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digitalmars.D - Anyone relying on signaling NaNs?

reply Martin Nowak <code+news.digitalmars dawg.eu> writes:
Just tried to fix the float/double initialization w/ signaling NaNs [¹],
but it seems we can't reliably do that for all backends/architectures.
Any additional move of float might convert SNaNs to QNaNs (quiet NaNs).
This has also been the finding of other people [²][³].

The biggest problem w/ the current situation is that float fields of a
struct sometimes are initialized to QNaNs and fail `s.field is float.init`.

We thought about giving up on SNaNs as default float init values. Is
anyone relying on them?

-Martin

[¹]: [fix Issue 15316 - different NaN value in struct
initializer](https://github.com/dlang/dmd/pull/6163#issuecomment-250929498)
[²]: [Issue 9813 – Signalling NaN initialization does not always work
correctly on x86](https://issues.dlang.org/show_bug.cgi?id=9813)
[³]: [Coding Castles: NaNs, Uninitialized Variables, and
C++](http://codingcastles.blogspot.de/2008/12/nans-in-c.html)
Oct 01 2016
next sibling parent reply Ilya Yaroshenko <ilyayaroshenko gmail.com> writes:
On Saturday, 1 October 2016 at 19:10:47 UTC, Martin Nowak wrote:
 Just tried to fix the float/double initialization w/ signaling 
 NaNs [¹], but it seems we can't reliably do that for all 
 backends/architectures. Any additional move of float might 
 convert SNaNs to QNaNs (quiet NaNs). This has also been the 
 finding of other people [²][³].

 [...]
Not Mir. Furthermore, default zero initialisation is better.
Oct 01 2016
parent Martin Nowak <code dawg.eu> writes:
On Saturday, 1 October 2016 at 19:35:54 UTC, Ilya Yaroshenko 
wrote:
 Not Mir. Furthermore, default zero initialisation is better.
Yes, initializing floats to a value that is useless, requiring additional setting before usage is a weird design, but such a big change is out of scope and you can provide the init value.
Oct 02 2016
prev sibling parent kinke <noone nowhere.com> writes:
On Saturday, 1 October 2016 at 19:10:47 UTC, Martin Nowak wrote:
 Just tried to fix the float/double initialization w/ signaling 
 NaNs [¹], but it seems we can't reliably do that for all 
 backends/architectures. Any additional move of float might 
 convert SNaNs to QNaNs (quiet NaNs). This has also been the 
 finding of other people [²][³].

 The biggest problem w/ the current situation is that float 
 fields of a struct sometimes are initialized to QNaNs and fail 
 `s.field is float.init`.

 We thought about giving up on SNaNs as default float init 
 values. Is anyone relying on them?
I just had the same 'fun' with LDC. Both LDC 1.3 and DMD 2.074.0 produce special quiet NaNs for float.init and double.init on Win64 (both most significant mantissa bits set). I also tried to fix it, but it seems impossible when the x87 FPU (and not SSE) is used. This leads to a Win64 LDC build using signalling inits when cross-compiling via `-m32`, while the native Win32 LDC compiler uses quiet ones etc. So I'm all in for consistent special quiet NaNs as init values for all 3 floating-point types (already implemented, https://github.com/ldc-developers/ldc/pull/2207). If someone relies on signalling NaNs and missed the original post, here's your chance to speak up.
Jul 26