That assumes the compiler reserves one continuous place for the value, which isn’t always true (hardly ever true in the case of registers). If the compiler is required to make all code paths result in the same uninitialized value, that can limit code generation options, which might reduce performance (and performance is the whole reason to use uninitialized values!).

Also, an uninitialized value might be in a memory page that gets reclaimed and then mapped in again, in which case (because it hasn’t been written to) the OS doesn’t guarantee it will have the same value the second time. There was recently a bug discovered in one of the few algorithms that uses uninitialized values, because of this effect.

But, I wonder how much it would reduce performance, if we only have to pick a value the first time the memory is read?

I would imagine there isn't that many cases where we are reading uninitalised memory and counting on that reading not saving a value. It would happen when reading in 8-byte blocks for alignment, but does it happen that much elsewhere?

if you pick a value you have to store it, and if you have to store it it might spill into memory when register allocation fails. Moving from register-only to stack/heap usage easily slows down your program by an order of magnitude or two. If this is in a hot path, which I'd argue it is since using uninitialized values seems senseless otherwise, it might have a big impact.

The only way to really know is to test this. Compilers and their optimizations depend on a lot of things. Even the order and layout of instructions can matter due to the instruction cache. You can always go and make the guarantee later on, but undoing it would be impossible.

> same uninitialized value, that can limit code generation options

it pretty much requires the compiler to initialize all values when they first "appear"

except that this is impossible and outright hazardous if pointers are involved

But doable for a small subset like e.g.

- stack values (but would inhibit optimizations, potentially pretty badly)

- some allocations e.g. I/O buffers, (except C alloc has no idea that you are allocating an I/O buffer)

> If the compiler is required to make all code paths result in the same uninitialized value, that can limit code generation options

Can you provide (on say x86_64) an example of this, other than the case where the compiler prunes cases based on characterizing certain paths as UB? In other words, a case where "an uninitialized value is well-defined but can be different on each read" allows more performance optimization than "the value will be the same on each read".

> Also, an uninitialized value might be in a memory page that gets reclaimed and then mapped in again, in which case (because it hasn’t been written to) the OS doesn’t guarantee it will have the same value the second time. There was recently a bug discovered in one of the few algorithms that uses uninitialized values, because of this effect.

This does not sound correct to me, at least for Linux (assuming one isn't directly requesting such behavior with madvise or something). Do you have more information?

I can’t find a citation, but my recollection is the problem happened with the Briggs-Torczon sparse set algorithm, which relies on uninitialized memory not changing. For performance, they were using MMAP_UNINITIALIZED (which has to be enabled with a kernel config).

The most obvious general case (to me) is reading an uninitialized local variable in a loop. If uninitialized has to be the same value every time, you’d have to allocate a register or stack space to ensure the value was the same on every iteration. Instead, you’d don’t have to allocate anything, just use whatever value is in any register that’s handy. (By this logic you can also start pruning code, by picking the “most optimal” value for the uninitialized variable.)

Uninitialized memory being UB isn’t an insane default imo (although it makes masked simd hard), nor is most UB. But the lack of escape hatches can be frustrating

Anything being UB is insane to me...

Nah it makes some sense for portability between architectures. Or at least it did back when C was invented and there were some wild architectures out there.

And it definitely does allow some optimisation. But probably nothing significant on modern out-of-order machines.

> there were some wild architectures out there.

what is out there is still pretty wield

just slightly less

> probably nothing significant on modern out-of-order machines.

having no UB at all will kill a lot of optimizations still relevant today (and won't match anymore to hardware as some UB is on hardware level)

out of order machines aren't magically fixing that, just makes some less optimized code work better, but not all

and a lot of low energy/cheap hardware does have no or very very limited out of order capabilities so it's still very relevant and likely will stay very relevant for a very long time

only until you get deeper into how the hardware actually work (and OS to some degree)

and realize sometimes the UB is even in the hardware registers

and that the same logical memory address might have 5 different values in hardware at the same time without you having a bug

and other fun like that

so the insanity is reality not the compiler

(through IMHO in C and especially C++ the insanity is how easily you might accidentally run into UB without doing any fancy trickery but just dumb not hot every day code)

I do not find it so easy to accidentally run into UB in C if you follow some basic rules. The exceptions are null pointer dereferences, out-of-bounds accesses for arrays, and signed overflow, all those can be turned into run-time traps. The rules include no pointer arithmetic, no type casts, and having some ownership strategy. None of those is difficult to implement and where exceptions are made, one should treat it carefully similar to using "unsafe" in Rust.

There is no UB in hardware registers or physical DRAM, I don't think you actually have familiarity with how the hardware works if you make this claim. (Or perhaps you aren't familiar with how crazy "UB" in the sense of the ISO C documentation is)

> There is no UB in hardware registers or physical DRAM

This seems very sensitive to specific definitions that others might not share. DRAM is provided with a spec sheet that defines its behavior (if you write to an address, you’ll read back the same value from the same address in the future) under certain conditions. If you violate those conditions, the behavior is… undefined. If you operate DRAM with the wrong refresh timing, or temperature, or voltage, or ionizing radiation level, you may see strange behavior. Even non-local behavior, where the value read from one cell depends on other cells (RowHammer). How is this not UB?

