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I have heard this reaction to this article a lot, but sorry, there is nothing in the C standard that says that objects should not overlap, except a rule that on

I have heard this reaction to this article a lot, but sorry, there is nothing in the C standard that says that objects should not overlap, except a rule that only apply to “lvalue = lvalue;” assignments and is not relevant here. Plus on a some 32-bit ISA, a long long and a double only need to be aligned to 32-bit boundaries, so I note that in the made-up C rules that you are referring to, “basic type” is not very well defined. > I believe this behavior is actually specified in the standard, actually, > in the same section that defines the aliasing rules. The strict aliasing rules are here: https://port70.net/~nsz/c/c11/n1570.html#6.5p7 https://port70.net/~nsz/c/c11/n1570.html#6.5p7 Go ahead and point to the rule that says that “basic types” cannot overlap with themselves.

ajross 6y ago
I genuinely can't follow you completely, but half suspect you're violently agreeing with me. Are you saying the optimization in the linked article is, or is not, in violation of the standard? Edit: this is the text I was remembering, from 6.5.16.1 ("Simple Assignment"): "If the value being stored in an object is read from another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have qualified or unqualified versions of a compatible type; otherwise, the behavior is undefined.". That pretty much matches exactly what I was saying: compilers are free to assume that basic types don't overlap, because if they do then any generated code will be undefined behavior anyway.
- pascal_cuoq 6y ago
  6.5.16.1 is the “rule that only apply to “lvalue = lvalue;” assignments and is not relevant here” It does not apply to “lvalue = 1;” or to “lvalue = 2;”, which are the two relevant assignments in the example in the article. For context, I think I made it clear in the article that the program being discussed is UB, and therefore that the compiler is not to blame. But since I wrote this article, I have had people telling me “The complaint isn't about alignment at all, it's that the optimizer assumes that two pointers to the same basic type cannot overlap in memory”. My reply to this specific sentence is: No. You are wrong. There are no words in the standard that say that “basic types cannot overlap in memory”. There is not even a notion of “basic type”. There are clauses about pointer alignment, that are explicitly cited in the article, and there are clauses about strict aliasing, that are shown in the article not to be the reason for GCC optimizing the program by using -fno-strict-aliasing. There are no rules about “basic types not overlapping” in the C standard. You only think there are. Or please cite them. (6.5.16.1 is a rule about assignment, it only applies for the code pattern lvalue1 = lvalue2;)
  - ajross 6y ago
    I'm sorry, can you explain how that's not relevant here? You're being incongruously combative, but I still think you're mostly agreeing with me. The section on "simple" assignments doesn't say that the rvalue must be an lvalue expression syntactically . I think it applies very well to "*p = 1;", which is the statement in the linked code. What am I missing? > There are no words in the standard that say that “basic types cannot overlap in memory”. I don't believe I said there were. I said the standard expressly allowed the optimization in the linked article. And as far as I can see, absent a clearer explanation for why that section doesn't apply, it does.
    - pascal_cuoq 6y ago
      1 is not an object, but even if it was one, it would not be an object that overlaps with “* p”. You are interpreting the C standard as if it were a philosophy text. It contains a rule that says that in very precise circumstances (for an assignment from one to the other) objects must not partially overlap, and you are claiming that it means that “two pointers to the same basic type cannot overlap in memory”. The clause does not say that, sorry. The clause applies to the objects that are on one side and the other of an assignment. > I said the standard expressly allowed the optimization in the linked article. I hope that the article makes it clear that the standard expressly allows the optimization. Specific, explicit rules, cited in the article, about pointer alignment, allow the optimization. For this reason, I, “violently” as you say, disagree with the sentence “The complaint isn't about alignment at all, it's that the optimizer assumes that two pointers to the same basic type cannot overlap in memory”. This sentence gets it all wrong. It is about alignment; it is not about “pointers to basic types”, whatever that is, not being allowed to overlap in memory; they are allowed to overlap for large enough “basic types” because it is about alignment, not overlap: https://gcc.godbolt.org/z/ZAMkeH https://gcc.godbolt.org/z/ZAMkeH You could argue that GCC 9.3 only missed the optimization in the example in this Compiler Explorer link for some other reason and that absence of optimization doesn't mean that p and q cannot overlap. This would be correct, this aspect is one of the difficulties in studying the rules that these compilers implement. However, what I am saying is that if you reported this missed optimization to GCC developers, they would tell you that GCC can't optimize the function f because p and q can overlap. There is no clause in the C standard that prevent them to (apart from strict aliasing rules, but I used the option to tell the compiler I didn't want it to take advantage of these ones). (Please do not bother them with this, or if you do, at least leave me out of it; I have nothing better to do than to write this because it's the week-end but they have better things to do.)

