================
@@ -1262,6 +1262,18 @@ bool MachineLICMBase::IsProfitableToHoist(MachineInstr
&MI,
return false;
}
+ // If we have a COPY with other uses in the loop, hoist to allow the users to
----------------
bjope wrote:
For info: Downstream this caused regressions in some of our benchmarks. Not
sure if you need to care about that (we will probably guard this with a check
for our downstream target), but thought it might be nice to mention it.
Haven't fully investigated what happens, but I think that hoisting the COPY
increases register pressure resulting in spill. The COPY instructions I see
that are hoisted in that benchmark can be mapped to two categories:
```
%228:gn32 = COPY %143:pn ; cross register bank copy
%245:gn16 = COPY %227.lo:gn32 ; extracting a subreg
```
So for example hoisting the cross register bank COPY results in increasing the
register pressure for the general gn registers in the path leading up to the
use. Similarly, by hoisting the subreg extract the register pressure on the gn
registers increase.
I wonder if the heuristic here perhaps should look closer at the using
instructions to see if they actually can be hoisted as well? After all, we are
in the path where CanCauseHighRegPressure has returned true. So traditinonally
we have been more careful here and only hoisted trivially rematerializable MI:s.
https://github.com/llvm/llvm-project/pull/71403
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