Intel® 64 and IA-32 Architectures Optimization Reference Manual

USING PERFORMANCE MONITORING EVENTSFrequent accesses to registers that cause partial stalls increase access latency anddecrease performance. Partial Register Stalls Ratio is the percentage of cycles whenpartial stalls occur.B.7.4.4Partial Flag Stalls22. Partial Flag Stalls Ratio:RAT_STALLS.FLAGS / CPU_CLK_UNHALTED.COREPartial flag stalls have high penalty and they can be easily avoided. However, in somecases, Partial Flag Stalls Ratio might be high although there are no real flag stalls.There are a few instructions that partially modify the RFLAGS register and may causepartial flag stalls. The most popular are the shift instructions (SAR, SAL, SHR, andSHL) and the INC and DEC instructions.B.7.4.5Bypass Between Execution Domains23. Delayed Bypass to FP Operation Rate: DELAYED_BYPASS.FP /CPU_CLK_UNHALTED.CORE24. Delayed Bypass to SIMD Operation Rate: DELAYED_BYPASS.SIMD /CPU_CLK_UNHALTED.CORE25. Delayed Bypass to Load Operation Rate: DELAYED_BYPASS.LOAD /CPU_CLK_UNHALTED.COREDomain bypass adds one cycle to instruction latency. To identify frequent domainbypasses in the code you can use the above ratios.B.7.4.6Floating Point Performance Ratios26. Floating Point Instructions Ratio: X87_OPS_RETIRED.ANY / INST_RETIRED.ANY* 100Significant floating-point activity indicates that specialized optimizations for floatingpointalgorithms may be applicable.27. FP Assist Performance Impact: FP_ASSIST * 80 / CPU_CLK_UNHALTED.CORE *100Floating Point assist is activated for non-regular FP values like denormals and NANs.FP assist is extremely slow compared to regular FP execution. Different assists incurdifferent penalties. FP Assist Performance Impact estimates the overall impact.28. Divider Busy: IDLE_DURING_DIV / CPU_CLK_UNHALTED.CORE * 100A high value for the Divider Busy ratio indicates that the divider is busy and no otherexecution unit or load operation is in progress for many cycles. Using this ratioignores L1 data cache misses and L2 cache misses that can be executed in paralleland hide the divider penalty.29. Floating-Point Control Word Stall Ratio: RESOURCE_STALLS.FPCW /CPU_CLK_UNHALTED.CORE * 100B-55