Reapply: [DebugInfo] Correctly handle salvaged casts and split fragments at ISel

This reverts commit 1f3dd83cc1f2b8f72b9d59e2b4221b12fb7f9a95, reapplying
commit bb1b0bc4e57428ce364d3d6c075ff03cb8973462.

The original commit failed on some builds seemingly due to the use of a
bracketed constructor with an std::array, i.e. `std::array<> arr({...})`.

1 files changed, 19 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 27ac489b35b..735ac67ebfc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -5571,8 +5571,26 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
       = [&](ArrayRef<std::pair<unsigned, unsigned>> SplitRegs) {
       unsigned Offset = 0;
       for (auto RegAndSize : SplitRegs) {
+        // If the expression is already a fragment, the current register
+        // offset+size might extend beyond the fragment. In this case, only
+        // the register bits that are inside the fragment are relevant.
+        int RegFragmentSizeInBits = RegAndSize.second;
+        if (auto ExprFragmentInfo = Expr->getFragmentInfo()) {
+          uint64_t ExprFragmentSizeInBits = ExprFragmentInfo->SizeInBits;
+          // The register is entirely outside the expression fragment,
+          // so is irrelevant for debug info.
+          if (Offset >= ExprFragmentSizeInBits)
+            break;
+          // The register is partially outside the expression fragment, only
+          // the low bits within the fragment are relevant for debug info.
+          if (Offset + RegFragmentSizeInBits > ExprFragmentSizeInBits) {
+            RegFragmentSizeInBits = ExprFragmentSizeInBits - Offset;
+          }
+        }
+
         auto FragmentExpr = DIExpression::createFragmentExpression(
-          Expr, Offset, RegAndSize.second);
+            Expr, Offset, RegFragmentSizeInBits);
+        Offset += RegAndSize.second;
         // If a valid fragment expression cannot be created, the variable's
         // correct value cannot be determined and so it is set as Undef.
         if (!FragmentExpr) {
@@ -5585,7 +5603,6 @@ bool SelectionDAGBuilder::EmitFuncArgumentDbgValue(
         FuncInfo.ArgDbgValues.push_back(
           BuildMI(MF, DL, TII->get(TargetOpcode::DBG_VALUE), false,
                   RegAndSize.first, Variable, *FragmentExpr));
-        Offset += RegAndSize.second;
       }
     };