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This is an issue that is ignored by just about everyone in practice. The reality is that most developers have subconsciously internalized the compiler behavior

This is an issue that is ignored by just about everyone in practice. The reality is that most developers have subconsciously internalized the compiler behavior and assume that will always hold. And they are mostly right, I’ve only seen a few cases where this has caused a bug in real systems over my entire career. I try, to the extent possible, to always satisfy the requirements of strict aliasing when writing code. It is difficult to determine if I’ve been successful in this endeavor. Here is why I don’t blame the developers: writing fast, efficient systems code that satisfies the requirements of strict aliasing as defined by C/C++ is surprisingly difficult. It has taken me years to figure out the technically correct incantations for every weird edge case such that they always satisfy the requirements of strict aliasing. The code gymnastics in some cases are entirely unreasonable. In fairness, recent versions of C++ have been adding ways to express each of these cases directly, eliminating the need to use obtuse incantations. But we still have huge old code bases that assume compiler behavior, as was the practice for decades. I am not here to attribute blame, I think it the causes are pretty diffuse honestly. This is just a part of the systems world we failed to do well, and it impacts the code we write every day. I see strict aliasing violations in almost every code base I look at.

082349872349872 2y ago
One of the issues that worked against Euclid's adoption was that its compiler strictly disallowed aliasing. That said, https://dl.acm.org/doi/pdf/10.5555/800078.802513 https://dl.acm.org/doi/pdf/10.5555/800078.802513 claims that while they tended to write potentially-aliased code at first, after one made the Euclid compiler happy, subsequent development wasn't likely to reintroduce it. https://en.wikipedia.org/wiki/Euclid_(programming_language) https://en.wikipedia.org/wiki/Euclid_(programming_language)
- tialaramex 2y ago
  The insight in languages like Rust is that aliasing is actually fine if we can guarantee all the aliases are immutable and that's facilitated by default reference immutability. [These alias related] Bugs only arise when you have mutable aliasing which is why that doesn't exist in safe Rust. That paper also highlights that checking is crucial, their initial Euclid compiler just required that there's no aliasing, but never checked. So of course programmers will make mistakes and without the checks those mistakes leak into running code. The finished compiler checked, which means the mistake won't even compile. Shifting left in this way is huge, WUFFS shifts bounds misses left - when you write code which can have a bounds miss in C of course it just does have a bounds miss at runtime, there's a stray read or overwrite and chaos results maybe it's Remote Code Execution, in Rust the miss panics at runtime - maybe a Denial of Service or at least a major inconvenience. But in WUFFS it won't compile - you find out about your bug likely before it gets sent out for code review. Most software can't be written in WUFFS, but "most" is doing a lot of work there, plenty of code which should be in WUFFS or an analogous language is not, meaning mistakes are not shifted left.
gpderetta 2y ago
Indeed, another problem is that we have no tools, other than very imperfect linters/compiler warnings, to identify aliasing violations. Even today I don't think sanitizers can catch most cases.
- RossBencina 2y ago
  More often than not when I realise that I am violating strict aliasing it is because I am doing something that I want to do and the language is not going to let me. Much hand wringing, language lawyering and time wasting typically follows.
  - atq2119 2y ago
    That is understood. However, it is generally too hard (in C and C++) for compilers to tell whether you were wanting to do the thing at any one particular place. So compilers have two options: Assume that you never do the thing, or always assume that you don't do the thing. The former is often better for performance in practice, and it's true most of the time, so here we are. As has been pointed out elsewhere, one of the strengths of Rust is that it shifts how pointers (references) work and allows the compiler to more often know for certain that you don't do the thing, without making assumptions.
Arch-TK 2y ago
> The reality is that most developers have subconsciously internalized the compiler behavior and assume that will always hold. I blame this on how people like to teach C and present C. It's very important that the second anyone conceives of the idea of learning C that they first off informed that trying things and seeing what happens is a highly unreliable method of learning how C programs behave and that C is not a high level assembly language. If you teach C in relation to the abstract machine instead of any real world machine you will understandably scare off most people. Which is good, since most people shouldn't be learning or writing C. It's a language which can barely be written correctly even by people with the necessary self discipline to only write code they're 100% certain is well defined. > It is difficult to determine if I’ve been successful in this endeavor. Why is your program so full of casts between pointer types that you have difficulty determining if you've avoided strict aliasing? Yes, if you treat C as a high level assembly language (like the linux kernel likes to do) then it becomes difficult to reason about the behaviour of your programs where 50% of them are in the grey area of uncertainty of whether they're well defined or not. If you are forced to write C in a non-learning context, don't write any line of code unless you're certain you could tell someone which parts of the standard describe its behaviour. > Here is why I don’t blame the developers: writing fast, efficient systems code that satisfies the requirements of strict aliasing as defined by C/C++ is surprisingly difficult. C/C++ isn't a language. So I will stick to C because I don't know nor care about C++. That being said, it's not hard to write efficient C which satisfies the requirements of strict aliasing except when you're dealing with idiotic APIs like bind or connect. Most code by default, assuming you use appropriate algorithms and data structures, is performant. The only time it becomes difficult with regards to strict aliasing is if you're micro optimizing. While non-trivial, the case of converting between unsigned long and float shown in the article is entirely possible to do with completely safe C constructs. Likewise serialization/deserialization of binary data never requires coming close to aliasing unless you're dealing with a "native" endian protocol. In the case of general serialisation and deserialisation, compilers will reliably optimise such operations into one or two instructions (depending on whether you're decoding same-endianness or not).
- gpderetta 2y ago
  Well, it is more complicated than that. First of all compilers disagree on many interpretations and consequences of abstract machine rules. Also compilers have bugs. So a proficient C/C++ programmer does have to learn what compilers actually do in practice and what they guarantee beyond the standard (or how they differ from it). > C/C++ isn't a language. It isn't, but it is a family of languages that share a lot of syntax and semantics.
spacechild1 2y ago
> In fairness, recent versions of C++ have been adding ways to express each of these cases directly, eliminating the need to use obtuse incantations. In particular, C++20 gave us std::bit_cast (https://en.cppreference.com/w/cpp/numeric/bit_cast https://en.cppreference.com/w/cpp/numeric/bit_cast) for type punning and C++23 added std::start_life_time_as (https://en.cppreference.com/w/cpp/memory/start_lifetime_as https://en.cppreference.com/w/cpp/memory/start_lifetime_as) for interpreting raw bytes as an object.
- helltone 2y ago
  std::start_life_time_as and std::launder honestly feel like a massive hack

