如何理解icc编译器在优化报告中的加速？

Question

环境是：

icc版本19.0.0.117 (gcc版本5.4.0兼容性)

英特尔并行工作室XE集群版2019

Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz

Ubuntu 16.04

编译器标志是：

-std=gnu11 -Wall -xHost -xCORE-AVX2 2 -O2 -fma -qopenmp -qopenmp-simd -qopt-report=5

我使用OpenMP simd或英特尔parama将我的循环矢量化以获得加速。在icc生成的优化报告中，我通常看到以下结果：

LOOP BEGIN at get_forces.c(3668,3)
   remark #15389: vectorization support: reference mon->fricforce[n1][d] has unaligned access   [ get_forces.c(3669,4) ]
   remark #15389: vectorization support: reference mon->vel[n1][d] has unaligned access   [ get_forces.c(3669,36) ]
   remark #15389: vectorization support: reference vel[n1][d] has unaligned access   [ get_forces.c(3669,51) ]
   remark #15389: vectorization support: reference mon->drag[n1][d] has unaligned access   [ get_forces.c(3671,4) ]
   remark #15389: vectorization support: reference mon->vel[n1][d] has unaligned access   [ get_forces.c(3671,40) ]
   remark #15389: vectorization support: reference vel[n1][d] has unaligned access   [ get_forces.c(3671,57) ]
   remark #15381: vectorization support: unaligned access used inside loop body
   remark #15305: vectorization support: vector length 2
   remark #15309: vectorization support: normalized vectorization overhead 0.773
   remark #15300: LOOP WAS VECTORIZED
   remark #15450: unmasked unaligned unit stride loads: 3 
   remark #15451: unmasked unaligned unit stride stores: 2 
   remark #15475: --- begin vector cost summary ---
   remark #15476: scalar cost: 21 
   remark #15477: vector cost: 11.000 
   remark #15478: estimated potential speedup: 1.050 
   remark #15488: --- end vector cost summary ---
   remark #25456: Number of Array Refs Scalar Replaced In Loop: 1
   remark #25015: Estimate of max trip count of loop=1
LOOP END

我的问题是:我不明白如何从

normalized vectorization overhead 0.773
scalar cost: 21 
vector cost: 11.000 

另一个更极端和困惑的情况可能是

LOOP BEGIN at get_forces.c(2690,8)
<Distributed chunk3>
   remark #15388: vectorization support: reference q12[j] has aligned access   [ get_forces.c(2694,19) ]
   remark #15388: vectorization support: reference q12[j] has aligned access   [ get_forces.c(2694,26) ]
   remark #15335: loop was not vectorized: vectorization possible but seems inefficient. Use vector always directive or -vec-threshold0 to override 
   remark #15305: vectorization support: vector length 2
   remark #15309: vectorization support: normalized vectorization overhead 1.857
   remark #15448: unmasked aligned unit stride loads: 1 
   remark #15475: --- begin vector cost summary ---
   remark #15476: scalar cost: 7 
   remark #15477: vector cost: 3.500 
   remark #15478: estimated potential speedup: 0.770 
   remark #15488: --- end vector cost summary ---
   remark #25436: completely unrolled by 3  
LOOP END

现在，3.5+1.857=5.357 <7

所以，我仍然可以simd这个循环，得到一个加速比，或者我应该把加速比号码0.770在报告中，而不是simd它？

如何理解icc编译器在优化报告中的加速比？

Answer 1

“标量成本”是指“标量循环一次迭代的成本”。

“向量代价”是指“向量化循环的一次迭代被vector_length*unroll_factor除以后的代价”，即与一次标量迭代相当的代价。

“向量化开销”表示循环前后向量初始化/终结的标准化成本(按向量迭代成本)。

计算整个循环执行的“估计的潜在加速比”。它显示了向量化循环执行的归一化(通过标量迭代代价)的潜在增益，包括对估计的环路行程计数的剥离、剩余和主循环。它不能从上面所示的标量和向量代价中显式地导出。