iOS Metal shader searching text naive

gistfile1.cpp

#include <metal_stdlib>
using namespace metal;

// Every kernel processes 1 row of the char table:
kernel void lmCharSearch(
                         
const device        char *charTable               [[ buffer(0) ]],
constant            uint &charsInRow              [[ buffer(1) ]],
constant            uint &rowCount                [[ buffer(2) ]],
const device        char *searchPhrase            [[ buffer(3) ]],
constant            uint &searchPhraseLenght      [[ buffer(4) ]],
device              bool *outMatchMasks           [[ buffer(5) ]],
                         
                    uint row                      [[ thread_position_in_grid ]])
{

    if(row >= rowCount)return;
    
    bool found = false;
    
    char tableCharacter;
    char searchedPhraseCharacter;
    
    uint phraseMatchCount = 0;
    
    uint indexRowStart    = row * charsInRow;
    uint indexRowEnd      = indexRowStart + charsInRow;
    
    // 48.3 % of the time spent in this line: !!! (even if it is outside the loop)
    searchedPhraseCharacter = searchPhrase[phraseMatchCount];
    
    uint firstMatchIndex = 0;
    
    for (uint i = indexRowStart; i < indexRowEnd; i++) {
        
            // 42.3 % of the time spent in this line: !!!
            tableCharacter = charTable[i];
        
            if (searchedPhraseCharacter == tableCharacter) {
    
                if(phraseMatchCount == 0)firstMatchIndex = i;
                phraseMatchCount++;
                searchedPhraseCharacter = searchPhrase[phraseMatchCount];
                
                if (phraseMatchCount == searchPhraseLenght) {
                    
                    found = true;
                    break;
                }
                
            }else{
                
                if(phraseMatchCount > 0){
                
                    i = firstMatchIndex + 1;
                    phraseMatchCount = 0;
                    searchedPhraseCharacter = searchPhrase[phraseMatchCount];
                }
            }
        }
    
    outMatchMasks[row] = found;
}