Enhancing Multiple-Path Speculative Execution with Predicate Window Shifting
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Enhancing Multiple-Path Speculative Execution with Predicate Window Shifting
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1997
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Abstract
Speculative execution has long been used as an approach to exploit instruction level parallelism
across basic block boundaries. Most existing speculative execution techniques only
support speculating along single control path, and heavily rely on branch prediction to choose
the right control path. In this paper, we propose an extended predicated execution mechanism,
called predicate shifting, to support speculating along multiple control paths. The predicate
shifting mechanism maintains a condition/predicate window for each basic block. With the
condition/predicate window, instructions ca.n be guarded by predicates related to current or
future branch conditions. The predicate shifting mechanism can reduce the number of required
tag bits by shifting conditions/predicates out of the condition/predicate window whenever they
are no longer in use. To incorporate the predicate shifting mechanism into a VLIW processor,
a new result-buffering structure, call future buffer, is used to buffer uncommitted results and
to evaluate predicates. The FIFO structure of the future buffer not only simplifies exception
handling but also allows multiple uncommitted writes to the same register. Experimental results
show that the predicate shifting mechanism can use predicate tag effectively and achieve 24%
performance improvement over the previous predicating mechanism [2] using a small predicate
tag.
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Tsai, Jenn-Yuan; Yew, Pen-Chung. (1997). Enhancing Multiple-Path Speculative Execution with Predicate Window Shifting. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/215314.
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