[Bug target/58416] Incorrect x87-based union copying code

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416

--- Comment #3 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Richard Biener from comment #2)

> where *movdf_internal simply doesn't properly preserve sNaN.
> 
> It looks wrong for DFmode move instructions to not preserve a IEEE defined
> flag.  I suppose it would be also wrong to not preserve the exact bit pattern
> (think of canonicalizing NaNs).

As said in PR57484, comment 11:

On an x86 target using the legacy x87 instructions and the 80-bit registers, a
load of a 64-bit or 32-bit value in memory into the 80-bit registers counts as
a format conversion and an signaling NaN input will turn into a quiet NaN in
the register format.