https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117266
--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to H. Peter Anvin from comment #6)
> And THAT is exactly the point: *the two aren't equivalent.* Only the
> programmer knows when this instruction is usable, and for performance
> reasons, you *really, really* want to be able to use it when you as the
> programmer know, a priori, that you can.
Actually the compiler could know based on the ranges. And it could techincally
optimize something like you gave for div2 into the instruction.
Like say:
```
typedef unsigned _BitInt(64) uint64_t;
uint64_t div2(uint64_t hi, uint64_t lo, uint64_t divisor)
{
unsigned _BitInt(128) dividend = ((unsigned _BitInt(128))hi << 64) | lo;
unsigned _BitInt(128) qq = dividend / divisor;
if (qq >> 64)
__builtin_unreachable();
return qq;
}
```
Could be optimized to using the 128/64->64 instruction since you say the upper
bits are 0; otherwise it is undefined.
Note a trap here could be how it is undefined.
