[Bug target/99434] std::bit_cast generates more instructions than __builtin_bit_cast and memcpy with -march=native

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to cqwrteur from comment #2)
> (In reply to Andrew Pinski from comment #1)
> > This is just a register allocation issue dealing with mulx and TImode.
> > 
> > If mulq was used instead (that is without -march=native), all of the
> > functions are done correctly.
> 
> I do not think so. I think GCC generally did things like this wrong. I have
> even found out how to produce different wrong results deterministically.
> 
> For example like this
> https://godbolt.org/z/PbobYG
> 
> Any time it deals with things like >>32 or >>64, it produces a slower result.
> This even compiles without -march=native.

This is still a register allocation issue, this time dealing with DImode on
32bit. GCC has a known issue with register allocation when dealing with values
stored into two registers.  See PR 21150, PR 43644, PR 50339, etc.