[danellis]

This was part of the beauty of ARM when I learned it as a teenager back in the early 90s. Very simple and elegant, and writing ARM code by hand was enjoyable. Coming back to ARM now, though, in this form of Cortex-M microcontrollers, I see that things have become muddied with things like if-then-else instructions and mixed 16-bit/32-bit Thumb-2 code.

[Someone]

Simple, elegant, and it eats an astonishing 12.5% of instruction bandwidth (4 bits out of 32). A branch will require less space as soon as you want to conditionally execute over 8 instructions. On top of that, for that is executed unconditionally (in practice, maybe not most of the instructions if you look at the binary, but almost certainly most of the instructions if you count ones executed multiple times)

So, a neat idea, but not for the long term, and certainly not for all CPUs. For example, ARM 64 ditches this feature (https://www.mikeash.com/pyblog/friday-qa-2013-09-27-arm64-an...)

[stephencanon]

arm64 ... sort of ditches conditional execution. It’s not on every instruction any more, but it’s still available on more instructions than on most other arches.

To the usual complement of typical conditional instructions (branch, add/sub with carry, select and set), arm64 adds select with increment, negate, or inversion, the ability to conditionally set to -1 as well as +1, and the ability to conditionally compare and merge the flags in a fairly flexible manner (it’s really a conditional select of condition flags between the result of a comparison and an immediate). This actually preserves most of the power of conditional execution (except for really exotic hand-coded usages), while taking up much less encoding space.