[Bug rtl-optimization/113280] Strange error for empty inline assembly with +X constraint

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113280

--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #6)
> From the context given in the gcc-help thread, the goal is to place an
> optimization barrier in a sequence of floating-point calculation. "+r" is
> inappropriate for floats, as it usually incurs a reload from a
> floating-point register to a GPR and back, and there's no universal
> constraint for FP regs (e.g. on amd64 it is "+x" for SSE registers, but "+t"
> for long double (or x87 FPU on 32-bit x86)).

Yup.  "Some target-specific register constraint" :-)