An Empirical Study of Data Speculation Use on the Intel Itanium 2 Processor

Abstract

The Intel Itanium architecture uses a dedicated 32entry hardware table, the Advanced Load Address Table (ALAT) to support data speculation via an instruction set interface. This study presents an empirical evaluation of the use of the ALAT and data speculative instructions for several optimizing compilers. We determined what and how often compilers generated the different speculative instructions, and used the Itanium's hardware performance counters to evaluate their run-time behavior. We also performed a limit study by modifying one compiler to always generate data speculation when possible. We found that this aggressive approach significantly increased the amount of data speculation and often resulted in performance improvements, of as much as 10% in one case. Since it worsened performance only for one application and then only for some inputs, we conclude that more aggressive data speculation heuristics than those employed by current compilers are desirable and may further improve performance gains from data speculation.

Widening the Memory Bottleneck by Automatically-Compiled Application-Specific Speculation Mechanisms

“…Mock et al were able to prove by means of a modified C compiler, which forced data speculation on an Intel Itanium 2 CPU architecture where possible, that performance increases due to load value speculation [22] of up to 10% were achievable. On the other hand, the Itanium 2 roll-back mechanism, which is based on the Advanced Load Address Table (ALAT), a dedicated hardware structure that usually needs to be explicitly controlled by the programmer [20], produces performance losses of up to 5% under adverse conditions with frequent misspeculations.…”

Memory Latency Hiding by Load Value Speculation for Reconfigurable Computers

“…Mock et al [2005] modified a compiler to force value speculation where possible on the Intel Itanium 2 CPU architecture. Their scheme relied on hardware support in the form of the Itanium's Advanced Load Address Table (ALAT) [McNairy and Soltis 2003].…”

Evaluation of speculative execution techniques for high-level language to hardware compilation

“…Mock et al modified a compiler to force value speculation where possible on the Intel Itanium 2 CPU architecture [18]. Their scheme relied on hardware support in the form of the Itanium's Advanced Load Address Table (ALAT) [17].…”