Den sön 26 maj 2019 kl 12:37 skrev René J. V. Bertin <rjvber...@gmail.com>: > > 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
If you look at the generated code for Guiseppe's example after removing the assume, you'll see that it'll use vector instructions for the addition/assignment in the loop body for as long as it can, but then there are a bunch of non-vector instructions for handling the remaining iterations (when (count % 16) != 0). With the added hint that (count % 16) == 0, the compiler was free to omit those instructions. At least that's how I interpret it. Elvis > > 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 _______________________________________________ Interest mailing list Interest@qt-project.org https://lists.qt-project.org/listinfo/interest
