[lights0123]

It appears that you didn't enable optimizations. That's needed for SIMD, which can only be taken advantage of with contiguously packed data.

[prirun]

Results with -O are below.

x1 (AoS) vs x2 (SoA): no performance difference x3 (arrays not in structure, both arrays in loop): slower x4 (arrays not in structure, one array in loop): faster

My advice is still not to assume that SoA is always faster than AoS without benchmarking.

  + cc -O -o x1 x1.c
  + ./x1
  s=1808348672

  real 0m11.775s
  user 0m3.540s
  sys 0m6.592s
  + ./x1
  s=1808348672

  real 0m5.427s
  user 0m2.727s
  sys 0m2.682s
  + cc -O -o x2 x2.c
  + ./x2
  s=1808348672

  real 0m5.185s
  user 0m2.296s
  sys 0m2.872s
  + ./x2
  s=1808348672

  real 0m5.140s
  user 0m2.273s
  sys 0m2.852s
  + cc -O -o x3 x3.c
  + ./x3
  s=1808348672

  real 0m6.423s
  user 0m3.745s
  sys 0m2.660s
  + ./x3
  s=1808348672

  real 0m6.485s
  user 0m3.741s
  sys 0m2.714s
  + cc -O -o x4 x4.c
  + ./x4
  s=1808348672

  real 0m4.875s
  user 0m2.205s
  sys 0m2.651s
  + ./x4
  s=1808348672

  real 0m4.894s
  user 0m2.189s
  sys 0m2.684s

[Narishma]

The reason you're not seeing much difference is that your struct is very small, just 16 bytes. In the cases where you're using both x and y in the loop (x1 and x2), you can fit 4 of them in a cache line and you're not wasting space since you need to use both. In the case you're only using one of the values (x3), you're wasting half a cache line and that shows in the benchmark. If you had a bigger struct and/or where you're not using all the members in the calculation, you'd see a much bigger difference in performance between SoA and AoS.

[rpep]

You need to add “-march=native” to turn on SIMD at the highest level your processor will support, otherwise it will just use SSE4.1 by default on most compilers (the lowest common denominator, as all x86-64 processors have it).