piwalker/dolphin
1
0
Fork 0
mirror of https://github.com/dolphin-emu/dolphin.git synced 2025-07-24 14:49:42 -06:00
Code Issues Packages Projects Releases Wiki Activity

Jit64[IL]: fix float conversions

Browse Source 
Floating-point is complicated...

Some background: Denormals are floats that are too close to zero to be
stored in a normalized way (their exponent would need more bits). Since
they are stored unnormalized, they are hard to work with, even in
hardware.  That's why both PowerPC and SSE can be configured to operate
in faster but non-standard-conpliant modes in which these numbers are
simply rounded ('flushed') to zero.

Internally, we do the same as the PowerPC CPU and store all floats in
double format. This means that for loading and storing singles we need a
conversion. The PowerPC CPU does this in hardware. We previously did
this using CVTSS2SD/CVTSD2SS. Unfortunately, these instructions are
considered arithmetic and therefore flush denormals to zero if non-IEEE
mode is active. This normally wouldn't be a problem since the next
arithmetic floating-point instruction would do the same anyway but as it
turns out some games actually use floating-point instructions for
copying arbitrary data.

My idea for fixing this problem was to use x87 instructions since the
x87 FPU never supported flush-to-zero and thus doesn't mangle denormals.
However, there is one more problem to deal with: SNaNs are automatically
converted to QNaNs (by setting the most-significant bit of the
fraction). I opted to fix this by manually resetting the QNaN bit of all
values with all-1s exponent.
This commit is contained in:
Tillmann Karras
2014-02-03 23:56:11 +01:00
parent c25c4a6e20
commit db196d8c5b
8 changed files with 173 additions and 37 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
Source/Core/Common/x64Emitter.h
View File

@ -433,9 +433,27 @@ public:
void REPNE();
// x87
enum x87StatusWordBits {
x87_InvalidOperation = 0x1,
x87_DenormalizedOperand = 0x2,
x87_DivisionByZero = 0x4,
x87_Overflow = 0x8,
x87_Underflow = 0x10,
x87_Precision = 0x20,
x87_StackFault = 0x40,
x87_ErrorSummary = 0x80,
x87_C0 = 0x100,
x87_C1 = 0x200,
x87_C2 = 0x400,
x87_TopOfStack = 0x2000 | 0x1000 | 0x800,
x87_C3 = 0x4000,
x87_FPUBusy = 0x8000,
};
void FLD(int bits, OpArg src);
void FST(int bits, OpArg dest);
void FSTP(int bits, OpArg dest);
void FNSTSW_AX();
void FWAIT();
// SSE/SSE2: Floating point arithmetic
@ -562,6 +580,7 @@ public:
void PUNPCKLWD(X64Reg dest, const OpArg &arg);
void PUNPCKLDQ(X64Reg dest, const OpArg &arg);
void PTEST(X64Reg dest, OpArg arg);
void PAND(X64Reg dest, OpArg arg);
void PANDN(X64Reg dest, OpArg arg);
void PXOR(X64Reg dest, OpArg arg);
@ -631,6 +650,8 @@ public:
void VMULSD(X64Reg regOp1, X64Reg regOp2, OpArg arg);
void VDIVSD(X64Reg regOp1, X64Reg regOp2, OpArg arg);
void VSQRTSD(X64Reg regOp1, X64Reg regOp2, OpArg arg);
void VPAND(X64Reg regOp1, X64Reg regOp2, OpArg arg);
void VPANDN(X64Reg regOp1, X64Reg regOp2, OpArg arg);
void RTDSC();