4 ms·

Yes, I had similar thoughts when I started reading, but I think only ARM32 has predication. (There's a prefix-instruction-based something or other in Thumb, I

Yes, I had similar thoughts when I started reading, but I think only ARM32 has predication. (There's a prefix-instruction-based something or other in Thumb, I think, but it doesn't devote part of the encoding space to predication bits like ARM32 does.) As I understand it they didn't carry predication across from ARM32 to ARM64 for various performance reasons (if you want to be able to re-order instructions, or even agressively pipeline them, you don't want them depending on the result of the immediately-prior instructon). Predication everywhere (i.e. orthogonal to the rest of the instruction set, and not special-cased) is certainly more RISC than CISC - but having removed it in general, bringing it back for a few specific instructions is arguably CISCy.

peterfirefly 3y ago
> There's a prefix-instruction-based something or other in Thumb, I think, but it doesn't devote part of the encoding space to predication bits like ARM32 does. Yes, the IT (if-then) instruction (prefix). It is not supported by Cortex-M0, Cortex-M0+, and Cortex-M1, though. Those are the smallest T32 (Thumb-2) microcontroller designs ARM has. IT can be followed by up to 4 instructions and encodes their predicate bits would have been in 32-bit ARM code (A32). There is not total freedom regarding their predicate bits: they all have to share the same ground condition (3 bits) and then get an individual bit that says whether to execute when that condition is met or when it isn't. The IT instruction is a 16-bit instruction that devotes 8 bits to this -- not 7, because the encoding is weird.
- peterfirefly 3y ago
  I should add that M0/M0+/M1 have very short pipelines: 2 and 3 stages. That means the cost of a branch isn't all that high so the benefit of predication is small. (They don't have branch predictors, either.)

peterfirefly 3y ago

> There's a prefix-instruction-based something or other in Thumb, I think, but it doesn't devote part of the encoding space to predication bits like ARM32 does. Yes, the IT (if-then) instruction (prefix). It is not supported by Cortex-M0, Cortex-M0+, and Cortex-M1, though. Those are the smallest T32 (Thumb-2) microcontroller designs ARM has. IT can be followed by up to 4 instructions and encodes their predicate bits would have been in 32-bit ARM code (A32). There is not total freedom regarding their predicate bits: they all have to share the same ground condition (3 bits) and then get an individual bit that says whether to execute when that condition is met or when it isn't. The IT instruction is a 16-bit instruction that devotes 8 bits to this -- not 7, because the encoding is weird.

peterfirefly 3y ago
I should add that M0/M0+/M1 have very short pipelines: 2 and 3 stages. That means the cost of a branch isn't all that high so the benefit of predication is small. (They don't have branch predictors, either.)