alt Hacker NewsAarch64 dropped thumb instructions.
I don't think you're going to find a single benchmark on the effectiveness of compressed instructions since it really depends deeply on both the workload and the whole system. For example, memory bandwidth and cache pressure are both important for whether smaller text sizes matter, and that may depend on what else is running at the same time.
Note your assembler may be automatically compressing instructions without you asking. You'll have to disassemble the binary to find out.
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Maybe the best approach is to remove C from RVA while keeping it around in the specs for niche applications where text size _really_ matters (with current silicon processes, I wonder how weird those niche applications have to be to require C). But it seems some would remove C even from the specs to free some ISA space. If arm removed thumb...
Don't worry, I would know if the assembler is producing C machine instructions, my rv64 interpreter on x86_64 does not support the C instructions at all.
You can benchmark stuff with and without RVC. Once there is faster hardware I want to do such a comparison with a full gentoo build for both sides.
However, quantifying what the result will actually mean is nearly impossible, because you don't know what the hardware cost was.