Reapply "[DebugInfo] Drop DBG_VALUE_LISTs with an excessive number of debug operands"

Reapply b623df3c, which was reverted while reverting a different patch
with a breaking change. There are no underlying issues with this patch,
so no changes have been made to the original patch.

This reverts commit b11e4c990771541e440861f017afea7b4ba162f4.

1 files changed, 27 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugVariables.cpp b/llvm/lib/CodeGen/LiveDebugVariables.cpp
index 100dacb..2761ee6 100644
--- a/llvm/lib/CodeGen/LiveDebugVariables.cpp
+++ b/llvm/lib/CodeGen/LiveDebugVariables.cpp
@@ -119,17 +119,33 @@ public:
             DIExpression::replaceArg(Expression, OpIdx, DuplicatingIdx);
       }
     }
-    // A debug value referencing 64+ unique machine locations is very likely
-    // to be the result of a bug earlier in the pipeline. If by some means this
-    // limit is validly reached, then we can add a byte to the size of
-    // LocNoCount.
-    assert(LocNoVec.size() < 64 &&
-           "debug value containing 64+ unique machine locations is not "
-           "supported by Live Debug Variables");
-    LocNoCount = LocNoVec.size();
-    if (LocNoCount > 0) {
-      LocNos.reset(new unsigned[LocNoCount]());
-      std::copy(LocNoVec.begin(), LocNoVec.end(), loc_nos_begin());
+    // FIXME: Debug values referencing 64+ unique machine locations are rare and
+    // currently unsupported for performance reasons. If we can verify that
+    // performance is acceptable for such debug values, we can increase the
+    // bit-width of LocNoCount to 14 to enable up to 16384 unique machine
+    // locations. We will also need to verify that this does not cause issues
+    // with LiveDebugVariables' use of IntervalMap.
+    if (LocNoVec.size() < 64) {
+      LocNoCount = LocNoVec.size();
+      if (LocNoCount > 0) {
+        LocNos = std::make_unique<unsigned[]>(LocNoCount);
+        std::copy(LocNoVec.begin(), LocNoVec.end(), loc_nos_begin());
+      }
+    } else {
+      LLVM_DEBUG(dbgs() << "Found debug value with 64+ unique machine "
+                           "locations, dropping...\n");
+      LocNoCount = 1;
+      // Turn this into an undef debug value list; right now, the simplest form
+      // of this is an expression with one arg, and an undef debug operand.
+      Expression =
+          DIExpression::get(Expr.getContext(), {dwarf::DW_OP_LLVM_arg, 0,
+                                                dwarf::DW_OP_stack_value});
+      if (auto FragmentInfoOpt = Expr.getFragmentInfo())
+        Expression = *DIExpression::createFragmentExpression(
+            Expression, FragmentInfoOpt->OffsetInBits,
+            FragmentInfoOpt->SizeInBits);
+      LocNos = std::make_unique<unsigned[]>(LocNoCount);
+      LocNos[0] = UndefLocNo;
     }
   }