# OS dependency of \`Float16(::BigFloat)\` on nightly

**URL:** <https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414>\
**Category:** New to Julia\
**Created:** [April 2, 2021, 6:08am UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414 "2021-04-02T06:08:07Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [April 2, 2021, 6:08am UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414/1 "2021-04-02T06:08:07Z")

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Is there any OS dependency regarding the `BigFloat` → `Float16` conversion? ( **Edit:** Apparently, the core problem may be with the `Float64` → `Float16` conversion.)  
I’m not sure the cause, but the output of CI (GitHub Actions) for macOS x86\_64 says:

```julia
julia> versioninfo()
Julia Version 1.7.0-DEV.796
Commit 82dc40264d (2021-04-01 23:25 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin18.7.0)
  CPU: Intel(R) Xeon(R) CPU E5-1650 v2 @ 3.50GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, ivybridge)

julia> Float16(big"16379" / 16383)
Float16(0.9995)

```

On Linux x86\_64, for example, it returns the more accurate `Float16(1.0)`.  
My concern is not the failure of correct rounding due to double rounding, etc., but the OS dependency.

I don’t have a local macOS environment, so I haven’t been able to track the details, but the change has occurred over the last about half a month, and there has been at least one implementation change.  
[https://github.com/JuliaLang/julia/pull/40245](https://github.com/JuliaLang/julia/pull/40245)  
However, PR #40245 itself should be OS-independent. 🤔

This was discovered from a CI failure in FixedPointNumbers: [Test failure on nightly (1.7.0-DEV) in macOS (x64) · Issue #246 · JuliaMath/FixedPointNumbers.jl · GitHub](https://github.com/JuliaMath/FixedPointNumbers.jl/issues/246)

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [April 2, 2021, 6:35am UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414/2 "2021-04-02T06:35:27Z")

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As the author of that or, I think it’s plausible today that might be the cause. If so, it would presumably be due to a bug in how mac converts Float64 to Float16.

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<div class="post-metadata">

**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [April 2, 2021, 6:49am UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414/3 "2021-04-02T06:49:16Z")

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I see, `Float64(::BigFloat)` seems to be ok. So, the difference seems to be in `Float16(::Float64)`.

```julia
julia> Float64(big"16378" / 16383)
0.9996948055911615

julia> Float64(big"16379" / 16383)
0.9997558444729292

julia> Float16(0.9996948055911615)
Float16(0.9995)

julia> Float16(0.9997558444729292) # `Float16(1.0)` on Linux and Windows
Float16(0.9995)

```

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<div class="post-metadata">

**Author:** ![kimikage](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kimikage/32/14534_2.png) [@kimikage](https://discourse.julialang.org/u/kimikage)\
**Post date:** [April 2, 2021, 7:49am UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414/4 "2021-04-02T07:49:30Z")

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It seems that `__truncdfhf2` is called when going through `Float64`.

```nohighlight
julia> @code_native Float16(1.0)
	.section	__TEXT,__ text,regular,pure_instructions
; ┌ @ float.jl:204 within `Float16'
	subq	$8, %rsp
	movabsq	$__truncdfhf2, %rax
	callq	*%rax
	popq	%rcx
	retq
	nopw	%cs:(%rax,%rax)
; └

julia> @code_native Float16(1.0f0)
	.section	__TEXT,__ text,regular,pure_instructions
; ┌ @ float.jl:203 within `Float16'
	vcvtps2ph	$4, %xmm0, %xmm0
	vmovd	%xmm0, %eax
	retq
	nopl	(%rax,%rax)
; └

```

> <https://github.com/JuliaLang/julia/blob/878e1cd52ebf14e8666d8d54a54a95ccee4405d3/src/intrinsics.cpp#L1478-L1498>

There is indeed a conditional branch on `_OS_DARWIN_` there.

```cpp
extern "C" JL_DLLEXPORT uint16_t __truncdfhf2(double param)
{
    return float_to_half((float)param);
}

```

Triple rounding? 😅

I think the concept of PR #40245 is very nice, but I think there are (at least two) problems with the current implementation.

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [April 2, 2021, 1:13pm UTC](https://discourse.julialang.org/t/os-dependency-of-float16-bigfloat-on-nightly/58414/5 "2021-04-02T13:13:11Z")

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Yeah, it would probably be a good idea to make a version that only uses arithmetic. Shouldn’t be too hard, and could get rid of the double rounding completely.