[nneonneo]

Even calling uninitialized data “garbage” is misleading. You might expect that the compiler would just leave out some initialization code and compile the remaining code in the expected way, causing the values to be “whatever was in memory previously”. But no - the compiler can (and absolutely will) optimize by assuming the values are whatever would be most convenient for optimization reasons, even if it would be vanishingly unlikely or even impossible.

As an example, consider this code (godbolt: https://godbolt.org/z/TrMrYTKG9):

    struct foo {
        unsigned char a, b;
    };

    foo make(int x) {
        foo result;
        if (x) {
            result.a = 13;
        } else {
            result.b = 37;
        }
        return result;
    }

At high enough optimization levels, the function compiles to “mov eax, 9485; ret”, which sets both a=13 and b=37 without testing the condition at all - as if both branches of the test were executed. This is perfectly reasonable because the lack of initialization means the values could already have been set that way (even if unlikely), so the compiler just goes ahead and sets them that way. It’s faster!

[titzer]

Indeed, UB is literally whatever the compiler feels like. A famous one [1] has the compiler deleting code that contains UB and falling through to the next function.

"But it's right there in the name!" Undefined behavior literally places no restrictions on the code generated or the behavior of the program. And the compiler is under no obligation to help you debug your (admittedly buggy) program. It can literally delete your program and replace it with something else that it likes.

[1] https://kristerw.blogspot.com/2017/09/why-undefined-behavior...

[jmgao]

There are some even funnier cases like this one: https://gcc.godbolt.org/z/cbscGf8ss

The compiler sees that foo can only be assigned in one place (that isn't called locally, but could called from other object files linked into the program) and its address never escapes. Since dereferencing a null pointer is UB, it can legally assume that `*foo` is always 42 and optimizes out the variable entirely.

[publicdebates]

To those who are just as confused as me:

Compilers can do whatever they want when they see UB, and accessing an unassigned and unassiganble (file-local) variable is UB, therefore the compiler can just decide that *foo is in fact always 42, or never 42, or sometimes 42, and all would be just as valid options for the compiler.

(I know I'm just restating the parent comment, but I had to think it through several times before understanding it myself, even after reading that.)

[recursivecaveat]

Even the notion that uninitialized memory contain values is kind of dangerous. Once you access them you can't reason about what's going to happen at all. Behaviour can happen that's not self-consistent with any value at all: https://godbolt.org/z/adsP4sxMT

[masklinn]

Is that an old 'bot? because I noticed it was an old version of Clang, and I tried switching to the latest Clang which is hilarious: https://godbolt.org/z/fra6fWexM

[userbinator]

If you don't initialise a variable, you're implicitly saying any value is fine, so this actually makes sense.

[pornel]

The difference is that it can behave as if it had multiple different values at the same time. You don't just get any value, you can get completely absurd paradoxical Schrödinger values where `x > 5 && x < 5` may be true, and on the next line `x > 5` may be false, and it may flip on Wednesdays.

This is because the code is executed symbolically during optimization. It's not running on your real CPU. It's first "run" on a simulation of an abstract machine from the C spec, which doesn't have registers or even real stack to hold an actual garbage value, but it does have magic memory where bits can be set to 0, 1, or this-can-never-ever-happen.

Optimization passes ask questions like "is x unused? (so I can skip saving its register)" or "is x always equal to y? (so I can stop storing it separately)" or "is this condition using x always true? (so that I can remove the else branch)". When using the value is an undefined behavior, there's no requirement for these answers to be consistent or even correct, so the optimizer rolls with whatever seems cheapest/easiest.

[like_any_other]

For some values of 'sense'.

[sethev]

That seems like a reasonable optimization, actually. If the programmer doesn’t initialize a variable, why not set it to a value that always works?

Good example of why uninitialized variables are not intuitive.

[masklinn]

Things can get even wonkier if the compiler keeps the values in registers, as two consecutive loads could use different registers based as you say on what's the most convenient for optimisation (register allocation, code density).

[quietbritishjim]

If I understand it right, in principle the compiler doesn't even need to do that.

It can just leave the result totally uninitialised. That's because both code paths have undefined behaviour: whichever of result.x or result.y is not set is still copied at "return result" which is undefined behaviour, so the overall function has undefined behaviour either way.

It could even just replace the function body with abort(), or omit the implementation entirely (even the ret instruction, allowing execution to just fall through to whatever memory happens to follow). Whether any computer does that in practice is another matter.

[masklinn]

> It can just leave the result totally uninitialised. That's because both code paths have undefined behaviour: whichever of result.x or result.y is not set is still copied at "return result" which is undefined behaviour, so the overall function has undefined behaviour either way.

That is incorrect, per the resolution of DR222 (partially initialized structures) at WG14:

> This DR asks the question of whether or not struct assignment is well defined when the source of the assignment is a struct, some of whose members have not been given a value. There was consensus that this should be well defined because of common usage, including the standard-specified structure struct tm.

As long as the caller doesn't read an uninitialised member, it's completely fine.

[arrowsmith]

How is this an "optimization" if the compiled result is incorrect? Why would you design a compiler that can produce errors?

[Negitivefrags]

It’s not incorrect.

The code says that if x is true then a=13 and if it is false than b=37.

This is the case. Its just that a=13 even if x is false. A thing that the code had nothing to say about, and so the compiler is free to do.

[throwatdem12311]

Because a could be 13 even if x is false because initialisation of the struct doesn’t have defined behavior of what the initial values of a and b need to be.

Same for b. If x is true, b could be 37 no matter how unlikely that is.

[xboxnolifes]

It is not incorrect. The values are undefined, so the compiler is free to do whatever it want to do with them, even assign values to them.

[tehjoker]

It's not incorrect. Where is the flaw?