bitmask.h

#ifndef SIMDJSON_ARM64_BITMASK_H
#define SIMDJSON_ARM64_BITMASK_H
 
namespace simdjson {
namespace SIMDJSON_IMPLEMENTATION {
namespace {
 
//
// Perform a "cumulative bitwise xor," flipping bits each time a 1 is encountered.
//
// For example, prefix_xor(00100100) == 00011100
//
simdjson_inline uint64_t prefix_xor(uint64_t bitmask) {
  // We could do this with PMULL, but it is apparently slow.
  //
  //#ifdef __ARM_FEATURE_CRYPTO // some ARM processors lack this extension
  //return vmull_p64(-1ULL, bitmask);
  //#else
  // Analysis by @sebpop:
  // When diffing the assembly for src/stage1_find_marks.cpp I see that the eors are all spread out
  // in between other vector code, so effectively the extra cycles of the sequence do not matter
  // because the GPR units are idle otherwise and the critical path is on the FP side.
  // Also the PMULL requires two extra fmovs: GPR->FP (3 cycles in N1, 5 cycles in A72 )
  // and FP->GPR (2 cycles on N1 and 5 cycles on A72.)
  bitmask ^= bitmask << 1;
  bitmask ^= bitmask << 2;
  bitmask ^= bitmask << 4;
  bitmask ^= bitmask << 8;
  bitmask ^= bitmask << 16;
  bitmask ^= bitmask << 32;
  return bitmask;
}
 
} // unnamed namespace
} // namespace arm64
} // namespace simdjson
 
#endif