Using this pointer causes strange deoptimization in hot loop

Question

I recently came across a strange deoptimization (or rather missed optimization opportunity).

Consider this function for efficient unpacking of arrays of 3-bit intege

Answer 1

Strict aliasing rules allows char* to alias any other pointer. So this->target may alias with this, and in your code method, the first part of the code,

target[0] = t & 0x7;
target[1] = (t >> 3) & 0x7;
target[2] = (t >> 6) & 0x7;

is in fact

this->target[0] = t & 0x7;
this->target[1] = (t >> 3) & 0x7;
this->target[2] = (t >> 6) & 0x7;

as this may be modified when you modify this->target content.

Once this->target is cached into a local variable, the alias is no longer possible with the local variable.