mirror of
https://github.com/pkivolowitz/asm_book.git
synced 2026-06-21 06:26:49 +08:00
43 lines
1.2 KiB
Markdown
43 lines
1.2 KiB
Markdown
# Section 1 / Half Precision Floats
|
|
|
|
TL;DR - don't use these in C and C++ without being willing to wade
|
|
through a great deal of muck. In assembly language, it is more
|
|
straight forward.
|
|
|
|
## Half Precision Formats in C and C++
|
|
|
|
Support for half precision (16 bit) floating point values does exist but
|
|
there is no complete agreement on how different compilers support them.
|
|
Indeed, there are not one but two competing half precision formats out
|
|
there. These are the IEEE and GOOGLE types. Further still, many open
|
|
source developers have created their own implementations with
|
|
potentially clashing naming conventions.
|
|
|
|
Finally, as of this writing, there is a performance penalty to using
|
|
half precision floating point values from C and C++ for ordinary math.
|
|
See below:
|
|
|
|
```c++
|
|
__fp16 Foo(__fp16 g, __fp16 f) {
|
|
return g + f;
|
|
}
|
|
```
|
|
|
|
compiles to:
|
|
|
|
```asm
|
|
fcvt s1, h1
|
|
fcvt s0, h0
|
|
fadd s0, s0, s1
|
|
fcvt h0, s0
|
|
ret
|
|
```
|
|
|
|
On the other hand, if you are willing to use *intrinsics* and one of
|
|
the SIMD instruction sets offered by ARM, then knock yourself out. Be
|
|
aware that doing so ties your code to the ARM processor in ways which
|
|
you might regret.
|
|
|
|
## Half Precision in Assembly Language
|
|
|
|
more to come
|