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Interpreter_FPUtils: Correct setting the FPSCR's zero divide exceptio…

…n flag in the 0/0 case

FPSCR[ZX] is the bit defined to represent the zero divide exception
condition bit, and is defined as (according to PowerPC Microprocessor
Family: The Programming Environments Manual for 32 and 64-bit
Microprocessors, which will be referred to as "PEM" for the rest of this
commit message) at section 3.3.6.1:

"
A zero divide exception condition occurs when a divide instructions is
executed with a zero divisor value and a finite, nonzero dividend value
or when a floating reciprocal estimate single (fres) or a floating
reciprocal square root estimate (frsqrte) instruction is executed with a
zero operand value.
"

Note that it states the divisor must be zero and the dividend must be
nonzero in order for ZX to be set. This means that the interpreter was
performing the wrong behavior for the case where 0/0 (with any sign on
the zeros) is performed. We would incorrectly set the ZX bit when only
the VXZDZ bit should be set.

It's also worth pointing out that N/0 (where N is any finite nonzero
value) and 0/0 are not within the same exception class. N/0 is a zero
divide exception case, while 0/0 is considered an invalid operation
exception case, which is also indicated in the PEM section 3.3.6.1 as
well where it lists the criteria for invalid operation exceptions.

Therefore we should only be setting the VXZDZ bit in the 0/0 case, not
VXZDZ and ZX. This was also verified via hardware tests to ensure that
this behavior indeed holds.
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lioncash committed May 28, 2018
1 parent b2d8d2a commit 3deadd1fffae81083bb30449b868f2de48e3505d
Showing with 6 additions and 1 deletion.
  1. +6 −1 Source/Core/Core/PowerPC/Interpreter/Interpreter_FPUtils.h
@@ -127,9 +127,14 @@ inline double NI_div(double a, double b)
if (b == 0.0)
{
SetFPException(FPSCR_ZX);
if (a == 0.0)
{
SetFPException(FPSCR_VXZDZ);
}
else
{
SetFPException(FPSCR_ZX);
}
}
else if (std::isinf(a) && std::isinf(b))
{

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