Before getting into detailed review, I'd like to check with other developers.

Some more general points:

• I find overall the readability of the code a little worse, you always need to beware that the second index needs to be used td->center can't be used as is, without td->center[index].
  
  Statements such as:
  
  copy_m3_m3(td->axismtx, td_table[td_index]->axismtx); get an extra level of indirection, becoming.
  
  copy_m3_m3(td->space[tdi].axismtx, td->space[td_table[td_index]].axismtx);

• We loose some type safety, passing int's between functions can be more easily swapped out with other arguments (boolean or other int's) without any errors, besides runtime errors.
• Before creating this patch you were looking into adding data which would have bloated the transform struct. I'd be interested to see a patch for this, showing the pain-point that this patch intends to solve, to see the trade off being made.
• While it's nice there is a speedup, this is a modest gain for a one-time cost, which I would guess isn't all that bad compared to updating the depsgraph & 3D viewport.

I do not have a strong opinion on this patch… Main points I see from a quick check:

1. Performances argument should be ignored imho, this is at best marginal improvement in the overall picture.
2. Readability: am not sure... in a way, passing around explicit index of each item that need to be processed makes thing a bit more clear? But it also does add some bloating…
3. Memory usage control is the main strong point here, would be interesting to have metrics about that?

In D7930#193443, @Campbell Barton (campbellbarton) wrote:

(...)

• Before creating this patch you were looking into adding data which would have bloated the transform struct. I'd be interested to see a patch for this, showing the pain-point that this patch intends to solve, to see the trade off being made.

(...)

I actually wanted to use the same struct TransData as TransDataMirror.
But another solution was commited (rB1c189aa70aff)
However the "void *extra" and "float center[3]" members should be optional

I think this makes sense if we can improve readability and type safety.

• Rename basic to base. ext and special could also get better names?
• Putting an index in base would help with type safe, you'd pass the base between functions then.
• Use more temporary variables rather than indexing multiple times.

So for example:

TransData *td = tc->data;
for (int tdi = 0; tdi < tc->data_len; tdi++) {
  if (td->basic[tdi].flag & (TD_MIRROR_EDGE_X | TD_MIRROR_EDGE_Y | TD_MIRROR_EDGE_Z)) {
    if (td->basic[tdi].flag & TD_MIRROR_EDGE_X) {
      td->basic[tdi].loc[0] = 0.0f;
    }
    if (td->basic[tdi].flag & TD_MIRROR_EDGE_Y) {
      td->basic[tdi].loc[1] = 0.0f;
    }
    if (td->basic[tdi].flag & TD_MIRROR_EDGE_Z) {
      td->basic[tdi].loc[2] = 0.0f;
    }
  }
}

To:

TransDataBase *tbase = td->base;
for (i = 0; i < tc->data_len; i++, tbase++) {
  if (tbase->flag & (TD_MIRROR_EDGE_X | TD_MIRROR_EDGE_Y | TD_MIRROR_EDGE_Z)) {
    if (tbase->flag & TD_MIRROR_EDGE_X) {
      tbase->loc[0] = 0.0f; 
    }
    if (tbase->flag & TD_MIRROR_EDGE_Y) {
      tbase->loc[1] = 0.0f; 
    }
    if (tbase->flag & TD_MIRROR_EDGE_Z) {
      tbase->loc[2] = 0.0f; 
    }
  }

  TranseDataSpecial *special = &td->special[tbase->index];
  ...
}