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c#.netfloating-pointdeterministic
kaalus
kaalus

Reputation: 4574

How are float/double literals stored in compiled .NET DLL files?

My application must produce exactly the same numeric results on all machines. I understand that float/double math in C# is not deterministic, but how about the binary representation of literal values?

float x = 1.23f;

How will 1.23f be stored in the compiled DLL file? As a binary representation of 32-bit IEEE-754 format float? Or as some intermediate representation that will need converting to IEEE-754 floating point by the jitter on target machine (a potential source of indeterminism)?

I understand why floating point operations in C# are not deterministic. I am asking ONLY whether the binary representation of literals is deterministic. Please, no answers about floating point determinism in general.

Upvotes: 1

Views: 168

Answers (2)

Matthew Watson
Matthew Watson

Reputation: 109587

They are stored as IEEE 754 format.

You can verify this by using ILDASM.EXE to disassemble the generated code and inspecting the binary values.

For example, float x = 1.23f; will generate:

IL_0000:  /* 22   | A4709D3F         */ ldc.r4     1.23

(Note that this is stored in little-endian format on Intel platforms.)

Upvotes: 2

Phoenix Stoneham
Phoenix Stoneham

Reputation: 147

The decimal type was introduced for this reason. Check it out in the official microsoft documentation here. https://learn.microsoft.com/en-us/dotnet/api/system.decimal?view=netframework-4.8

A decimal number is a floating-point value that consists of a sign, a numeric value where each digit in the value ranges from 0 to 9, and a scaling factor that indicates the position of a floating decimal point that separates the integral and fractional parts of the numeric value.

The binary representation of a Decimal value consists of a 1-bit sign, a 96-bit integer number, and a scaling factor used to divide the 96-bit integer and specify what portion of it is a decimal fraction. The scaling factor is implicitly the number 10, raised to an exponent ranging from 0 to 28. Therefore, the binary representation of a Decimal value the form, ((-296 to 296) / 10(0 to 28)), where -(296-1) is equal to MinValue, and 296-1 is equal to MaxValue. For more information about the binary representation of Decimal values and an example, see the Decimal(Int32[]) constructor and the GetBits method.

The scaling factor also preserves any trailing zeros in a Decimal number. Trailing zeros do not affect the value of a Decimal number in arithmetic or comparison operations. However, trailing zeros might be revealed by the ToString method if an appropriate format string is applied.

Upvotes: -1

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