Linus Torvalds on aliasing

Post by brazzledazzle » Fri, Jun 08, 2018, 1:44 PM UTC

rainbowmverse wrote: post #220

cryon wrote: post #141

Earlier quoted context omitted.

I can't stand when you're expected to wrap every sentence in some sugar coated fake 'niceness' just to not step on somebody's toes. If I have a certain way of expressing myself... Well that's how it is.

>> If I have a certain way of expressing myself... Well that's how it is. You shouldn't sell yourself short. Changing the way you communicate is a huge but worthwhile challenge. In a different time, I would have been mean to you over your comment instead of offering supportive words. It took as much effort to be nice as it used to take to be mean.

Post by boomlinde » Fri, Jun 08, 2018, 1:54 PM UTC

minipci1321 wrote: post #224

boomlinde wrote: post #215

Earlier quoted context omitted.

They are optional, but only in the sense that if you can't exactly represent e.g. uint64_t, there can't be a uint64_t type. So the example above is kosher in the sense that it's standards compliant, even if it isn't perfectly portable (for reasons unrelated to memset type punning; that you're using uint64_t in the first place). If an implementation can't represent uint64_t exactly, it can still have a uint_least64_t…

I certainly agree with what you said as you said it. However, as usual, it is the implementation that kills it. a) I believe sizeof (double) * 8 == 64 is not enforced by the standard either. One could argue that it is hard to find anything else these days, but (for a contrived example) I believe nothing prevents you from making your own port of GCC for some purpose, and make 'double' be 'float'. (From my memories, it…

Post by [deleted] » Fri, Jun 08, 2018, 2:00 PM UTC

mannykannot wrote: post #214

ralfn wrote: post #181

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When somebody speaks their mind, everybody around them knows where they stand. I doubt there is difference between about how often people resent other people around the world. But I do consider it more humane if resentment isn't hidden under the surface. It's easier to trust Linus than a tactfull politician. That should make us all think.

One circumstance where insults might be justified is where one party has ignored valid objections to his position. I mention this here because kristianic made the valid point that you have set up a false dichotomy, and you replied with a statement that keeps it going. Is the habit of not paying attention to the actual issues also part of your superior Northern European culture, and, in fact, is it perhaps a reason wh…

Post by acqq » Fri, Jun 08, 2018, 2:26 PM UTC

boomlinde wrote: post #201

acqq wrote: post #68

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AFAIK even using union for the above purpose can be seen as "undefined behavior" when reading standard, and that's why the standard is especially wrong, as it makes some typical desired (and needed) outcomes impossible to implement, and at the moment clang still has problems even with the union variant whereas gcc allows you to use union by passing the compiler switch which effectively has the semantics "we promise w…

What part of the standard should I read to come to the conclusion that type punning with unions is undefined behavior? C99 and C11 both explicitly mention that it's not, but maybe you're talking C89/C90? IMO there are 4 highly relevant paragraphs in C99: 6.2.6.1: When a value is stored in an object of structure or union type, including in a member object, the bytes of the object representation that correspond to any…

Post by BruiseLee » Fri, Jun 08, 2018, 2:32 PM UTC

boomlinde wrote: post #193

TorKlingberg wrote: post #166

Earlier quoted context omitted.

Type punning through a union is allowed, buy you have to be careful so the compiler can see the union. union { int i; float f; } u; You can't safely call a function with pointers to the members like func(&u.i, &u.f), because when func was compiled, the compiler couldn't know that the arguments are pointers to union members. This sort of thing scares me, because what the compiler can "see" isn't well defined. Is it ok…

Assuming we're talking C, not C++, the compiler doesn't need to know that the arguments are pointers to union members, because the union members are all aligned with the union itself and have the same address. So &u.f is not some magic special pointer, it's the address of a float. So is (float *)&u. The example in the standard I think you're referring to relates to a special guarantee made about unions of structs tha…

    float func(int *i, float *f) {
      *i = 42;
      return *f;
    }

    float func(int *i, float *f) {
      float result = *f;
      *i = 42;
      return result;
    }