going through it.
-It was possible to remove devices belonging to other sources by adding a device with the same name and then removing it.
LC