On Tue, 24 Jun 2025 08:17:14 -0500
Segher Boessenkool <seg...@kernel.crashing.org> wrote:
> On Tue, Jun 24, 2025 at 07:27:47AM +0200, Christophe Leroy wrote:
> > Ah ok, I overlooked that, I didn't know the cmove instruction, seem
> > similar to the isel instruction on powerpc e500.
>
> cmove does a move (register or memory) when some condition is true.
The destination of x86 'cmov' is always a register (only the source can be
memory - an is probably always read).
It is a also a computational instruction.
It may well always do the register write - hard to detect.
There is a planned new instruction that would do a conditional write
to memory - but not on any cpu yet.
> isel (which is base PowerPC, not something "e500" only) is a
> computational instruction, it copies one of two registers to a third,
> which of the two is decided by any bit in the condition register.
Does that mean it could be used for all the ppc cpu variants?
> But sure, seen from very far off both isel and cmove can be used to
> implement the ternary operator ("?:"), are similar in that way :-)
Which is exactly what you want to avoid speculation.
David
>
>
> Segher