I've edited my claim to be a bit more clear, however in the context of parent's claim we are talking about bug-free code on an non-buggy physical processor, and I think implicitly we are talking about user-mode code where one does not have the ability to alter any DRAM timing configuration registers anyway.

If a C program accesses uninitialized memory (UB), it is perfectly compliant with the language spec for the compiler to reformat your hard drive and replace your OS code with crypto mining.

I'm not exaggerating by a legalistic interpretation, and I'm only slightly exaggerating in practice. UB can do some really weird, unintuitive stuff in practice on real hardware:

https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-...

The point is that this extreme UB should never happen. It was a choice of the compiler implementors, and rather than fix this, they allowed the escape hatch of UB in the spec. It would be more sensible for the compiler to say that, e.g., accessing uninitialized memory results in a nonspecified value, or even possibly multiple different nonspecified values if accessed from different threads. That captures what we expect to happen, but would be (according to C language spec lawyers) defined behavior.

In practice, it would mean that compliant compilers will ensure that any situation in which uninitialized memory could be accessed would not result in weird edge case page faults on certain architectures or whatever that could in fact lead to wacky UB situations.

This is not an unreasonable ask.

> This is not an unreasonable ask.

But isn't this exactly parallel to the Rowhammer case in DRAM? When operating at the edge of the spec, the behavior of DRAM becomes undefined. (And, of course, one challenge with Rowhammer was about /which/ edge of the spec this happened on.) In this case, writing one physical address altered the contents of other physical addresses. This is "really weird, unintuitive stuff … on real hardware." And of course we can (and do) ask DRAM vendors to not take advantage of this undefined behavior; but they do so as an optimization, allowing slightly smaller and more closely spaced DRAM cells, and thus higher density DRAM dice for the same price. Just like it's possible to work with a language with fully-defined semantics at the cost of performance, it's possible to buy DRAM with much wider specifications and more clearly defined behavior up to the edges of those specifications… at the cost of performance.

Extreme UB in both hardware and software is a choice of priorities. You may favor giving up performance capabilities to achieve a more understandable system (so do I! I do most of my work on in-order lockstep processors with on-die ECC SRAM to maximize understandability of failure modes), but the market as a whole clearly does not, in both hardware and software.

Legitimate bugs in hardware is probably out of scope for compilers. That would be an unreasonable ask, yes. But I believe it should be the job of the compiler to make sure that a correctly executed program on a reference machine strictly adhering to the architecture specs should not result in UB.

I'm not saying that a computer architecture should be UB-free. That would be awesome if it could be done, but in practice probably a bridge too far. But a compiler should map high-level directives into low-level implementations on a specific architecture using constructs that do not result in UB. This is not too much to ask.

A compiler can't reasonably protect you from rowhammer attacks. But it should guarantee that, barring hardware errors, accessing uninitialized memory has no effect other than something sensible like returning unspecified contents, or causing a memory access exception, or whatever. It should be defined up front what the behavior is, even if some of the runtime values are unpredictable.

As a more concrete example, most languages these days clearly define what happens in signed integer overflow: the thing that you expect to happen in any two's complement machine (char)127 + (char)1 == -128. C treats this as undefined behavior, and as mentioned in my link above that can cause what should be a finite loop (with or without overflow) to compile as an infinite loop. This "optimization" step by the compiler should never have happened. C resists changing this because C compilers exist for non-twos-complement architectures where the behavior would be different. IMHO the correct approach would be to require THOSE weird esoteric architectures to compile in extra overflow checks (possibly disabled with opt-in compiler flags that explicitly violate the standard), rather than burden every C developer everywhere with the mess that is signed arithmetic overflow UB.

It's a matter of expectations. Any but the most junior programmers expect that signed integer overflow will not be portable. That's fine. But they expect a sensible result (e.g. wrap around on two's complement machines), even if it is non-portable. They don't expect the compiler to silently and sneakily change the logic of their program to something fundamentally different, because UB means the compiler can do whatever tf it wants.

> Legitimate bugs in hardware is probably out of scope for compilers.

But that's exactly the point. The "bug" of RowHammer was that it occurred slightly on the "allowed" side of the envelope, at acceptably-low refresh rates. The "UB" of RowHammer and a hundred other observable effects is that, on the "disallowed" side of the envelope, the behavior is undefined. The system designer gets to choose at what probability they are on each side of the envelope, and the trade-offs are very much optimization opportunities.

Writing software in C that may exhibit undefined behavior is exactly this -- it's choosing, as a software engineer, to be on the far side of the specification envelope. In exchange, you get access to several powerful optimizations, some at the compiler level, and some at the career level (if you think that not needing to learn to use understand your language properly is at time optimization, at least).

> There is no UB in hardware registers

There most definitely is.

In the ARM documentation this is referred to as “UNPREDICTABLE”. The outcome is not defined. It may work. It may not. It may put garbage data in a register.

