6 ms·

And this half of my main beef with the language. To be clear, I absolutely agree that this aspect of it is wonderful, but it is a double-edged sword, and the ed

And this half of my main beef with the language. To be clear, I absolutely agree that this aspect of it is wonderful, but it is a double-edged sword, and the edge pointing at the programmer's face is very sharp. The second half of that beef is that most C++ learning journeys bring the learner up to the point of creativity without addressing concepts like Strict Aliasing which are not useful until you start getting "creative". That's big a problem because code containing strict aliasing violations (and/or any of a myriad of other UB) is often fundamentally broken while giving the illusion of behaving as intended.

synergy20 4y ago
or use -fno-strict-aliasing and profile the code to lift it for bottleneck code carefully? linux kernel does not allow strict-aliasing for example. but I agree yes all c and c++ course should teach strict aliasing. in modern c++, also teach RAII and unique_ptr, combined they can eliminate so many memory safety problems.
- Chabsff 4y ago
  C++ without strict aliasing is not C++ anymore. Furthermore, that was just an example. There are other such concepts, such as the illegality of accessing memory as anything else than raw bytes or the type of the object that resides there (which also applies to union fields). Another one would be the fact that signed integer overflow is UB. C++ is so tightly bound to memory layouts that once you start getting a solid mental model of what's going on, it's really easy to come up with legitimately neat and creative ideas that work very well and fast on a test bench environment without any complaint from modern compilers, but are fundamentally broken. I feel we do not do a good job at equipping up-and-coming programmers so that they can benefit from that mental model without writing code that only works out of sheer luck. In the extreme, you can write code that does stuff like dynamically retype an object by swapping out its vtable when vtables are not even thing as far as the language is concerned. It's astounding what it sometimes feels like you are capable of doing in C++.
  - synergy20 4y ago
    I understand compiler likes strict aliasing for performance optimizations, use it carefully but _only_ at bottleneck-ed areas seems like a good trade-off between safety and performance, why without-it is no c++ anymore? why do I need do pointer aliasing randomly?
    - Chabsff 4y ago
      Strict aliasing is what prevents compilers from having to sprinkle Load-Hit-Stores all over the place. This is not a marginal performance difference, and doubly-so for CPUs with deep execution pipelines. It also has massive code-reordering implications. In any case, Strict Aliasing is not a flippant rule that compilers like to apply. It's a formal component of the ISO standard that C++ is. You can chose to make a deal with a compiler (via a flag) to compile otherwise non-compliant code in a consistent way, but that does not change the fact that the code is non-compliant.
      - kazinator 4y ago
