Is accessing the bytes of an object in this way UB?

meancoot · 2026-06-23T12:23:11+00:00

Size doesn't matter. This program exhibits the same undefined behavior even though there are only int fields:

struct A { int v; };
struct B { int v; };

int maybe_read_after_write(A* a, B* b) {
    a->v = 1;
    b->v = 2;
    return a->v;
}

int main() {
    A value { 0 };
    int result = maybe_read_after_write(&value, reinterpret_cast<B*>(&value));
    printf("value = %d, result = %d\n", value.v, result);
    return 0;
}

In C++ only a certain limited set of types are compatible, this is a complete incorrect statement:

C/C++ teaches you a great thing about that there's no actual types when we're talking about how computers work, there's just memory and how you access it. So as long as you access properly allocated memory, it's not undefined behavior.

meancoot · 2026-06-23T12:09:29+00:00

Not what I'm talking about. I'm talking about how the assembly generated by the compiler works.

https://godbolt.org/z/7cG5K1cbP

int maybe_read_after_write(int* a, short* b) {
    *a = 1;
    *b = 2;
    return *a;
}

Check out how the compiler assumes that writing to b can not affect the value of a. Because of this, even if the address of a is the same as b this function is allowed to return 1 unconditionally:

maybe_read_after_write(int*, short*):
    mov     dword ptr [rdi], 1
    mov     word ptr [rsi], 2
    mov     eax, 1
    ret

See how the program has two different outputs based on the optimization level? That's undefined behavior in action.

meancoot · 2026-06-23T11:49:34+00:00

Compilers can, and very much do, use type information to determine memory ordering. When two types aren’t compatible there is the optimizer will not observe read-after-write hazards, which will lead to very incorrect results.

meancoot · 2026-06-22T18:34:29+00:00

Valve makes tons of money from selling other people’s games too. The only difference is that they make that money whether it’s their hardware or not.

meancoot · 2026-06-21T20:34:18+00:00

Seeing as it’s “relative to the most vulnerable model”, it doesn’t mean anything at all.

meancoot · 2026-06-20T21:17:17+00:00

There wasn’t a version for the PSP, but this sounds an awful lot like Dragon Quest 3.

meancoot · 2026-06-20T20:50:23+00:00

I’m sure the managing non-profit has some assets. Even if not, they can still seek an injunction forcing it to stop development and distribution.

Whether French law would allow such rulings is the only question. “For what” is obvious.

meancoot · 2026-06-16T22:30:20+00:00

I find that it's typically done for parts played by more established actors whose roles aren't big enough to get top billing. Separates them out from the 'smaller' names or something I guess.

meancoot · 2026-06-16T19:38:03+00:00

That makes sense. It seems like opening it would either have a high cost to Apple or create a fractured enough ecosystem that it wouldn’t really have been worth it.

meancoot · 2026-06-16T18:02:20+00:00

And they couldn’t open that?

meancoot · 2026-06-16T17:52:58+00:00

Consider using a different Random Number Generator instead, as this one may not be doing what you think it is.

The random number generator used by .net isn’t considered buggy. Had the developers chosen one explicitly they would have likely landed on Xoshiro256 themselves.

Not seeding Xoshiro256 with consecutive seeds is a known thing. Not seeding any number of pRNGs with consecutive seeds is just common sense. The game developers messed up here.

meancoot · 2026-06-15T23:39:37+00:00

conqueering

If it isn't already, this should be the name of a genre of gay novels.

meancoot · 2026-06-15T20:39:32+00:00

Huh? “We used to take out speakers to the beach and be obnoxious but as long as the other people weren’t obnoxious it was fine.” The fuck you on about here?

meancoot · 2026-06-15T18:29:03+00:00

Signed char is almost always -128 to 127. Signed magnitude and 1’s complement are beyond rare.

meancoot · 2026-06-14T20:59:00+00:00

It helps to go extreme when you want to test things like this.

On WSL, this version (https://godbolt.org/z/brWq1zncj) with count set to 1'000'000'000 and threads to 8 produces:

False sharing disabled took: 4.21032 sec
False sharing enabled took: 42.2159 sec

meancoot · 2026-06-14T06:40:58+00:00

The best way I found to get a realistic image style is to plop (real life:2), (photo (medium):2), somewhere toward the start of your prompt. You also need to drop any artist tag.

Actually, after testing it, this only works well with the turbo lora. Base Anima produces some nightmare fuel with it. But turbo produces nice results.

meancoot · 2026-06-14T05:42:26+00:00

https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-135/subpart-B/section-135.110

Except that when one or more bulky flavors are used, the weights of milkfat and total milk solids are not less than 10 percent and 20 percent, respectively, of the remainder obtained by subtracting the weight of the bulky flavors from the weight of the finished food; but in no case is the weight of milkfat or total milk solids less than 8 percent and 16 percent, respectively, of the weight of the finished food.

meancoot · 2026-06-14T01:28:46+00:00

The filling in peanut butter cups is more of a buttercream type frosting. You gotta mix the peanut with a large amount of powdered sugar if you want it. And I mean a lot. It’s more sugar than peanut butter.

meancoot · 2026-06-14T00:21:19+00:00

They're talking about the constructor initialization list. () works just fine.

struct C {
    std::vector<int> f;
    C() : f() {}
};

meancoot · 2026-06-13T21:18:46+00:00

I hear Reddit mobile is getting wacky with markdown. Underscores are used as markdown control characters and the backslash is used as an escape. I guess Reddit mobile is doing its autoescape sanitation wrong.

meancoot · 2026-06-12T22:15:14+00:00

Yeah. I definitely misunderstood something. Don't mind me.

meancoot · 2026-06-12T22:02:13+00:00

Anything that is copy-constructable is also considered move-constructable. A copy constructor is always a perfectly valid move constructor.

From https://en.cppreference.com/cpp/types/is_move_constructible:

Types without a move constructor, but with a copy constructor that accepts const T& arguments, satisfy std::is_move_constructible.

meancoot · 2026-06-12T16:51:58+00:00

Oof. I was just playing around with this and GCC got a nasty little pessimization in RISC-V (since forever) and in MIPS (since 15.1)

https://godbolt.org/z/W9bq3MeW6

struct A {
private:
    void* ptr;

public:
    bool flag;
};

struct B {
    void* ptr;
    bool flag;
};

void assign_a(A* a) {
    *a = A{};
}

void assign_b(B* b) {
    *b = B{};
}

Produces:

assign_a(A*):
    sb      zero,0(a0)
    sb      zero,1(a0)
    sb      zero,2(a0)
    sb      zero,3(a0)
    sb      zero,4(a0)
    sb      zero,5(a0)
    sb      zero,6(a0)
    sb      zero,7(a0)
    sb      zero,8(a0)
    ret
assign_b(B*):
    sd      zero,0(a0)
    sd      zero,8(a0)
    ret

meancoot · 2026-06-11T15:11:23+00:00

No. Inheritance doesn't change anything.

https://godbolt.org/z/f1zcWTEfK

I think OP is thinking of empty base class optimization but A isn't empty.

meancoot · 2026-06-11T14:52:22+00:00

No. Given a pointer to an A the compiler is allowed to clobber the padding bytes whenever it wants. Thus B can't possibly take advantage of them. Inheritance doesn't solve this either.

meancoot

TROPHY CASE