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While architects understand how to build cost-effective parallel machines across a wide spectrum of machine sizes (ranging from within a single chip to large-scale servers), the real challenge is how to easily create parallel software to effectively exploit all of this raw performance potential. One promising technique for overcoming this problem is Thread-Level Speculation (TLS), which enables the compiler to optimistically create parallel threads despite uncertainty as to whether those threads are actually independent. In this paper we propose and evaluate a design for supporting TLS that seamlessly scales to any machine size because it is a straightforward extension of writeback invalidation-based cache coherence (which itself scales both up and down). Our experimental results demonstrate that our scheme performs well on both single-chip multiprocessors and on larger-scale machines where communication latencies are twenty times larger.