        There are ways of coding coding in C which makes those optimizations unnecessary. The basic idea is: don't load your block of code with lots of pointer dereferences. Load the values you need into local variables. Don't proliferate common expressions which dereference the same pointer to get at the same value. Consolidate the assignments through pointers. Don't do this in three places: (*ptr)++. Have that value in a local variable var, and do var++ in three places, then assign it *ptr = var. Even without no-strict aliasing, a compiler can still assume that local variables whose addresses are not taken are not the targets of any pointers. In C (99 or later), you have restrict also. restrict is independent of strict aliasing, because it's not based on type. Furthermore, speaking of restrict, aliasing between like typed objects matters for optimization. It's good and well to optimize based on the idea that a double * cannot be aiming at an object whose declared type is long. But it's insufficient, because code that is manipulating double * pointers is likely working with objects of type double which could be the targets of those pointers. So without some combination of tight coding and possibly using restrict, you will end up with those load-hit-stores in all sorts of code. This code for removing from a linked list: node->prev->next = node->next; node->next->prev = node->prev; still isn't as good as this: node *next = node->next; node *prev = node->prev; next->prev = prev; prev->next = next; The problem is that the common expressions can't be eliminated based on aliasing, and the aliasing is between like types, so strict aliasing doesn't help. With gcc (x86) I get: movl (%eax), %edx movl 4(%eax), %ecx movl %ecx, 4(%edx) movl 4(%eax), %eax movl %eax, (%eax) versus: movl (%eax), %edx movl 4(%eax), %eax movl %eax, 4(%edx) movl %edx, (%eax) We shaved off an instruction through tighter coding, and IMHO (in this case) improved the readability also. That was gcc 7 on Ubuntu. The result is exactly the same like what I saw under gcc 2.7.x a quarter century ago. How about gcc 11, x86-64? I should probably be using godbolt, but anyway, also five instructions down to four: movq (%rdi), %rax movq 8(%rdi), %rdx movq %rdx, 8(%rax) movq 8(%rdi), %rax movq %rax, (%rax) vs: movq 8(%rdi), %rax movq (%rdi), %rdx movq %rax, 8(%rdx) movq %rdx, (%rax) (I think in this particular case the compiler could do a better job because even if the assignment "node->prev->next = node->next" clobbers the value of "node->next" due to the nodes being aliases, the assignment can only clobber it with the value that node->next already has! The compiler doesn't analyze it that far though.) C was designed from the start as a language which the programmer does the optimizing, and regardless of the advancements in compilers, that has not been entirely eliminated. How you write C still makes a difference, even at the microscopic level of individual statements and expressions, not just the level of overall program organization and use of algorithms. If you write tight code, you can turn off strict aliasing optimizations globally and it won't matter. But you don't have to do that globally. You may be able to confine your type punning hack in its own source file, and just turn it off for that file. (Or may be even on a finer granularity if you have such compiler support.)
rramadass 4y ago
https://blog.sigplan.org/2021/11/18/undefined-behavior-deserves-a-better-reputation/ https://blog.sigplan.org/2021/11/18/undefined-behavior-deser...
- Chabsff 4y ago
  Just in case you posted this as a counterpoint to what I'm saying... This blog effectively agrees with my position: Undefined Behavior is just "Stuff the compiler is allowed to assume never happens". And C++'s set of Undefined Behaviour is a fundamental part of what makes it valuable. The issue I have is not with them, but rather with the fact that it's too easy to become competent, if not talented, way past the point where one should take them into account without having to even know of their existence.
  - rramadass 4y ago
    Discussion here: https://news.ycombinator.com/item?id=32908724 https://news.ycombinator.com/item?id=32908724

