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JitArm64: Fix frsqrte misclassifying large negative inputs as NaN

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The frsqrte routine has a little trick where it uses
`TBNZ(ARM64Reg::X1, 62)` to check if the input is NaN/Inf. This only
works if we've checked that the input isn't normal. While we have
checked that the input isn't a positive normal at this point, it could
still be a negative normal. Because of this, the frsqrte routine would
incorrectly treat certain negative normal inputs as NaNs, causing it to
output the wrong exception in FPSCR (while still producing the correct
numerical value).

To fix the problem, I'm swapping the order of the lines
`FixupBranch nan_or_inf = TBNZ(ARM64Reg::X1, 62);` and
`FixupBranch negative = TBNZ(ARM64Reg::X1, 63);`, and then instead of
having the `nan_or_inf` case do a special check for negative infinity,
I'm making the `negative` case do a special check for NaN. The total
instruction count is the same.
This commit is contained in:
JosJuice 2024-12-26 12:01:03 +01:00
parent 1a8619c85b
commit 709b64d310
1 changed files with 3 additions and 4 deletions
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7
Source/Core/Core/PowerPC/JitArm64/JitAsm.cpp
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@ -363,8 +363,8 @@ void JitArm64::GenerateFrsqrte()
RET();
SetJumpTarget(not_positive_normal_not_zero);
FixupBranch nan_or_inf = TBNZ(ARM64Reg::X1, 62);
FixupBranch negative = TBNZ(ARM64Reg::X1, 63);
TBNZ(ARM64Reg::X1, 62, done); // Branch to done if NaN
// "Normalize" denormal values.
// The simplified calculation used here results in the upper 11 bits being incorrect,
@ -375,12 +375,11 @@ void JitArm64::GenerateFrsqrte()
BFI(ARM64Reg::X1, ARM64Reg::X3, 52, 12);
B(positive_normal);
SetJumpTarget(nan_or_inf);
SetJumpTarget(negative);
MOVI2R(ARM64Reg::X2, std::bit_cast<u64>(-std::numeric_limits<double>::infinity()));
CMP(ARM64Reg::X1, ARM64Reg::X2);
B(CCFlags::CC_NEQ, done);
B(CCFlags::CC_HI, done); // Branch to done if NaN
SetJumpTarget(negative);
TBNZ(ARM64Reg::W3, 9, done);
ORRI2R(ARM64Reg::W3, ARM64Reg::W3, FPSCR_FX | FPSCR_VXSQRT, ARM64Reg::W2);
B(store_fpscr);