Why is your "High-Level Shading Language" lower-level in some places than the "assembly" you compile to?

gistfile1.txt

int bitfieldInsert(int base, int insert, int offset, int bits)
{
    int mask = (-1 >> (32 - bits)) << offset;
    return (base & ~mask) | ((insert << offset) & mask);
}

---> (in usage)

    bfi r7.yz, l(0, 16, 16, 0), l(0, 16, 16, 0), r8.yywy, r8.xxzx
    
sigh.