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/trunk/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/trunk/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/trunk/source/Core/ValueObjectChild.cpp

if (parent->UpdateValueIfNeeded(false)) {
           if (addr == LLDB_INVALID_ADDRESS) {
             m_error.SetErrorString("parent address is invalid.");
           } else if (addr == 0) {
             m_error.SetErrorString("parent is NULL");
           } else {
             // Set this object's scalar value to the address of its value by
             // adding its byte offset to the parent address
             m_value.GetScalar() += GetByteOffset();
+
+            // If a bitfield doesn't fit into the child_byte_size'd
+            // window at child_byte_offset, move the window forward
+            // until it fits.  The problem here is that Value has no
+            // notion of bitfields and thus the Value's DataExtractor
+            // is sized like the bitfields CompilerType; a sequence of
+            // bitfields, however, can be larger than their underlying
+            // type.
+            if (m_bitfield_bit_offset) {
+              const bool thread_and_frame_only_if_stopped = true;
+              ExecutionContext exe_ctx(GetExecutionContextRef().Lock(
+                  thread_and_frame_only_if_stopped));
+              if (auto type_bit_size = GetCompilerType().GetBitSize(
+                      exe_ctx.GetBestExecutionContextScope())) {
+                uint64_t bitfield_end =
+                    m_bitfield_bit_size + m_bitfield_bit_offset;
+                if (bitfield_end > *type_bit_size) {
+                  uint64_t overhang_bytes =
+                      (bitfield_end - *type_bit_size + 7) / 8;
+                  m_value.GetScalar() += overhang_bytes;
+                  m_bitfield_bit_offset -= overhang_bytes * 8;
+                }
+              }
+            }
           }
         } break;
 
         case Value::eValueTypeScalar:
           // try to extract the child value from the parent's scalar value
           {
             Scalar scalar(m_value.GetScalar());
             if (m_bitfield_bit_size)