4 ms·

I agree, I'd go further and say I wonder why primitive types aren't "frozen" by default. I totally understand not wanting to promise things get zeroed, but I d

I agree, I'd go further and say I wonder why primitive types aren't "frozen" by default. I totally understand not wanting to promise things get zeroed, but I don't really understand why full UB, instead of just "they have whatever value is initially in memory / the register / the compiler chose" is so much better. Has anyone ever done a performance comparison between UB and freezing I wonder? I can't find one.

wrs 1y ago
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.
- CJefferson 1y ago
  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?
  - ablob 1y ago
    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.
- dathinab 1y ago
  > 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)
- dooglius 1y ago
  > 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?
  - wrs 1y ago
    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).
  - wrs 1y ago
    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.)
vgatherps 1y ago
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
- adastra22 1y ago
  Anything being UB is insane to me...
  - IshKebab 1y ago
    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.
    - dathinab 1y ago
      > 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
  - dathinab 1y ago
    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)
dathinab 1y ago
> 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...

wrs 1y ago

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.

CJefferson 1y ago
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?
- ablob 1y ago
  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.
dathinab 1y ago
> 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)
dooglius 1y ago
> 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?
- wrs 1y ago
  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).
- wrs 1y ago
  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.)

vgatherps 1y ago

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

adastra22 1y ago
Anything being UB is insane to me...
- IshKebab 1y ago
  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.
  - dathinab 1y ago
    > 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
- dathinab 1y ago
  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)

dathinab 1y ago

> 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...