builder: improve reading of DWARF variables

The compiler now uses DW_OP_plus_uconst in the address attribute of
DWARF variables. To be able to understand these addresses, we need to be
able to parse this opcode.

This commit should also improve the reliability of address parsing a
bit.

builder/sizes.go

@@ -219,20 +219,9 @@ func readProgramSizeFromDWARF(data *dwarf.Data, codeOffset uint64, skipTombstone
 			// Try to parse the location. While this could in theory be a very
 			// complex expression, usually it's just a DW_OP_addr opcode
 			// followed by an address.
-			locationCode := location.Val.([]uint8)
-			if locationCode[0] != 3 { // DW_OP_addr
-				continue
-			}
-			var addr uint64
-			switch len(locationCode) {
-			case 1 + 2:
-				addr = uint64(binary.LittleEndian.Uint16(locationCode[1:]))
-			case 1 + 4:
-				addr = uint64(binary.LittleEndian.Uint32(locationCode[1:]))
-			case 1 + 8:
-				addr = binary.LittleEndian.Uint64(locationCode[1:])
-			default:
-				continue // unknown address
+			addr, err := readDWARFConstant(r.AddressSize(), location.Val.([]uint8))
+			if err != nil {
+				continue // ignore the error, we don't know what to do with it
 			}
 
 			// Parse the type of the global variable, which (importantly)
@@ -256,6 +245,52 @@ func readProgramSizeFromDWARF(data *dwarf.Data, codeOffset uint64, skipTombstone
 	return addresses, nil
 }
 
+// Parse a DWARF constant. For addresses, this is usually a very simple
+// expression.
+func readDWARFConstant(addressSize int, bytecode []byte) (uint64, error) {
+	var addr uint64
+	for len(bytecode) != 0 {
+		op := bytecode[0]
+		bytecode = bytecode[1:]
+		switch op {
+		case 0x03: // DW_OP_addr
+			switch addressSize {
+			case 2:
+				addr = uint64(binary.LittleEndian.Uint16(bytecode))
+			case 4:
+				addr = uint64(binary.LittleEndian.Uint32(bytecode))
+			case 8:
+				addr = binary.LittleEndian.Uint64(bytecode)
+			default:
+				panic("unexpected address size")
+			}
+			bytecode = bytecode[addressSize:]
+		case 0x23: // DW_OP_plus_uconst
+			offset, n := readULEB128(bytecode)
+			addr += offset
+			bytecode = bytecode[n:]
+		default:
+			return 0, fmt.Errorf("unknown DWARF opcode: 0x%x", op)
+		}
+	}
+	return addr, nil
+}
+
+// Source: https://en.wikipedia.org/wiki/LEB128#Decode_unsigned_integer
+func readULEB128(buf []byte) (result uint64, n int) {
+	var shift uint8
+	for {
+		b := buf[n]
+		n++
+		result |= uint64(b&0x7f) << shift
+		if b&0x80 == 0 {
+			break
+		}
+		shift += 7
+	}
+	return
+}
+
 // Read a MachO object file and return a line table.
 // Also return an index from symbol name to start address in the line table.
 func readMachOSymbolAddresses(path string) (map[string]int, []addressLine, error) {