PreTrans: reducing TLB CAM-search via page number prediction and speculative pre-translation

The need for fast address translation within tight time constraints (before L1 tag check but after effective address computation) imposes many design constraints. The freedom from such constraints can potentially lead to lower TLB energy costs. In this paper, we observe that (1) data accesses commonly use base-displacement addressing modes in which the effective address is computed as the sum of a base and a displacement, and (2) the effective page numbers are predictable once the base address is known. Further, it is easy to cache address translations alongside the predicted page numbers thus enabling speculative address translation that can filter accesses to the TLB. The two observations enable our PreTrans design in which (a) a speculative translation is available based solely on the base address, and (b) the translation is available simultaneously with the effective (virtual) address. PreTrans replaces most of the energy-expensive CAM-lookups for TLB access with RAM lookups, which translates to significant power improvements in the TLB.