A far better place than the generation transaction (which I assume means
coinbase transaction?) is the last transaction in the block. That allows
you to save, on average, half of the hashes in the Merkle tree.

On Tue, Dec 8, 2015 at 11:55 PM, Justus Ranvier via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On 12/08/2015 09:12 AM, Gavin Andresen via bitcoin-dev wrote:
> > Stuffing the segwitness merkle tree in the coinbase
>
> If such a change is going to be deployed via a soft fork instead of a
> hard fork, then the coinbase is the worst place to put the segwitness
> merkle root.
>
> Instead, put it in the first output of the generation transaction as an
> OP_RETURN script.
>
> This is a better pattern because coinbase space is limited while output
> space is not. The next time there's a good reason to tie another merkle
> tree to a block, that proposal can be designated for the second output
> of the generation transaction.
>
>
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