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[docs] Clarify that Float16 is supported natively when possible #57725

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Merged
merged 4 commits into from
Mar 15, 2025
Merged

[docs] Clarify that Float16 is supported natively when possible #57725

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174531d
[docs] Clarify that Float16 is supported natively when possible
giordano Mar 11, 2025
f8512be
[docs] Add reference to `-fexcess-precision=standard` flag
giordano Mar 12, 2025
2b4105b
Further clarify implementation details of `Float16`
giordano Mar 12, 2025
f07fb35
Update doc/src/manual/integers-and-floating-point-numbers.md
giordano Mar 13, 2025
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3 changes: 1 addition & 2 deletions doc/src/manual/integers-and-floating-point-numbers.md
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Expand Up @@ -334,8 +334,7 @@ julia> typeof(x)
Float64
```

Half-precision floating-point numbers are also supported ([`Float16`](@ref)), but they are
implemented in software and use [`Float32`](@ref) for calculations.
Half-precision floating-point numbers are also supported ([`Float16`](@ref)) on all platforms, with native instructions used on hardware which supports this number format, otherwise operations are implemented in software and use [`Float32`](@ref) for intermediate calculations.
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Maybe point out that this equivalent to gcc -fexcess-precision=ieee

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I presume you meant -fexcess-precision=standard?

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I think we actually implement -fexcess-precision=16 https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fexcess-precision

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On x86 targets with SSE2 enabled, without -mavx512fp16, all operations will be emulated by software emulation and the float instructions. The default behavior for FLT_EVAL_METHOD is to keep the intermediate result of the operation as 32-bit precision. This may lead to inconsistent behavior between software emulation and AVX512-FP16 instructions. Using -fexcess-precision=16 will force round back after each operation.

Yeah we force round back after each operation.

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We techically just do the LLVM default nowadays, the demote float16 pass is basically unused since #55486

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Ok, I tried to incorporate both comments above, explaining this is an implementation detail (but I think some non-Julia people reading the manual may be interested in knowing it)


```jldoctest
julia> sizeof(Float16(4.))
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