[WayneS]

> A sticky overflow flag would be great... "if properly scheduled"

That is the rub right there. Think about what this means. Every ALU operation needs to read whatever register contains this state and write a new value. So every ALU operation becomes serially dependent all all previous operations. You have just totally killed performance on out-of-order processors.

Processor designers work really hard to avoid these kinds of operations. In fact between the Pentium and the Pentium Pro (aka p6) the behavior of condition codes on several x86 instructions were changed from "keep old value" to "always cleared". We couldn't find any code that cared and it was never previously documented.

This code doesn't have to use branches, it could also OR the condition codes to some register after every operation. This would also hurt performance, but only for code that cares instead of all code.

[stephencanon]

Floating-point flags behave in exactly this "sticky" fashion on every mainstream architecture, and processor designers don't have trouble breaking the apparent serial dependency. It's annoying, but it's well-understood, and there are widely-used techniques that address it.