I've edited/clarified the claim above. Parent did say "without you having a bug" and entering an UNPREDICTABLE state would be buggy code. Also, "maybe puts garbage data in a register" is not UB, it's a much more reasonable thing in UB (the definition of UNPREDICTABLE prior to ARMv8 does seem to allow for UB, however). I don't believe that such an UNPREDICTABLE state is reachable from user-space/unprivileged code (or else it would violate the chip's security properties) -- but if I'm wrong on that I'd be interested in an example.

Here are just a few examples:

https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-...

Those are examples regarding UB in the C++ language not the UNPREDICTABLE state in the ARM ISA which is what I asked for examples of.

None of what you said makes any sense. You're mixing "UB" and bugs.

UB is about declaring programs invalid with a snide "don't do that", not about incorrect execution due to an incorrect specification. E.g. speculative execution running in privileged mode due to a prior syscall is just a plain hardware bug. It's not undefined behavior. In fact, the bug in question is extremely well defined.

The closest thing to reading undefined behavior is reading a "don't care" or VHDL's 'U' value from a std_ulogic and even those are well defined in simulation, just not in physical hardware, but even there they can only ever be as bad as reading a garbage value. Since a lot of the hardware design is non-programmable, there is also usually no way to exploit it.

How would you implement integer-to-pointer conversions without UB?

What is UB about integer-to-pointer conversions?

Plenty of things! The resulting pointer may not be pointing to an object that is currently live, for example. It may not even be pointing to an object that makes any sense in the language's object model. It might be pointing to a return address on the stack, for example. Or a constant in the constant pool. Or a saved register in the middle of a computation that corresponds to no variables in the original program.

In short, the moment you enable integer-to-pointer conversions (assuming your target has a flat address space), you create pointer provenance problems whose only resolution is that some things have to be UB.

I don't think any of those are undefined behavior in the strict sense in which the term is defined in the C/C++ standards. Pointer casts are defined behavior. I believe the things you point to are either implementation-defined or unspecified, which is different from UB.

It may seem nitpicky, but the downside of relying on implementation defined or unspecified behavior is largely boxed and contained. E.g you might get a memory access error. UB is, in principle, completely unlimited in downside. And because of that, it often interacts badly with optimization passes, resulting in very strange bugs.

Pointer provenance is UB in the C/C++ sense, although it's one that largely lurks in the implementation-defined behavior of "integer/pointer casts are implementation-defined." The closest you're going to find to a full specification of pointer provenance is TS 6010 (https://www.open-std.org/jtc1/sc22/WG14/www/docs/n3226.pdf), but it should be noted that no compilers actually implement provenance strictly along the lines of TS 6010. Instead, they implement a poorly-documented, buggy, internally incoherent definition of provenance...

> It may seem nitpicky, but the downside of relying on implementation defined or unspecified behavior is largely boxed and contained.

... that is incoherent precisely because the interactions of provenance with optimizations isn't boxed and contained.

It's really not until people started putting the model into formal semantics that they realized "hey, wait a second, this means either most of our optimizations are wrong or our semantics are wrong" and the consensus is that it was the semantics that were broken.

jcranmer is correct and pointer provenance-related issues are not "boxed and contained". Start here: https://www.ralfj.de/blog/2020/12/14/provenance.html

> why primitive types aren't "frozen" by default.

it kills _a lot_ of optimizations leading to problematic perf. degredation

TL;DR: always freezing I/O buffers => yes no issues (in general); freezing all primitives => perf problem

(at lest in practice in theory many might still be possible but with a way higher analysis compute cost (like exponential higher) and potentially needing more high level information (so bad luck C)).

still for I/O buffers of primitive enough types `frozen` is basically always just fine (I also vaguely remember some discussion about some people more involved into rust core development to probably wanting to add some functionality like that, so it might still happen).

To illustrate why frozen I/O buffers are just fin: Some systems do already anyway always (zero or rand) initialize all their I/O buffers. And a lot of systems reuse I/O buffers, they init them once on startup and then just continuously re-use them. And some OS setups do (zero or rand) initialize all OS memory allocations (through that is for the OS granting more memory to your in process memory allocator, not for every lang specific alloc call, and it doesn't remove UB for stack or register values at all (nor for various stations related to heap values either)).

So doing much more "costly" things then just freezing them is pretty much normal for I/O buffers.

Through as mentioned, sometimes things are not frozen undefined on a hardware level (things like every read might return different values). It's a bit of a niche issue you probably won't run into wrt. I/O buffers and I'm not sure how common it is on modern hardware, but still a thing.

But freezing primitives which majorly affect control flows is both making some optimizations impossible and other much harder to compute/check/find, potentially to a point where it's not viable anymore.

This can involve (as in freezing can prevent) some forms of dead code elimination, some forms of inlining+unrolling+const propagation etc.. This is mostly (but not exclusively) for micro optimizations but micro optimizations which sum up and accumulate leading to (potentially but not always) major performance regressions. Frozen also has some subtle interactions with floats and their different NaN values (can be a problem especially wrt. signaling NaNs).

Through I'm wondering if a different C/C++ where arrays of primitives are always treated as frozen (and no signaling NaNs) would have worked just fine without any noticeable perf. drawback. And if so, if rust should adopt this...