Jit64: Fix FinalizeCarryOverflow XER[OV/SO] #9429
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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.
lioncash
approved these changes
Jan 10, 2021
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FinalizeCarryOverflow didn't maintain XER[OV/SO] properly due to an oversight. Here's the code it would generate:
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.