[webaholic]

One of the main reasons for speculative execution is to fetch data into the caches ahead of them being needed. If you don't modify the cache, then you throw that away.

May be one way would be to use a smaller, separate cache for speculative execution and then copy that value to the regular cache once speculation is confirmed? This would add a one cycle latency for cache-to-cache transfer but there might be better ways.

[caf]

This might actually improve performance because it would prevent actually-hot data being evicted from the cache in favour of cold data that was loaded in a not-taken speculated branch.

[rasz]

no need for cache-to-cache transfer, just like with invisible registers we could have cache line renaming

[gpderetta]

It is not enough. The cache line still need be evicted (or at least marked as no longer exclusive) from other cpus, so the side effect is still visible.