llvm dev - Aug 2021 - A question about support RV32E in the riscv target

Hello,


Though the ABI / call-convention for rv32e has not be defined clear by the riscv
organization.


But besides that, is it possible to partial support it by adding target feature
FeatureReserveX16---FeatureReserveX31 to the FeatureRV32E ?
Just like 
def FeatureStdExtD
    : SubtargetFeature<"d", "HasStdExtD",
"true",
                       "'D' (Double-Precision
Floating-Point)",
                       [FeatureStdExtF]>;


FeatureStdExtD implies FeatureStdExtF, we can make FeatureRV32E implies
FeatureReserveX16---FeatureReserveX31 ?


Is this a proper way to tell the register allocator do not use X16-X31 ?


Ben

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I would probably not do that. It creates a reverse relationship that can
create unexpected surprises, for example passing
"-mattr=-reserved-x31"
would clear the RV32E feature.

I would handle it in RISCVRegisterInfo::getReservedRegs or maybe
RISCVSubtarget::isRegisterReservedByUser. I would use the latter if it is
important that the other callers of isRegisterReservedByUser care about
this. Though I think most of those generate an error which might not be
what you want.

~Craig


On Mon, Aug 9, 2021 at 5:43 AM Ben Shi via llvm-dev <llvm-dev at
lists.llvm.org>
wrote:
> Hello,
>
> Though the ABI / call-convention for rv32e has not be defined clear by the
> riscv organization.
>
> But besides that, is it possible to partial support it by adding target
> feature FeatureReserveX16---FeatureReserveX31 to the FeatureRV32E ?
> Just like
> def FeatureStdExtD
>     : SubtargetFeature<"d", "HasStdExtD",
"true",
>                        "'D' (Double-Precision
Floating-Point)",
>                        [FeatureStdExtF]>;
>
> FeatureStdExtD implies FeatureStdExtF, we can make FeatureRV32E implies
> FeatureReserveX16---FeatureReserveX31 ?
>
> Is this a proper way to tell the register allocator do not use X16-X31 ?
>
> Ben
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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