3 ms·

> If the pointed to object has been replaced by a register, there might not even be a pointer to check. I dunno if that is relevant. In the example you posted

> If the pointed to object has been replaced by a register, there might not even be a pointer to check. I dunno if that is relevant. In the example you posted, simply removing the `static inline` leaves a pointer to check. The value isn't moved to a register. We are arguing the cases when the source lines are there, but then are not emitted. In the code samples under discussion, the pointer values are still there and can be checked. Their values are not sitting in some register. > (getter+ctx is just an hand built closure, so the code is far from non-realistic). Qualifying with `static inline` is unusual, and none of the objections are using static+inlined code as samples of poor code generation. The examples being presented are a lot more real (as they are taken from existing project, not contrived to make an argument).

gpderetta 2y ago
The important bit is that the functions are inlined into main, so for that specific code path the check is removed. The fact it would be left there in the non-called function is not really relevant. I used static inline (just inline would have been enough) just to get rid of irrelevant code. The code under discussion does not compile, so it is hard to discuss it. Please consider this variant: https://godbolt.org/z/EMzr4naMP https://godbolt.org/z/EMzr4naMP As you can see, while the check is still present in the non-inlined foo, main doesn't actually call it and omits the check. There is nothing left in main to check as there is no myvar object left. This example and my previous one are similar: in both examples, not only the compiler could statically compute the value of the pointer, but it can track the pointee directly ('int x' in my example, the null pointer in your), hence it doesn't need to allocate any memory or registers for the pointer. Separately, in both cases the compiler was able to statically prove that the pointer must be pointing to something (in my example because it knows the target, in yours because of the assignment through it), so the null check can be removed as the value of the condition is statically known to be always true. Finally, I believe that in your example GCC found the contradiction with the pointer being both null and not null and realized that the whole main function is not possibly reachable, hence it isolates it with ud2 (clang is just silly). What code would you expect GCC to generate in main to test the condition? Should it allocate a register, initialize it just for the purpose of performing the branch? Should it do it just for your example or also for mine?

gpderetta 2y ago

The important bit is that the functions are inlined into main, so for that specific code path the check is removed. The fact it would be left there in the non-called function is not really relevant. I used static inline (just inline would have been enough) just to get rid of irrelevant code. The code under discussion does not compile, so it is hard to discuss it. Please consider this variant: https://godbolt.org/z/EMzr4naMP https://godbolt.org/z/EMzr4naMP As you can see, while the check is still present in the non-inlined foo, main doesn't actually call it and omits the check. There is nothing left in main to check as there is no myvar object left. This example and my previous one are similar: in both examples, not only the compiler could statically compute the value of the pointer, but it can track the pointee directly ('int x' in my example, the null pointer in your), hence it doesn't need to allocate any memory or registers for the pointer. Separately, in both cases the compiler was able to statically prove that the pointer must be pointing to something (in my example because it knows the target, in yours because of the assignment through it), so the null check can be removed as the value of the condition is statically known to be always true. Finally, I believe that in your example GCC found the contradiction with the pointer being both null and not null and realized that the whole main function is not possibly reachable, hence it isolates it with ud2 (clang is just silly). What code would you expect GCC to generate in main to test the condition? Should it allocate a register, initialize it just for the purpose of performing the branch? Should it do it just for your example or also for mine?