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RE: IRA preferencing issues


Wilco Dijkstra <wdijkstr@arm.com> writes:
> While investigating why the IRA preferencing algorithm often chooses
> incorrect preferences from the costs, I noticed this thread:
> https://gcc.gnu.org/ml/gcc/2011-05/msg00186.html
> 
> I am seeing the exact same issue on AArch64 - during the final
> preference selection ira-costs takes the union of any register classes
> that happen to have equal cost. As a result many registers get ALL_REGS
> as the preferred register eventhough its cost is much higher than either
> GENERAL_REGS or FP_REGS. So we end up with lots of scalar SIMD
> instructions and expensive int<->FP moves in integer code when register
> pressure is high. When the preference is computed correctly as in the
> proposed patch (choosing the first class with lowest cost, ie.
> GENERAL_REGS) the resulting code is much more efficient, and there are
> no spurious SIMD instructions.
> 
> Choosing a preferred class when it doesn't have the lowest cost is
> clearly incorrect. So is there a good reason why the proposed patch
> should not be applied? I actually wonder why we'd ever need to do a
> union - if there are 2 classes with equal cost, you'd use the 2nd as the
> alternative class.
> 
> The other question I had is whether there is a good way to get improve
> the preference in cases like this and avoid classes with equal cost
> altogether. The costs are clearly not equal: scalar SIMD instructions
> have higher latency and require extra int<->FP moves. It is possible to
> mark variants in the MD patterns using '?' to discourage them but that
> seems like a hack, just like '*'. Is there a general way to say that
> GENERAL_REGS is preferred over FP_REGS for SI/DI mode?

MIPS has the same problem here and we have been looking at ways to address
it purely via costings rather than changing IRA. What we have done so
far is to make the cost of a move from GENERAL_REGS to FP_REGS more
expensive than memory if the move has an integer mode. The goal for MIPS
is to never allocate an FP register to an integer mode unless it was
absolutely necessary owing to an integer to fp conversion where the
integer has to be put in an FP register. Ideally I'd like a guarantee
that FP registers will never be used unless a floating point type is
present in the source but I haven't found a way to do that given the
FP-int conversion issue requiring SImode to be allowed in FP regs.

The patch for MIPS is not submitted yet but has eliminated the final
two uses of FP registers when building the whole Linux kernel with
hard-float enabled. I am however still not confident enough to say
you can build integer only code with hard-float and never touch an FP
register.

Since there are multiple architectures suffering from this I guess we
should look at properly addressing it in generic code.

Thanks,
Matthew


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