synergy20 4y ago

or use -fno-strict-aliasing and profile the code to lift it for bottleneck code carefully? linux kernel does not allow strict-aliasing for example. but I agree yes all c and c++ course should teach strict aliasing. in modern c++, also teach RAII and unique_ptr, combined they can eliminate so many memory safety problems.

Chabsff 4y ago
C++ without strict aliasing is not C++ anymore. Furthermore, that was just an example. There are other such concepts, such as the illegality of accessing memory as anything else than raw bytes or the type of the object that resides there (which also applies to union fields). Another one would be the fact that signed integer overflow is UB. C++ is so tightly bound to memory layouts that once you start getting a solid mental model of what's going on, it's really easy to come up with legitimately neat and creative ideas that work very well and fast on a test bench environment without any complaint from modern compilers, but are fundamentally broken. I feel we do not do a good job at equipping up-and-coming programmers so that they can benefit from that mental model without writing code that only works out of sheer luck. In the extreme, you can write code that does stuff like dynamically retype an object by swapping out its vtable when vtables are not even thing as far as the language is concerned. It's astounding what it sometimes feels like you are capable of doing in C++.
- synergy20 4y ago
  I understand compiler likes strict aliasing for performance optimizations, use it carefully but _only_ at bottleneck-ed areas seems like a good trade-off between safety and performance, why without-it is no c++ anymore? why do I need do pointer aliasing randomly?
  - Chabsff 4y ago
    Strict aliasing is what prevents compilers from having to sprinkle Load-Hit-Stores all over the place. This is not a marginal performance difference, and doubly-so for CPUs with deep execution pipelines. It also has massive code-reordering implications. In any case, Strict Aliasing is not a flippant rule that compilers like to apply. It's a formal component of the ISO standard that C++ is. You can chose to make a deal with a compiler (via a flag) to compile otherwise non-compliant code in a consistent way, but that does not change the fact that the code is non-compliant.
    - kazinator 4y ago
      There are ways of coding coding in C which makes those optimizations unnecessary. The basic idea is: don't load your block of code with lots of pointer dereferences. Load the values you need into local variables. Don't proliferate common expressions which dereference the same pointer to get at the same value. Consolidate the assignments through pointers. Don't do this in three places: (*ptr)++. Have that value in a local variable var, and do var++ in three places, then assign it *ptr = var. Even without no-strict aliasing, a compiler can still assume that local variables whose addresses are not taken are not the targets of any pointers. In C (99 or later), you have restrict also. restrict is independent of strict aliasing, because it's not based on type. Furthermore, speaking of restrict, aliasing between like typed objects matters for optimization. It's good and well to optimize based on the idea that a double * cannot be aiming at an object whose declared type is long. But it's insufficient, because code that is manipulating double * pointers is likely working with objects of type double which could be the targets of those pointers. So without some combination of tight coding and possibly using restrict, you will end up with those load-hit-stores in all sorts of code. This code for removing from a linked list: node->prev->next = node->next; node->next->prev = node->prev; still isn't as good as this: node *next = node->next; node *prev = node->prev; next->prev = prev; prev->next = next; The problem is that the common expressions can't be eliminated based on aliasing, and the aliasing is between like types, so strict aliasing doesn't help. With gcc (x86) I get: movl (%eax), %edx movl 4(%eax), %ecx movl %ecx, 4(%edx) movl 4(%eax), %eax movl %eax, (%eax) versus: movl (%eax), %edx movl 4(%eax), %eax movl %eax, 4(%edx) movl %edx, (%eax) We shaved off an instruction through tighter coding, and IMHO (in this case) improved the readability also. That was gcc 7 on Ubuntu. The result is exactly the same like what I saw under gcc 2.7.x a quarter century ago. How about gcc 11, x86-64? I should probably be using godbolt, but anyway, also five instructions down to four: movq (%rdi), %rax movq 8(%rdi), %rdx movq %rdx, 8(%rax) movq 8(%rdi), %rax movq %rax, (%rax) vs: movq 8(%rdi), %rax movq (%rdi), %rdx movq %rax, 8(%rdx) movq %rdx, (%rax) (I think in this particular case the compiler could do a better job because even if the assignment "node->prev->next = node->next" clobbers the value of "node->next" due to the nodes being aliases, the assignment can only clobber it with the value that node->next already has! The compiler doesn't analyze it that far though.) C was designed from the start as a language which the programmer does the optimizing, and regardless of the advancements in compilers, that has not been entirely eliminated. How you write C still makes a difference, even at the microscopic level of individual statements and expressions, not just the level of overall program organization and use of algorithms. If you write tight code, you can turn off strict aliasing optimizations globally and it won't matter. But you don't have to do that globally. You may be able to confine your type punning hack in its own source file, and just turn it off for that file. (Or may be even on a finer granularity if you have such compiler support.)

rramadass 4y ago

https://blog.sigplan.org/2021/11/18/undefined-behavior-deserves-a-better-reputation/ https://blog.sigplan.org/2021/11/18/undefined-behavior-deser...

Chabsff 4y ago
Just in case you posted this as a counterpoint to what I'm saying... This blog effectively agrees with my position: Undefined Behavior is just "Stuff the compiler is allowed to assume never happens". And C++'s set of Undefined Behaviour is a fundamental part of what makes it valuable. The issue I have is not with them, but rather with the fact that it's too easy to become competent, if not talented, way past the point where one should take them into account without having to even know of their existence.
- rramadass 4y ago
  Discussion here: https://news.ycombinator.com/item?id=32908724 https://news.ycombinator.com/item?id=32908724