[Bug c++/98829] New: Different results with -O3 and custom quiet NaN

[gnu at nemanjaboric dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98829

Bug ID: 98829
   Summary: Different results with -O3 and custom quiet NaN
   Product: gcc
   Version: 10.2.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: gnu at nemanjaboric dot com
  Target Milestone: ---

Created attachment 50047
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50047&action=edit
Example of the code giving different results

Hi, see the attached code which generates the different code with -O3 (compiled
with various GCC version). Two workarounds are commented in the code: to use
`std::isnan` and to copy the source object.

I couldn't find anything undefined that I'm doing here but I might be wrong.

-ffast-math _is not_ used. Just -O3 yields different result:
https://www.godbolt.org/z/nxPd8W

[Bug middle-end/98829] Different results with -O3 and custom quiet NaN

[gnu at nemanjaboric dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98829

--- Comment #2 from Nemanja Boric  ---
Indeed, but there's a barrier in the code (is_empty) which doesn't let NaN
values to enter the computation, so they shouldn't propagate.

[Bug middle-end/98829] Different results with -O3 and custom quiet NaN

[gnu at nemanjaboric dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98829

--- Comment #3 from Nemanja Boric  ---
Changing:

static constexpr std::uint64_t kMagicNumber = 1730;
static constexpr std::uint64_t kCustomNaN = 0x7ff0 |
kMagicNumber;

to

static inline std::uint64_t kMagicNumber = 1730;
static inline std::uint64_t kCustomNaN = 0x7ff0 | kMagicNumber;

also avoids the issue.

[Bug middle-end/98829] Different results with -O3 and custom quiet NaN

[gnu at nemanjaboric dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98829

--- Comment #5 from Nemanja Boric  ---
Yes, it seems that if the constants are `constexpr` or `static inline const`
the custom payload is gone. 

I guess this is aligned to
https://en.cppreference.com/w/cpp/types/numeric_limits/quiet_NaN

"A NaN never compares equal to itself. Copying a NaN may not preserve its bit
representation."