[LLVMdev] Named Register Implementation

[Renato Golin]

On 28 March 2014 16:12, Krzysztof Parzyszek <kparzysz at codeaurora.org> wrote:
> I mean how do you make sure that the "write" builtin does not look like dead
> code, and at the same time it's not treated as something that "changes
> everything".

On the IR level, I expect a call to an intrinsic to never be pruned.
But I also need more guarantees regarding code movement, which I'm not
there is. Function calls that are not const can't be moved around, so
I expected that an intrinsic (being a function call in the IR level)
without any annotation regarding its safety would guarantee that it
doesn't happen.

That could be very optimistic, I know, thus my new thread asking for
those hard questions. ;)


>  Do you expect a read of "sp" to have a dependency with a write
> of "ax"?

No. On the IR level you don't have to worry about registers yet, and
at the DAG level, that intrinsic will be converted to a
CopyToReg/CopyFromReg, so the dependency is clearly on that register
only from there onwards.


>  If not, how is that going to be communicated to the optimizer?

The high-level IR optimisation passes should treat the intrinsics as
unassuming function calls, and the low-level pattern-matching
optimizations will already have the register in hand.

If you're not concerned about those, can you be more specific?

thanks,
--renato