CODEWITHSUNDEEP
c++optimizationreferencethis
Pedro V. G.
Pedro V. G.

Reputation: 353

Keeping a reference to the object itself inside member functions

Is it useful, in terms of saving time and reducing number of operations, to create a reference to (*this) in the beginning of a member function where using of this-> is recurrent? Is it something that compilers (gcc interests me the most) already optimize for me? Are there reasons not to do this?

Example:

void A::checkBytes( const byte * dataChunk, uint32_t chunkSize )
{
    A & self = (*this);
    bool UTF8Valid = self.InvalidSequences & 1;
    byte current, expectedUTF8Bytes = 0;
    for (uint32_t i = 0; i < chunkSize; i++)
    {
        current = dataChunk[i];

        // many tests with 'current' and 'this->InvalidSequences'

        self.Count[current]++;
        self.ChunkSize++;
    }
    if (!UTF8Valid) self.InvalidSequences |= 1;
}

I know every non-static member function takes its own hidden this. I know I'll have both the hidden A * this and the A & self. What I do not know is whether many this->someMember will cost more than many referenceToThis.someMember or not at all.

Upvotes: 0

Views: 450

Answers (1)

Fire Lancer
Fire Lancer

Reputation: 30145

No. worst case it would consume extra stack space and need some extra instructions, best case it gets optimized back to this->.

this will almost certainly live in a register where possible, and a reference like A& is basically a pointer at the implementation level.

There probably isn't even a gain in trying to store a direct pointer/reference to this->Count etc., as on most platforms this->Count[n] can be a single instruction (e.g. on x86 I believe LEA will be used, you might check the disassembly).

Upvotes: 5

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