IEEE754 non-conformance: is_sign_negative does not apply on NANs #42425

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1. nagisa

   added

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   T-libs-api[DEPRECATED; DO NOT USE][DEPRECATED; DO NOT USE]

   I-wrong

   on Jun 4, 2017
2. Thiez commented on Jun 4, 2017

   

   Thiez

   on Jun 4, 2017

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   This program:

   fn main() {
       let nums = [::std::f64::NEG_INFINITY, -1.0, -0.0, ::std::f64::NAN, 0.0, 1.0, ::std::f64::INFINITY];
       for &num in &nums[..] {
           println!("Number: {}\tSign negative: {}\tSign positive: {}", num, num.is_sign_negative(), num.is_sign_positive());
       }
   }

   prints the following output when compiled with current master and nightly:

   Number: -inf	Sign negative: true	Sign positive: false
   Number: -1	Sign negative: true	Sign positive: false
   Number: 0	Sign negative: true	Sign positive: false
   Number: NaN	Sign negative: false	Sign positive: false
   Number: 0	Sign negative: false	Sign positive: true
   Number: 1	Sign negative: false	Sign positive: true
   Number: inf	Sign negative: false	Sign positive: true
   

   What would be the expected result according to the standard? I would expect something that isn't a number to be neither negative nor positive, so I think the current results are correct?
3. nagisa commented on Jun 4, 2017

   

   nagisa

   on Jun 4, 2017

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   My reading of the spec suggests the answer should be equivalent to is_sign_negative = signbit(x) != 0, where signbit is non-zero if the binary representation has sign bit set.

   E.g. Section 6.2.1:

   When encoded, all NaNs have a sign bit and a pattern of bits necessary to identify the encoding as a NaN and which determines its kind (sNaN vs. qNaN).

   alludes to NaNs having a sign bit. The description of isSignMinus seems to quite clearly signal towards the fact that for NaNs the implementation ought to be inspecting this sign bit.

   You can set the sign of NaN in rust with -::std::f64::NAN as well.
4. alexcrichton

   added

   P-mediumMedium priorityMedium priority

   and removed

   I-nominated

   on Jun 7, 2017
5. bors

   added a commit that references this issue on Jun 28, 2017

   Auto merge of #42431 - nagisa:core-float-2, r=alexcrichton

   ...

   88c3242
6. Mark-Simulacrum

   added

   C-bugCategory: This is a bug.Category: This is a bug.

   and removed

   I-wrong

   on Jul 27, 2017
7. hanna-kruppe commented on Nov 20, 2017

   

   hanna-kruppe

   on Nov 20, 2017

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   Isn't this fixed since #42431 was merged?
8. nagisa

   closed this as completedon Nov 20, 2017
9. bors

   added a commit that references this issue on Mar 1, 2018

   Merge #41

   ...

   5f90623