IEEE 754和浮点精度

Question

Does it make any difference in having a 32 or 64-bit CPU in the amount of precision that IEEE 754 provides?

I mean when programming in C whether the size of float , double and long double are different between a 32 or 64-bit CPU.

Answer 1

In most architectures that use IEEE-754, float and double are exact 32 and 64-bit types corresponding to single and double precision respectively. Therefore the precision is the same whether you're on a 32 or a 64-bit computer . The exceptions are some microcontrollers with non-standard-compliant C compilers where both double and float are the same and contain 32 bits

OTOH long double support varies depending on system . On x86 most implementations will utilize the hardware 80-bit extended precision type (often padded to 12 or 16 bytes in order to maintain alignment), except MSVC where long double is just an alias for double . On other architectures long double are often implemented as either

1. The same type as double , or
2. 128-bit IEEE 754-2008 quadruple precision , or
3. 128-bit double-double

While the second way increases both range and precision significantly compared to double , it's also often significantly slower due to the lack of hardware support

The double-double method results in a type with in the same range but twice the precision of double , with the advantage of hardware double support, ie you don't need to implement entirely in software like quadruple precision. However it's not IEEE-754 compliant

If you're doing a lot of math on x86 or arm , moving to 64-bit would benefit because of the increased number of registers, SSE2/Neon available by default... which improves performance compared to the 32-bit version, unlike most other architectures where 64-bit programs often run slower due to bigger pointers.

Answer 2

It is common to most 32-bit and 64-bit machines for float to be IEEE-754 32-bit floating point, and double to be IEEE-754 64-bit floating point. Some implementations might use the IEEE-754 80-bit type as double (or long double).

Answer 3

No, there is no difference, you can confirm this by checking sizeof(float) across both architectures. If you need greater precision use double .

Answer 4

Assuming float and double map to IEEE-754 single-precision and double-precision numbers respectively, then no, there is no difference.

long double may be a different story, however, since compilers may choose to pad it to an even size.