ajross 6y ago

I genuinely can't follow you completely, but half suspect you're violently agreeing with me. Are you saying the optimization in the linked article is, or is not, in violation of the standard? Edit: this is the text I was remembering, from 6.5.16.1 ("Simple Assignment"): "If the value being stored in an object is read from another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have qualified or unqualified versions of a compatible type; otherwise, the behavior is undefined.". That pretty much matches exactly what I was saying: compilers are free to assume that basic types don't overlap, because if they do then any generated code will be undefined behavior anyway.

pascal_cuoq 6y ago
6.5.16.1 is the “rule that only apply to “lvalue = lvalue;” assignments and is not relevant here” It does not apply to “lvalue = 1;” or to “lvalue = 2;”, which are the two relevant assignments in the example in the article. For context, I think I made it clear in the article that the program being discussed is UB, and therefore that the compiler is not to blame. But since I wrote this article, I have had people telling me “The complaint isn't about alignment at all, it's that the optimizer assumes that two pointers to the same basic type cannot overlap in memory”. My reply to this specific sentence is: No. You are wrong. There are no words in the standard that say that “basic types cannot overlap in memory”. There is not even a notion of “basic type”. There are clauses about pointer alignment, that are explicitly cited in the article, and there are clauses about strict aliasing, that are shown in the article not to be the reason for GCC optimizing the program by using -fno-strict-aliasing. There are no rules about “basic types not overlapping” in the C standard. You only think there are. Or please cite them. (6.5.16.1 is a rule about assignment, it only applies for the code pattern lvalue1 = lvalue2;)
- ajross 6y ago
  I'm sorry, can you explain how that's not relevant here? You're being incongruously combative, but I still think you're mostly agreeing with me. The section on "simple" assignments doesn't say that the rvalue must be an lvalue expression syntactically . I think it applies very well to "*p = 1;", which is the statement in the linked code. What am I missing? > There are no words in the standard that say that “basic types cannot overlap in memory”. I don't believe I said there were. I said the standard expressly allowed the optimization in the linked article. And as far as I can see, absent a clearer explanation for why that section doesn't apply, it does.
  - pascal_cuoq 6y ago
    1 is not an object, but even if it was one, it would not be an object that overlaps with “* p”. You are interpreting the C standard as if it were a philosophy text. It contains a rule that says that in very precise circumstances (for an assignment from one to the other) objects must not partially overlap, and you are claiming that it means that “two pointers to the same basic type cannot overlap in memory”. The clause does not say that, sorry. The clause applies to the objects that are on one side and the other of an assignment. > I said the standard expressly allowed the optimization in the linked article. I hope that the article makes it clear that the standard expressly allows the optimization. Specific, explicit rules, cited in the article, about pointer alignment, allow the optimization. For this reason, I, “violently” as you say, disagree with the sentence “The complaint isn't about alignment at all, it's that the optimizer assumes that two pointers to the same basic type cannot overlap in memory”. This sentence gets it all wrong. It is about alignment; it is not about “pointers to basic types”, whatever that is, not being allowed to overlap in memory; they are allowed to overlap for large enough “basic types” because it is about alignment, not overlap: https://gcc.godbolt.org/z/ZAMkeH https://gcc.godbolt.org/z/ZAMkeH You could argue that GCC 9.3 only missed the optimization in the example in this Compiler Explorer link for some other reason and that absence of optimization doesn't mean that p and q cannot overlap. This would be correct, this aspect is one of the difficulties in studying the rules that these compilers implement. However, what I am saying is that if you reported this missed optimization to GCC developers, they would tell you that GCC can't optimize the function f because p and q can overlap. There is no clause in the C standard that prevent them to (apart from strict aliasing rules, but I used the option to tell the compiler I didn't want it to take advantage of these ones). (Please do not bother them with this, or if you do, at least leave me out of it; I have nothing better to do than to write this because it's the week-end but they have better things to do.)