[DebugInfo] Fixing CodeView assert related to lowerBound field of DISubrange.

[alok]

Summary

This is to fix CodeView build failure https://bugs.llvm.org/show_bug.cgi?id=47287
after DIsSubrange upgrade D80197

Assert condition is now removed and Count is calculated in case LowerBound
is absent or zero and Count or UpperBound is constant. If Count is unknown
it is later handled as VLA (currently Count is set to zero).

[rnk]

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

1588–1589

I think we should handle the presence of anything we don't understand (non-zero lower bound, stride, and upper bound) as a VLA. As in, translate it to an array count of -1. That's how we can eliminate the assertion. If the user tries to emit codeview when compiling fortran with these types of arrays, they would probably prefer to have an array bound of zero in their debug info than for the compiler to crash.

I should've provided that guidance during the original review when this code landed. The author was trying to be cautious.

[alok]

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

1588–1589

Thanks for your comment. I shall update the patch and request you to have a look at updated patch and share your feedback.

[rnk]

I would ask you to add a test, but I can't recommend the existing test, llvm/test/DebugInfo/COFF/types-array.ll, as a good basis for a test. I'll look into rewriting it and adding coverage after you land.

[alok]

In D87406#2266722, @rnk wrote:

I would ask you to add a test, but I can't recommend the existing test, llvm/test/DebugInfo/COFF/types-array.ll, as a good basis for a test. I'll look into rewriting it and adding coverage after you land.

Thanks a lot for all your guidance and review.

llvm/lib/CodeGen/AsmPrinter/CodeViewDebug.cpp

TypeIndex CodeViewDebug::lowerTypeArray(const DICompositeType *Ty) {
 
   // Add subranges to array type.
   DINodeArray Elements = Ty->getElements();
   for (int i = Elements.size() - 1; i >= 0; --i) {
     const DINode *Element = Elements[i];
     assert(Element->getTag() == dwarf::DW_TAG_subrange_type);
 
     const DISubrange *Subrange = cast<DISubrange>(Element);
-    assert(!Subrange->getRawLowerBound() &&
-           "codeview doesn't support subranges with lower bounds");
+    // Codeview supports Subranges with either absent lowerBound or lowerBound
+    // with constant zero value. Everyting else will be rejected.
+    auto *LI = Subrange->getLowerBound().dyn_cast<ConstantInt *>();
+    assert(
+        (!Subrange->getRawLowerBound() || (LI && (LI->getSExtValue() == 0))) &&
+        "codeview doesn't support subranges with non zero lower bounds");
+
     int64_t Count = -1;
     if (auto *CI = Subrange->getCount().dyn_cast<ConstantInt*>())
       Count = CI->getSExtValue();
 
     // Forward declarations of arrays without a size and VLAs use a count of -1.
     // Emit a count of zero in these cases to match what MSVC does for arrays
     // without a size. MSVC doesn't support VLAs, so it's not clear what we
     // should do for them even if we could distinguish them.
     if (Count == -1)
       Count = 0;