I really hate to see this rush to "make sure our code doesn't use branch prediction". It may be the best fix we have available now, but it's going to create a legacy of bizarre inefficient machine code that will last forever.
For most things the CPU is doing, we can ignore in high level languages because the compiler/optimizer can turn our high level code into the appropriate byte code to take advantage of deep hardwar magic. Not branch prediction, though, no matter how high level my language, I still must consider pipeline size, cache alignment and so on when designing a novel new container or algorithm or risk it being inexplicitly slow.
Branch prediction is, and always was a hack. Let it die.
The effect of this seems to be "I wish everything was slower and then the naive implementation would be the best implementation".
No, a stall is only a thing because branch prediction exists. Get rid of it and I can go back to not caring about the deep underlying architecture of the hardware I may potentially be running on. And not doing branch prediction isn't inefficient... that's a bizare statement. Branch prediction and speculative execution are themselves inefficiencies (i.e. spending power to do things that will turn out to not be needed fairly often).
>the naive implementation would be the best implementation
Not the naive implementation. A well thought out, developed implementation that is defeated by deep hardware details. The promise of high level languages was always to avoid having to know these sorts of details. In designing an algorithm I should be worried about things like how often I access data (e.g. can I cut down on how many accesses with some clever structure?) and the like. Now that's all trumped by deeply knowing the low level implementation.
EDIT: I just wanted to clarify here that I did not mean “all branches” just some of them, hence the name of the technique which is “branchless security” (which is a really good piece of engineering). The frustration is basically we have to bake in CPU model specific mitigation’s into binaries. You can of course patch it out, but the affected CPUs will in theory be in the wild for a long time to come even once corrected ones are released.
To fix this in hardware without software changes you would have to disable branch prediction. That would perform worse than what we are doing.
It is a really cool piece of engineering but we can still be sad we have to implement it.
(On the hardware side I was assuming sommeone clever at Intel will come up with some security non Perf killing change to make branches trustable again).
Similarly, where we use pointer poisoning, it is in addition to some other security check, which is almost always a branch.