082349872349872 2y ago

One of the issues that worked against Euclid's adoption was that its compiler strictly disallowed aliasing. That said, https://dl.acm.org/doi/pdf/10.5555/800078.802513 https://dl.acm.org/doi/pdf/10.5555/800078.802513 claims that while they tended to write potentially-aliased code at first, after one made the Euclid compiler happy, subsequent development wasn't likely to reintroduce it. https://en.wikipedia.org/wiki/Euclid_(programming_language) https://en.wikipedia.org/wiki/Euclid_(programming_language)

tialaramex 2y ago
The insight in languages like Rust is that aliasing is actually fine if we can guarantee all the aliases are immutable and that's facilitated by default reference immutability. [These alias related] Bugs only arise when you have mutable aliasing which is why that doesn't exist in safe Rust. That paper also highlights that checking is crucial, their initial Euclid compiler just required that there's no aliasing, but never checked. So of course programmers will make mistakes and without the checks those mistakes leak into running code. The finished compiler checked, which means the mistake won't even compile. Shifting left in this way is huge, WUFFS shifts bounds misses left - when you write code which can have a bounds miss in C of course it just does have a bounds miss at runtime, there's a stray read or overwrite and chaos results maybe it's Remote Code Execution, in Rust the miss panics at runtime - maybe a Denial of Service or at least a major inconvenience. But in WUFFS it won't compile - you find out about your bug likely before it gets sent out for code review. Most software can't be written in WUFFS, but "most" is doing a lot of work there, plenty of code which should be in WUFFS or an analogous language is not, meaning mistakes are not shifted left.

gpderetta 2y ago

Indeed, another problem is that we have no tools, other than very imperfect linters/compiler warnings, to identify aliasing violations. Even today I don't think sanitizers can catch most cases.

