Adjust a ValueObjectChild's offset when the child is a bitfield

[aprantl]

Summary

If a bitfield doesn't fit into the child_byte_size'd window at child_byte_offset, move the window (= child_byte_offset) forward until it fits.

This was not in the big-endian-derived DWARF 2 bitfield attributes because their offsets were counted from the end of the window, so they always fit.

rdar://problem/53132189

[aprantl]

ping.

[jasonmolenda]

I'm sure this is fine, I read the code through and I've realized I haven't had to do anything with bitfields in lldb up to now.

lldb/packages/Python/lldbsuite/test/lang/c/bitfields/TestBitfields.py

def test_and_run_command(self):
                 '(uint8_t:1) b12 = \'\\0\'',
                 '(uint8_t:1) b13 = \'\\x01\'',
                 '(uint8_t:1) b14 = \'\\x01\'',
                 '(uint8_t:1) b15 = \'\\0\'',
                 '(uint8_t:1) b16 = \'\\0\'',
                 '(uint8_t:1) b17 = \'\\0\'',
                 ])
 
+        self.expect("v/x large_packed", VARIABLES_DISPLAYED_CORRECTLY,
+                    substrs=["a = 0x0000000cbbbbaaaa", "b = 0x0000000dffffeee"])
+
 
     @add_test_categories(['pyapi'])
     # BitFields exhibit crashes in record layout on Windows
     # (http://llvm.org/pr21800)
     @skipIfWindows
     @expectedFailureNetBSD
     def test_and_python_api(self):
         """Use Python APIs to inspect a bitfields variable."""

lldb/packages/Python/lldbsuite/test/lang/c/bitfields/main.c

struct PackedBits
                  c : 27;
     };
 #pragma pack()  
     struct PackedBits packed;
     packed.a = 'a';
     packed.b = 10;
     packed.c = 0x7112233;
 
+    struct LargePackedBits {
+        unsigned long a: 36;
+        unsigned long b: 36;
+    } __attribute__((packed));
+
+    struct LargePackedBits large_packed =
+      (struct LargePackedBits){ 0xcbbbbaaaa, 0xdffffeeee };
+    
     return 0;               //// Set break point at this line.
 
 }

lldb/source/Core/ValueObject.cpp

ValueObject *ValueObject::CreateChildAtIndex(size_t idx,
   if (child_compiler_type) {
     if (synthetic_index)
       child_byte_offset += child_byte_size * synthetic_index;
 
     ConstString child_name;
     if (!child_name_str.empty())
       child_name.SetCString(child_name_str.c_str());
 
+    // If a bitfield doesn't fit into the child_byte_size'd window at
+    // child_byte_offset, move the window (= child_byte_offset)
+    // forward until it fits.
+    uint32_t child_bit_size = child_byte_size * 8;
+    if (child_bitfield_bit_size + child_bitfield_bit_offset > child_bit_size) {
+      uint32_t overhang_bits =
+          (child_bitfield_bit_size + child_bitfield_bit_offset) - child_bit_size;
+      uint32_t overhang_bytes = (overhang_bits + 7) / 8;
+      child_byte_offset += overhang_bytes;
+      child_bitfield_bit_offset -= overhang_bytes * 8;
+    }
+
     valobj = new ValueObjectChild(
         *this, child_compiler_type, child_name, child_byte_size,
         child_byte_offset, child_bitfield_bit_size, child_bitfield_bit_offset,
         child_is_base_class, child_is_deref_of_parent, eAddressTypeInvalid,
         language_flags);
   }
 
   return valobj;