https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113280
--- Comment #7 from David Brown <david at westcontrol dot com> ---
Yes, the goal is an optimisation barrier with the least possible impact on the
code. For most uses, asm("" : "+g" (x)) has been ideal, as far as I have
tested. Typically it ensures "x" is evaluated in a register before the
assembly "executes", and uses that register for "x" afterwards. But "g" also
matches memory, so if "x" happens to be in a stack slot, it gets left there -
there is no extra movement in and out of a register.
But "g" does not match floating point or SIMD registers. So if "x" is in one
of these registers, extra moves are generated, and I'd like to avoid them. So
the ideal constraint would be one that matches memory or any register that can
contain data. That's not quite the same as "match anything" - it should not
match immediate values, nor "special" registers such as PC, SP, or a flags
register, or the extra registers many processors have for floating point
settings, link registers, interrupt state registers, and so on. But it might
include registers for other accelerators that some chips have.