RossBencina 2y ago
More often than not when I realise that I am violating strict aliasing it is because I am doing something that I want to do and the language is not going to let me. Much hand wringing, language lawyering and time wasting typically follows.
- atq2119 2y ago
  That is understood. However, it is generally too hard (in C and C++) for compilers to tell whether you were wanting to do the thing at any one particular place. So compilers have two options: Assume that you never do the thing, or always assume that you don't do the thing. The former is often better for performance in practice, and it's true most of the time, so here we are. As has been pointed out elsewhere, one of the strengths of Rust is that it shifts how pointers (references) work and allows the compiler to more often know for certain that you don't do the thing, without making assumptions.

Arch-TK 2y ago

> The reality is that most developers have subconsciously internalized the compiler behavior and assume that will always hold. I blame this on how people like to teach C and present C. It's very important that the second anyone conceives of the idea of learning C that they first off informed that trying things and seeing what happens is a highly unreliable method of learning how C programs behave and that C is not a high level assembly language. If you teach C in relation to the abstract machine instead of any real world machine you will understandably scare off most people. Which is good, since most people shouldn't be learning or writing C. It's a language which can barely be written correctly even by people with the necessary self discipline to only write code they're 100% certain is well defined. > It is difficult to determine if I’ve been successful in this endeavor. Why is your program so full of casts between pointer types that you have difficulty determining if you've avoided strict aliasing? Yes, if you treat C as a high level assembly language (like the linux kernel likes to do) then it becomes difficult to reason about the behaviour of your programs where 50% of them are in the grey area of uncertainty of whether they're well defined or not. If you are forced to write C in a non-learning context, don't write any line of code unless you're certain you could tell someone which parts of the standard describe its behaviour. > Here is why I don’t blame the developers: writing fast, efficient systems code that satisfies the requirements of strict aliasing as defined by C/C++ is surprisingly difficult. C/C++ isn't a language. So I will stick to C because I don't know nor care about C++. That being said, it's not hard to write efficient C which satisfies the requirements of strict aliasing except when you're dealing with idiotic APIs like bind or connect. Most code by default, assuming you use appropriate algorithms and data structures, is performant. The only time it becomes difficult with regards to strict aliasing is if you're micro optimizing. While non-trivial, the case of converting between unsigned long and float shown in the article is entirely possible to do with completely safe C constructs. Likewise serialization/deserialization of binary data never requires coming close to aliasing unless you're dealing with a "native" endian protocol. In the case of general serialisation and deserialisation, compilers will reliably optimise such operations into one or two instructions (depending on whether you're decoding same-endianness or not).

gpderetta 2y ago
Well, it is more complicated than that. First of all compilers disagree on many interpretations and consequences of abstract machine rules. Also compilers have bugs. So a proficient C/C++ programmer does have to learn what compilers actually do in practice and what they guarantee beyond the standard (or how they differ from it). > C/C++ isn't a language. It isn't, but it is a family of languages that share a lot of syntax and semantics.

spacechild1 2y ago

> In fairness, recent versions of C++ have been adding ways to express each of these cases directly, eliminating the need to use obtuse incantations. In particular, C++20 gave us std::bit_cast (https://en.cppreference.com/w/cpp/numeric/bit_cast https://en.cppreference.com/w/cpp/numeric/bit_cast) for type punning and C++23 added std::start_life_time_as (https://en.cppreference.com/w/cpp/memory/start_lifetime_as https://en.cppreference.com/w/cpp/memory/start_lifetime_as) for interpreting raw bytes as an object.

helltone 2y ago
std::start_life_time_as and std::launder honestly feel like a massive hack