Arguably x86 only has a compressed encoding in the sense that super common instructions often fit in fewer than 4 bytes and some of the most used ones are 1 byte. I don’t think the compression is deliberate or optimal though.

Arguably most of the 1 byte instructions are garbage, though ;)

What, no love for AAA :-?

(IIRC they removed that one on x86-64)

Sounds pretty similar to thumb mode on ARM, no?

And ARM licensed the Hitachi SuperH patents when implementing Thumb. It's not exactly a new concept.

In addition to Thumb there are apparently at least MIPS16e ASE and PowerPC VLE.

ARM had Thumb, right? Is that a deadend at this point?

Thumb was removed from ARMv8 (AArch64). If you use 32-but legacy, it may exist on some chips; but it’s no longer the benefit it once was.

It's always been of the same value. You exchange performance for code density. They want aarch64 to be high-performance, so they removed thumb.

RISC-V compact instructions don't require special modes and run in fully-mixed mode with 32-bit instructions without all the penalties thumb has (they are literally just extended into their 32-bit counterparts internally).

> It's always been of the same value. You exchange performance for code density.

High code density became less valuable, was the point.