__builtin_ia32_cvtb2mask512() is the GNU C builtin for vpmovb2m k, zmm.
The Intel intrinsic for it is _mm512_movepi8_mask.
It extracts the most-significant bit from each byte, producing an integer mask.
The SSE2 and AVX2 instructions pmovmskb and vpmovmskb do the same thing for 16 or 32-byte vectors, producing the mask in a GPR instead of an AVX-512 mask register. (_mm_movemask_epi8 and _mm256_movemask_epi8).
I have attached a basic scalar implementation in C. For those trying to implement this in ARM, we care about the high bit, but each byte's high bit (in a 128bit vector), can be easily shifted to the low bit using the ARM NEON intrinsic: vshrq_n_u8(). Note that I would prefer not to store the bitmap to memory, it should just be the return value of the function similar to the following function.
#define _(n) __attribute((vector_size(1<<n),aligned(1)))
typedef char V _(6); // 64 bytes, 512 bits
typedef unsigned long U;
#undef _
U generic_cvtb2mask512(V v) {
U mask=0;int i=0;
while(i<64){
// shift mask by 1 and OR with MSB of v[i] byte
mask=(mask<<1)|((v[i]&0x80)>>7);
i++;}
return mask;
}
This is one possible algorithm for 16 bytes (128b vector), it would just need to be put into a loop for 64 bytes (512b vector):
#define _(n) __attribute((vector_size(1<<n),aligned(1)))
typedef char g4 _(4); // 16 bytes, 128 bits
typedef char g3 _(3); // 8 bytes, 64 bits
typedef unsigned long U;
#undef _
unsigned short get_16msb(g4 v) {
unsigned short = ret;
// per byte, make every bit same as msb
g4 msb = vdupq_n_u8(0x80);
g4 filled = vceqq_u8(v, msb);
// create a mask of each bit value
g4 b = {0x80, 0x40, 0x20, 0x01, 0x08, 0x04, 0x02, 0x01,
0x80, 0x40, 0x20, 0x01, 0x08, 0x04, 0x02, 0x01};
// and vectors together
g4 z = vandq_u8 (filled,b);
// extract lower 8 bytes, hi 8 bytes
g3 lo = vget_low_u8(z);
g3 hi = vget_high_u8(z);
// 'or' the 8 bytes of lo together ...
// put in byte 1 of ret
// 'or' the 8 bytes of hi together ...
// put in byte 2 of ret
return ret;
}
Here is another solution using ARM NEON.
#include <stdio.h>
// clang -Ofast -ob0 b0.c -funsigned-char -fno-unwind-tables -w
#define _(n) __attribute((vector_size(1<<n),aligned(1)))
typedef char g4 _(4),g6 _(6); // 64 * 8b
typedef unsigned long U,j6 _(6); // 8 * 64b
#undef _
#define ATM __attribute((minsize,noinline))
ATM void p4(g4 v) { for(int i=0;i<sizeof(g4);i++) { printf("%u,%s", v[i], (i+1)%8?"":" ");} putchar('\n');}
ATM void pu(U u) {for(int i=63;i>=0;i--){putchar((u & (1ULL<<i))?'1':'0'); if(0==(i)%8) putchar(' ');} putchar('b'); putchar('\n');} //print
#define bu(f) __builtin_neon_v##f
#define xi ((g4*)&x)[i]
#define vadd bu(addv_u8)
#define vbsl bu(bslq_v)
#define vs __builtin_shufflevector
//static g4 MB={0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80,0x80};
static g4 MC={0x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80,0x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80};
static g4 M0={0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0};
ATM static U getmsb_neon(g6 x) {
U u;g4 c;
for (int i = 0; i < 4; ++i) {
c = vbsl(xi>=128,MC,M0,48); // if >128, lookup //p4(c);
((char*)&u)[i*2] = vadd(vs(c,c,0,1,2,3,4,5,6,7));
((char*)&u)[i*2+1] = vadd(vs(c,c,8,9,10,11,12,13,14,15));
}
return u;
}
int main(int argc, char *argv[]) {
g6 a = {255,255,2,3,4,5,255,7, 0,1,2,3,4,5,6,7, 0,1,2,3,4,5,6,7, 0,1,2,3,4,5,6,7,
0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0, 0,0,0,0,0,0,0,0, 255,255,255,128,128,128,128,128};
U u = getmsb_neon(a);
pu(u);
return 0;
}