Nested loop vectorisation issue

[Thomas Womack]

Good morning.

I have some code that looks like

typedef unsigned long long uint64;
typedef unsigned int uint32;

typedef struct { uint64 x[8]; } __attribute__((aligned(64))) v_t;

inline v_t xor(v_t a, v_t b)
{
  v_t Q;
  for (int i=0; i<8; i++) Q.x[i] = a.x[i] ^ b.x[i];
  return Q;
}

void xor_matrix_precomp(v_t* __restrict__ a, v_t* __restrict__ c, v_t* __restrict__ d, int n)
{
  uint32 i,j;
  for (i=0; i<n; i++)
    {
      v_t vi = a[i];
      v_t acc = c[0*256 + (vi.x[0] & 0xff)];
      for (j=1; j<64; j++)
	{
	  uint32 w = j>>3, b=j&7;
	  acc = xor(acc, c[j*256 + ((vi.x[w] >> (8*b))&0xff)]);
	}
      d[i] = xor(d[i], acc);
    }
}

built with 

/home/nfsworld/tooling/gcc-9.1-isl16/bin/gcc -O3 -fomit-frame-pointer -march=skylake-avx512 -mprefer-vector-width=512 -S badeg.c


and the inner xor_matrix loop is not vectorised at all: it carries ‘acc’ in eight x86-64 registers rather than one ZMM.  What am I missing?

-fopt-info-all-vec is not helping me, it points out that it can’t vectorise the i or j loop but says nothing about the loop in the inlined xor() call.

I’m sure I’m missing something obvious; many thanks for your help.

Tom