incorrect result for fp16 fract function for small negative inputs

[bashbaug]

Found during a review of new fp16 CTS tests:

KhronosGroup/OpenCL-CTS#1681 (review)

IGC is currently generating incorrect results for fract for small negative inputs. Specifically, it is returning 1.0, rather than a number close to but not quite 1.0.

For -0.000061 (8400): fract() returned 1.000000 (3C00) + -1.000000 (BC00)

It looks like this is because IGC is using to the fp32 constant 0x1.fffffep-1f rather than the fp16 constant 0x1.ffcp-1f:

intel-graphics-compiler/IGC/BiFModule/Implementation/Math/fract.cl

Lines 288 to 295 in d9e9d29

	INLINE half SPIRV_OVERLOADABLE SPIRV_OCL_BUILTIN(fract, _f16_p0f16, )( half x,
	__private half* iptr )
	{
	*iptr = SPIRV_OCL_BUILTIN(select, _f16_f16_i16, )( SPIRV_OCL_BUILTIN(floor, _f16, )( x ), SPIRV_OCL_BUILTIN(nan, _i16, )( (short)0 ), (short)(SPIRV_BUILTIN(IsNan, _f16, )( x )) );
	half temp = x - *iptr;
	temp = SPIRV_OCL_BUILTIN(select, _f16_f16_i16, )( (half)SPIRV_OCL_BUILTIN(fmin, _f16_f16, )( temp, (half)(0x1.fffffep-1f)), (half)SPIRV_OCL_BUILTIN(copysign, _f16_f16, )((half)0.0f, x), (short)SPIRV_BUILTIN(IsInf, _f16, )(x));
	return SPIRV_OCL_BUILTIN(select, _f16_f16_i16, )( temp, SPIRV_OCL_BUILTIN(nan, _i16, )((short)0), (short)(SPIRV_BUILTIN(IsNan, _f16, )(x)) );
	}

If I use the fp16 constant instead:

half my_fract_fix(half x, private half* iptr) {
    *iptr = select(floor(x), nan((ushort)0), (short)isnan(x));
    half temp = x - *iptr;
    temp = select(fmin(temp, (half)(0x1.ffcp-1f)), (half)copysign((half)0.0f, x), (short)isinf(x));
    return select(temp, nan((ushort)0), (short)isnan(x));
}

Then I get the right results:

For -0.000061 (8400): fract() returned 0.999512 (3BFF) + -1.000000 (BC00)

[pszymich]

Fixed in 0db1db3
Thanks @bashbaug for the bug submission and thanks @MichalMroz12 for the fix.

[bashbaug]

It looks like the fix for this issue was backed out in 3cf68d7

So, it is still occurring. Can this issue be re-opened?