[ARM][MVE] tail-predication: predicate new checks on force-enabled option

[SjoerdMeijer]

Summary

Additional sanity checks were added to get.active.lane.mask's second argument, the loop tripcount/elementcount, in rG676febc044ecbb27f8. Like the other (overflow) checks, skip this if tail-predication is forced.

[dmgreen]

Sounds like a good idea in the short term. Thanks

[SjoerdMeijer]

Thanks Dave, just for completeness, uploading a new diff with the codegen changes gone, which shouldn't have been there.

llvm/lib/Target/ARM/MVETailPredication.cpp

if ((ConstElemCount = dyn_cast<ConstantInt>(ElemCount))) {
     uint64_t TC2 = ConstElemCount->getZExtValue() + 1;
 
     if (TC1 != TC2) {
       LLVM_DEBUG(dbgs() << "ARM TP: inconsistent constant tripcount values: "
                  << TC1 << " from set.loop.iterations, and "
                  << TC2 << " from get.active.lane.mask\n");
       return false;
     }
-  } else {
+  } else if (!ForceTailPredication) {
     // Smoke tests if the element count is a runtime value. I.e., this isn't
     // fully generic because that would require a full SCEV visitor here. It
     // would require extracting the variable from the elementcount SCEV
     // expression, and match this up with the tripcount SCEV expression. If
     // this matches up, we know both expressions are bound by the same
     // variable, and thus we know this tripcount belongs to this loop. The
     // checks below will catch most cases though.
     if (isa<SCEVAddExpr>(EC) || isa<SCEVUnknown>(EC)) {