The effectiveness of at-speed silicon debug is constrained by the limited trace buffer size and/or trace port bandwidth, requiring highly-efficient trace data compression solutions. As it is usually inevitable to have unknown ‘X’ values during silicon debug, trace compressor should be equipped with X-tolerance feature in order not to significantly degrade error detection capability. To tackle this problem, this paper presents a novel reconfigurable X-tolerant trace compressor, namely X-Tracer, which is able to tolerate as many X-bits as possible in the trace streams while guaranteeing high compression ratio, at the cost of little extra design-for-debug hardware.