[Lldb-commits] [lldb] [lldb] Add pointer arithmetics for addition and subtraction to DIL (PR #184652)

[Michael Buch via lldb-commits]

================
@@ -602,6 +642,62 @@ llvm::Expected<lldb::ValueObjectSP> Interpreter::EvaluateBinarySubtraction(
   if (result_type.IsScalarType())
     return EvaluateScalarOp(BinaryOpKind::Sub, lhs, rhs, result_type, location);
 
+  auto lhs_type = lhs->GetCompilerType();
+  auto rhs_type = rhs->GetCompilerType();
+
+  // "pointer - integer" operation.
+  if (lhs_type.IsPointerType() && rhs_type.IsInteger())
+    return PointerAdd(lhs, -rhs->GetValueAsUnsigned(0), location);
+
+  // "pointer - pointer" operation.
+  if (lhs_type.IsPointerType() && rhs_type.IsPointerType()) {
+    if (lhs_type.IsPointerToVoid() && rhs_type.IsPointerToVoid()) {
+      return llvm::make_error<DILDiagnosticError>(
+          m_expr, "arithmetic on pointers to void", location);
+    }
+    // Compare canonical unqualified pointer types.
+    CompilerType lhs_unqualified_type =
+        lhs_type.GetCanonicalType().GetFullyUnqualifiedType();
+    CompilerType rhs_unqualified_type =
+        rhs_type.GetCanonicalType().GetFullyUnqualifiedType();
+    bool comparable = lhs_unqualified_type.CompareTypes(rhs_unqualified_type);
+    if (!comparable) {
+      std::string errMsg = llvm::formatv(
+          "'{0}' and '{1}' are not pointers to compatible types",
+          orig_lhs_type.GetTypeName(), orig_rhs_type.GetTypeName());
+      return llvm::make_error<DILDiagnosticError>(m_expr, errMsg, location);
+    }
+
+    llvm::Expected<uint64_t> lhs_byte_size =
+        lhs_type.GetPointeeType().GetByteSize(m_exe_ctx_scope.get());
+    if (!lhs_byte_size)
+      return lhs_byte_size.takeError();
+    // Since pointers have compatible types, both have the same pointee size.
+    int64_t item_size = *lhs_byte_size;
+    int64_t diff = static_cast<int64_t>(lhs->GetValueAsUnsigned(0) -
+                                        rhs->GetValueAsUnsigned(0));
+    if (diff % item_size != 0) {
+      // If address difference isn't divisible by pointee size then performing
+      // the operation is undefined behaviour.
+      return llvm::make_error<DILDiagnosticError>(
+          m_expr, "undefined pointer arithmetic", location);
+    }
+    diff /= item_size;
----------------
Michael137 wrote:

can we add an `assert (item_size > 0)`?

https://github.com/llvm/llvm-project/pull/184652