Giuseppe D'Angelo via Interest wrote: Hi,
> On the other hand, Q_ASSUME(cond) tells the compiler that cond is true, After reading the MS doc I sort of understand how you can use the construct to implement a Q_UNREACHABLE (but in the example given I don't see the codegen advantage of `default: Q_UNREACHABLE` vs. not adding a `default:` at all). > Look here at a possible example at how it can improve codegen: > > https://gcc.godbolt.org/z/KlWBRY Not really, I'm afraid. The only thing that's evident to me from there is that there is much fewer generated machine code when you add the assume statement. I don't see at all why that would be, what difference it would make for the loop that it is always iterated over a multiple of 16. I thought the difference might be in evaluating the `i < count` expression, but even after trying to factor that out the difference remains: https://gcc.godbolt.org/z/2Zclp5 Take home message for me is that this is a construct that's probably useful only if you have very intimate knowledge about code generation, and thus not very cross-platform/compiler (and even less cross-architecture). Except for the Q_UNREACHABLE thing. What I was hoping it might do is what the Qt documentation suggests, a more graceful version of a Q_ASSERT. That is, a version that does the usual abort in debug builds, but becomes a regular if in production code. I've seen too many examples of coding where a Q_ASSERT is used to guard against situations that are *assumed* never to occur, and then forgotten (or the devs assume everyone else uses debug/development builds). In many if not most of those cases it's trivial to add a graceful handling of the situation. R _______________________________________________ Interest mailing list Interest@qt-project.org https://lists.qt-project.org/listinfo/interest
