Why are conditionally executed instructions not present in later ARM instruction sets?

Question

Naively, conditionally executed instructions seem like a great idea to me.

As I read more about ARM (and ARM-like) instruction sets (Thumb2, Unicore, AArch64) I find

Answer 1

It's somewhat misleading to say that conditional execution is not present in ARMv8. The issue is to understand why you don't want to execute some instructions. Perhaps in the early ARM days, the actual non-execution of instructions mattered (for power or whatever) but today the significance of this feature is that it allows you to avoid branches for small dumb jumps, for example code like a=(b>0? 1: 2). This sort of thing is more common than you might imagine --- conceptually it's things like MAX/MIN or ABS (though for some CPUs there may be instructions to do these particular tasks).

In ARMv8, while there are not general conditionally executed instructions there are a few instructions that perform the specific task I am describing, namely allowing you to avoid branching for short dumb jumps; CSEL is the most obvious example, though there are other cases (e.g. conditional setting of conditions) to handle other common patterns (in that case the pattern of C short-circuited expression evaluation).

IMHO what ARM has done here is what makes the most sense. They've extracted the feature of conditional execution that remains valuable on modern CPUs (avoid many branches) while changing the details of the implementation to match the micro-architecture of modern CPUs.