Intel® 64 and IA-32 Architectures Optimization Reference Manual

GENERAL OPTIMIZATION GUIDELINESStoreAlignmentTable 3-1. Store Forwarding Restrictions of ProcessorsBased on Intel Core Microarchitecture (Contd.)Width ofStore(bits)Load Alignment(byte)Width ofLoad (bits)StoreForwardingRestrictionTo Natural size 64 not qword aligned 8, 16 stalledTo Natural size 64 dword aligned 32 not stalledTo Natural size 64 not dword aligned 32 stalledTo Natural size 128 dqword aligned 8, 16, 128 not stalledTo Natural size 128 not dqword aligned 8, 16 stalledTo Natural size 128 dword aligned 32 not stalledTo Natural size 128 not dword aligned 32 stalledTo Natural size 128 qword aligned 64 not stalledTo Natural size 128 not qword aligned 64 stalledUnaligned, start byte 1 32 byte 0 of store 8, 16, 32 not stalledUnaligned, start byte 1 32 not byte 0 of store 8, 16 stalledUnaligned, start byte 1 64 byte 0 of store 8, 16, 32 not stalledUnaligned, start byte 1 64 not byte 0 of store 8, 16, 32 stalledUnaligned, start byte 1 64 byte 0 of store 64 stalledUnaligned, start byte 7 32 byte 0 of store 8 not stalledUnaligned, start byte 7 32 not byte 0 of store 8 not stalledUnaligned, start byte 7 32 don’t care 16, 32 stalledUnaligned, start byte 7 64 don’t care 16, 32, 64 stalled3.6.4.2 Store-forwarding Restriction on Data AvailabilityThe value to be stored must be available before the load operation can be completed.If this restriction is violated, the execution of the load will be delayed until the data isavailable. This delay causes some execution resources to be used unnecessarily, andthat can lead to sizable but non-deterministic delays. However, the overall impact ofthis problem is much smaller than that from violating size and alignment requirements.In processors based on Intel NetBurst microarchitecture, hardware predicts whenloads are dependent on and get their data forwarded from preceding stores. Thesepredictions can significantly improve performance. However, if a load is scheduledtoo soon after the store it depends on or if the generation of the data to be stored isdelayed, there can be a significant penalty.3-55