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0 pointsbrandmeyer4y ago0 comments

Comparing RISC-V's "C" extension to classic Thumb when Thumb-2 is 17 years old is like comparing RISC-V's "V" extension to classic SSE when AVX-512 and SVE2 are already available. Its an insidious form of straw-man attack that preys on the reader's ignorance.

> [Thumb-2] doesn't do everything, and switching has a significant cost.

Technically true, but irrelevant. Cortex-M is thumb-only and can't switch. Cortex-A processors that support both Thumb and ARM instructions almost never actually switch at all.

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AshamedCaptain4y ago

> Cortex-A processors that support both Thumb and ARM instructions almost never actually switch at all.

That is not correct. At least before ARMv8, most processors that could run both Thumb and ARM switch very frequently, up the point some libraries could be Thumb while others were ARM (i.e. within the same task!). A lot (but not all) of Android for ARMv7 is actually Thumb(-2). This is why "interworking" is such a hot topic.

Also, contrary to what the above poster says, switching does not have a "high cost", it is rather similar to the cost of a function call.

stephencanon4y ago

> it is rather similar to the cost of a function call.

It literally is a function call, most of the time.

And yeah, thumb-2 was the preferred encoding for 32b iOS and Android, and the only supported encoding for Windows phone, so it was used on billions of devices.

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