#QUIET MEANING HOW TO#
#QUIET MEANING MANUAL#
#QUIET MEANING SOFTWARE#

Now that we know what qNaNs and sNaNs look like, and how to manipulate them, we are finally ready to try and make sNaNs do their thing and blow some programs up!Īssert(std::fetestexcept(FE_ALL_EXCEPT) = 0) Produce exactly the same bits as std::numeric_limits::quiet_NaN().īoth of those operations compile to a single x86 assembly instruction that generates the qNaN directly in the hardware (TODO confirm with GDB).

This can be seen from our example where both: float div_0_0 = 0.0f / 0.0f

#QUIET MEANING SOFTWARE#

They must be inserted by software that is, the processor never generates an SNaN as a result of a floating-point operation. SNaNs are typically used to trap or invoke an exception handler.

#QUIET MEANING MANUAL#

The Intel manual clearly states this principle however at 4.8.3.4 'NaNs': I could not find clear IEEE 754 or C11 quotes for that, but neither can I find any built-in operation that generates sNaNs -)

sNaN is never generated by built-in operations, it can only be explicitly added by programmers, e.g.

qNaN is generated by regular built-in (software or hardware) arithmetic operations with weird values.

One major difference between qNaNs and sNaNs is that: The sign bit and remaining fraction bits can take any value.

The sign bit can take any value, and the remaining fraction bits can take any value except all zeros. The two types of NaN are distinguished by their most significant fraction bit, bit: A QNaN is a NaN with the most significant fraction bit set an SNaN is a NaN with the most significant fraction bit clear.Īnd so does the ARM Architecture Reference Manual - ARMv8, for ARMv8-A architecture profile - DDI 0487C.a A1.4.3 'Single-precision floating-point format':įraction != 0: The value is a NaN, and is either a quiet NaN or a signaling NaN. The IA-32 architecture defines two classes of NaNs: quiet NaNs (QNaNs) and signaling NaNs (SNaNs). The Intel 64 and IA-32 Architectures Software Developer’s Manual - Volume 1 Basic Architecture - 253665-056US September 2015 4.8.3.4 'NaNs' confirms that x86 follows IEEE 754 by distinguishing NaN and sNaN by the highest fraction bit: In the preferred encoding just described, a signaling NaN shall be quieted by settingĭ1 to 1, leaving the remaining bits of T unchanged.įor binary formats, the payload is encoded in the p−2 least significant bits of the trailing significand field Trailing significand field is 0, some other bit of the trailing significand field must be non-zero to distinguish A signaling NaNīit string should be encoded with the first bit of the trailing significand field being 0. String should be encoded with the first bit (d1) of the trailing significand field T being 1. The remaining bits, which are in the trailing significandįield, encode the payload, which might be diagnostic information (see above).Īll binary NaN bit strings have all the bits of the biased exponent field E set to 1 (see 3.4). When encoded, all NaNs have a sign bit and a pattern of bits necessary to identify the encoding as a NaNĪnd which determines its kind (sNaN vs. This subclause further specifies the encodings of NaNs as bit strings when they are the results of operations.

and that the highest fraction bit differentiates qNaN from sNaNīut it does not seem to say which bit is preferred to differentiate infinity from NaN.Ħ.2.1 'NaN encodings in binary formats' says:.

anything with exponent = 0xFF and fraction != 0 is a NaN.

IEEE 754 2008 recommends that (TODO mandatory or optional?): How qNaNs and sNaNs look like in the manuals?

The expected form the meaning of the n-char sequence is implementation-defined. NaN, if supported in the return type, else like a subject sequence part that does not have The C11 N1570 standard draft confirms that nanf() generates quiet NaNs, because nanf forwards to strtod and 7.22.1.3 'The strtod, strtof, and strtold functions' says:Ī character sequence NAN or NAN(n-char-sequence opt ) is interpreted as a quiet Std::cout ::quiet_NaN(), we deduce that they are all different quiet NaNs. Static_assert(std::numeric_limits::has_infinity, '') įloat qnan = std::numeric_limits::quiet_NaN() įloat snan = std::numeric_limits::signaling_NaN() įloat inf = std::numeric_limits::infinity() Static_assert(std::numeric_limits::has_signaling_NaN, '') I could not however find a function to classify if a NaN is sNaN or qNaN, so let's just print out the NaN raw bytes: I'll be using C++ in this answer instead of C because it offers the convenient std::numeric_limits::quiet_NaN and std::numeric_limits::signaling_NaN which I could not find in C conveniently. Let's first learn how to identify if we have an sNaN or a qNaN. How qNaNs and sNaNs look like experimentally?

Quiet meaning