[llvm-commits] [Patch] Use TargetTransformInfo to control switch-to-lookup table transform

[Hans Wennborg]

On Mon, Oct 29, 2012 at 3:29 PM, Duncan Sands <baldrick at free.fr> wrote:
>> When the switch-to-lookup tables transform landed in SimplifyCFG, it
>> was pointed out [1] that this could be inappropriate for some targets.
>> Since there was no way at the time for the pass to know anything about
>> the target, an awkward reverse-transform was added in CodeGenPrepare
>> that turned lookup tables back into switches for some targets.
>>
>> The attached patch uses the new TargetTransformInfo to determine if a
>> switch should be transformed, and removes
>> CodeGenPrepare::ConvertLoadToSwitch. Please take a look!
>
>
> personally I think it is completely wrong to have codegen concepts such as
> ISD::BR_JT and ISD::BRIND leaking into the IR level:
>
>> +bool ScalarTargetTransformImpl::shouldBuildLookupTables() const {
>> +  return TLI->supportJumpTables() &&
>> +      (TLI->isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
>> +       TLI->isOperationLegalOrCustom(ISD::BRIND, MVT::Other));

But as Hal pointed out below, those codegen concepts don't leak into
the IR level. The only thing that goes into the IR level is a "yes" or
"no" on whether the IR level should do this switch transformation or
not. Is that what you're opposing, or are you opposing the way
ScalarTargetTransformImpl::shouldBuildLookupTables is implemented?

Thanks,
Hans