[The_Colonel]

This idea isn't about compressing in-flight, but in the instruction cache, so that more instructions will fit into the cache, and you don't need to fetch as often (and thus incur latency) from main memory / L2.

Zstd is impractical, but I can imagine some sort of storage efficient microcode? (current Intel CPUs store original x86 instructions in the L1 instruction cache).

[simiones]

You then need extra memory to store the de-compressed instructions, since you can't predict before running the decompression how many instructions you'll get. And you still the problem of an unpredictably-sized instruction cache.

Plus, the larger the instruction cache is, the worse every branch mis-prediction is. As far as I know, the size of the instruction cache is not really limited because of space issues, it's limited for precisely this reason.