Efficient Neon Implementation Of Clipping

Question

Within a loop i have to implement a sort of clipping

if ( isLast )
{
    val = ( val < 0 ) ? 0 : val;
    val = ( val > 255 ) ? 255 : val;        
}

However this "clipping" takes up almost half the time of execution of the loop in Neon . This is what the whole loop looks like-

for (row = 0; row < height; row++)
{
  for (col = 0; col < width; col++)
  {
      Int sum;
      //...Calculate the sum
   
      Short val = ( sum + offset ) >> shift;
      if ( isLast )
      {
           val = ( val < 0 ) ? 0 : val;
           val = ( val > 255 ) ? 255 : val;        
      }
      dst[col] = val;
   }

}

This is how the clipping has been implemented in Neon

     cmp       %10,#1                           //if(isLast)         
     bne       3f                                         
     vmov.i32   %4, d4[0]                       //put val in %4
     cmp       %4,#0                            //if( val < 0 )
     blt       4f                               
     b         5f                               
     4:                                         
     mov       %4,#0                             
     vmov.i32   d4[0],%4                        
     5:                                         
     cmp       %4,%11                          //if( val > maxVal )
     bgt       6f                               
     b         3f                               
     6:                                         
     mov       %4,%11                            
     vmov.i32   d4[0],%4                       
     3:  

     

This is the mapping of variables to registers-

isLast-    %10
maxVal-    %11

Any suggestions to make it faster ? Thanks

EDIT-

The clipping now looks like-

     "cmp       %10,#1                            \n\t"//if(isLast)      
     "bne       3f                                \n\t"          
     "vmin.s32   d4,d4,d13                        \n\t"
     "vmax.s32   d4,d4,d12                        \n\t"
     "3:                                          \n\t" 

//d13 contains maxVal(255)
//d12 contains 0

Time consumed by this portion of the code has dropped from 223ms to 18ms

Answer 1

Using normal compares with NEON is almost always a bad idea because it forces the contents of a NEON register into a general purpose ARM register, and this costs lots of cycles.

You can use the vmin and vmax NEON instructions. Here is a little example that clamps an array of integers to any min/max values.

void clampArray (int minimum,
                 int maximum,
                 int * input,
                 int * output,
                 int numElements)
{
  // get two NEON values with your minimum and maximum in each lane:
  int32x2_t lower  = vdup_n_s32 (minimum);
  int32x2_t higher = vdup_n_s32 (maximum);
  int i;

  for (i=0; i<numElements; i+=2)
  {
    // load two integers
    int32x2_t x = vld1_s32 (&input[i]);

    // clamp against maximum:
    x = vmin_s32 (x, higher);

    // clamp against minimum
    x = vmax_s32 (x, lower);

    // store two integers
    vst1_s32 (&output[i], x);
  }
} 

Warning: This code assumes the numElements is always a multiple of two, and I haven't tested it.

You may even make it faster if you process four elements at a time using the vminq / vmaxq instructions and load/store four integers per iteration.

Answer 2

If maxVal is UCHAR_MAX, CHAR_MAX, SHORT_MAX or USHORT_MAX, you can simply convert with neon from int to your desired datatype, by casting with saturation.

By example

// Will convert four int32 values to signed short values, with saturation.
int16x4_t vqmovn_s32 (int32x4_t) 

// Converts signed short to unsgigned char, with saturation
uint8x8_t vqmovun_s16 (int16x8_t) 

If you do not want to use multiple-data capabilities, you can still use those instructions, by simply loading and reading one of the lanes.