Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESvalues to spill to memory when there are too many live values to fit in registers.Consider the code in Example 3-17, where it is necessary to spill either A, B, or C.Example 3-17. Spill Scheduling CodeLOOPC := ...B := ...A := A + ...For modern microarchitectures, using dependence depth information in spill schedulingis even more important than in previous processors. The loop-carried dependencein A makes it especially important that A not be spilled. Not only would astore/load be placed in the dependence chain, but there would also be a data-notreadystall of the load, costing further cycles.Assembly/Compiler Coding Rule 41. (H impact, MH generality) For smallloops, placing loop invariants in memory is better than spilling loop-carrieddependencies.A possibly counter-intuitive result is that in such a situation it is better to put loopinvariants in memory than in registers, since loop invariants never have a loadblocked by store data that is not ready.3.5.2 Avoiding Stalls in Execution CoreAlthough the design of the execution core is optimized to make common casesexecutes quickly. A μop may encounter various hazards, delays, or stalls whilemaking forward progress from the front end to the ROB and RS. The significant casesare:• ROB Read Port Stalls• Partial Register Reference Stalls• Partial Updates to XMM Register Stalls• Partial Flag Register Reference Stalls3.5.2.1 ROB Read Port StallsAs a μop is renamed, it determines whether its source operands have executed andbeen written to the reorder buffer (ROB), or whether they will be captured “in flight”in the RS or in the bypass network. Typically, the great majority of source operandsare found to be “in flight” during renaming. Those that have been written back to theROB are read through a set of read ports.Since the Intel Core Microarchitecture is optimized for the common case where theoperands are “in flight”, it does not provide a full set of read ports to enable allrenamed μops to read all sources from the ROB in the same cycle.3-32