Compiler-assisted, selective out-of-order commit

This paper proposes an out-of-order instruction commit mechanism using a novel compiler/architecture interface. The compiler creates instruction “blocks” guaranteeing some commit conditions and the processor uses the block information to commit certain instructions out of order. Micro-architectural support for the new commit mode is made on top of the standard, ROB-based processor and includes out-of-order instruction commit with register and load queue entry release. The commit mode may be switched multiple times during execution. Initial results for a 4-wide processor show that, on average, 52% instructions are committed out of order resulting in 10% to 26% speedups over in-order commit, with minimal hardware overhead. The performance improvement is a result of an effectively larger instruction window that allows more cache misses to be overlapped for both L1 and L2 caches.