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Looking at the C code from the Fast Inverse Square Root, the casting of a float to a long is done via pointer arithmetic:

i  = * ( long * ) &y;  // evil floating point bit level hacking

The hacking in question isn't of immediate interest to me. The method, using pointer arithmetic to access the memory values of one type and assign them to another is where my question lies. Later in the article they note that this behavior is now proscribed in the C standard, but which standard is not specified.

We can see from this StackOverflow answer that in C++, "dereferencing a pointer that aliases an object that is not of a compatible type or one of the other types allowed by C 2011 6.5 paragraph 7 is undefined behavior." (internal footnotes and links omitted) That's C++, but this answer (on the same issue, the strict aliasing rule) in C notes that a way around the problem of producing undefined behavior appeared with unions in C99.

From K&R C (2nd edition), page 103 we have:

“The valid pointer operations are assignments of pointers to the same time, adding or subtracting a pointer and an integer, subtracting or comparing two pointers to members of the same array, and assigning or comparing to zero. All other pointer arithmetic is illegal. It is not legal to add two pointers, or to multiple or divide or shift or mask them, or to add float or double to them, or even, except for void*, to assign a pointer of one type to a pointer of another type without a cast.”

This seems pretty explicitly to disallow the trick above. However I don't know what force this statement had in terms of code in the wild. Worth noting that Ritchie, writing in 1993, remarks "Compilers in 1977, and even well after, did not complain about usages such as assigning between integers and pointers or using objects of the wrong type to refer to structure members. Although the language definition presented in the first edition of K&R was reasonably (though not completely) coherent in its treatment of type rules, that book admitted that existing compilers didn't enforce them."

I recognize that this behavior has always been undefined. What I am trying to figure out is when and where is became specifically disallowed, meaning:

  1. When did we start modifying compilers to emit errors or warnings for this behavior? GCC will still compile the Quake FISR with arguments -O0
  2. Was this behavior identified in the c99 standard? in the c90 standard?
  3. Apart from reading the standard, how might a C programmer learn that this was behavior to be avoided and not the good kind of cleverness? K&R C strikes me as a good example, are there others?

That's three sub-questions, so I apologize for that. I'm looking for information on this pattern and so any help will be useful.

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2 Answers 2

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As mentioned here for instance, the aliasing rule was present in the first ANSI C standard (C89).

The second edition of K&R is based on ANSI C. The first edition doesn't seem to discuss the issue, but it also seems to contain no code that violates the ANSI C aliasing rule. The only examples of access through multiple types involve alloc/calloc and read/write, and the other type in all cases is char. (I didn't read the whole book, but I looked in the index and searched for *) among other things.)

I think it was never explicitly allowed, and I disagree with Justme's comment suggesting that it's by-the-book K&R C. But was never exactly forbidden either; it was a nonissue since sophisticated optimizing compilers didn't exist.

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  • 5
    it was a nonissue since sophisticated optimizing compilers didn't exist. Precisely. A plausible answer to the OP is "When compilers started to be really optimizing, and those hacks became tedious to handle for the compiler writers". Regarding the original hack, the guy was either clueless or too lazy to write it the right way, using a union to start with.
    – Leo B.
    Jan 31 at 2:47
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    If people had known how today's compilers would interpret the "Strict Aliasing Rule", the Standard would have been soundly rejected, or at least recognized by programmers as not being a valid description of the language the Committee was chartered to describe.
    – supercat
    Jan 31 at 3:20
  • Because nothing in C89 suggests that an assignment could cause the lifetime of an object to begin or end, the aliasing rules were widely interpreted prior to C99 as applying only to objects with lifetimes that were established by the rules of the language, and compilers were expected to process function calls between translation units according to a platform's calling conventions and semantics, rather than those of the C language.
    – supercat
    Jan 31 at 17:58
  • 2
    @KarlKnechtel: The Standard states that UB occurs as a result of "non-portable or erroneous" constructs. The authors never imagined that anyone would take seriously the notion that such a phrase means "non-portable, and therefore erroneous".
    – supercat
    Jan 31 at 19:52
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    @supercat Your comments suggest there is some easy way to fix this problem (just stop interpreting the standard in a stupid way), but I'm not seeing how. Dealing with platform differences in the encodings of float and long is the easy part, you just need a nightmarish thicket of #ifdefs. The hard part is (assuming sizeof(x) == sizeof(y) and proper alignment and no traps) how can the standard, or even an implementation, guarantee x = *(X *)&y is equivalent to memcpy(&x, &y, sizeof(x)) without precluding important optimizations?
    – benrg
    Feb 1 at 0:34
3

When the C89 Standard was ratified, programmers and compiler writers alike recognized that it didn't partition constructs into categories that were "allowed" and "forbidden", but instead those over which it for which it would mandate support and those over which it would waive jurisdiction.

Type punning constructs were allowed in most dialects of the language the Standard was chartered to describe. The authors of the Standard, however, did not want to forbid a compiler from optimizing code like

int x;
int test(double *p)
{
    x=1;
    *p = 1.0;
    return x;
}

by consolidating the load of x with the preceding store. They recognized that there would be situations where this would be incorrect (and they used that term in the published Rationale), but there was no need to forbid compilers from performing such optimizations in cases where they wouldn't affect practical programs.

Note that the C89 Standard was never intended to imply that compiler writers should be willfully blind to situations where such optimizations would be likely to affect program behavior. Judging from the published Rationale, it would have been sufficiently mind numbingly obvious to the Committee that only an obtusely designed compiler would look at something like:

unsigned get_float_bits(float *fp)
{ return *(unsigned*)fp; }

and decide to assume that such a function could never actually access an object of type float whose address was passed to it in a pointer of type float*, that there was no need to try to forbid implementations from doing such silly things.

A key thing to understand about the Standard is that there are two categories of conformance for programs; while the Standard does not allow type punning in Strictly Conforming C Programs, it does not forbid type punning in Conforming C Programs. If the Standard had forbidden type punning in Conforming C Programs, it would have been soundly rejected.

Further, there has never been a consensus understanding as to what the type aliasing rules mean. In the absence of the aliasing rules, any region of storage which could be accessed by a program could be viewed as simultaneously holding every kind of object that could fit therein, with the value of all of those objects being encapsulated within the contents of those bits. An action which modified the value of value would change the bits to match, and changes to the bits would change the value represented by an object.

Under that view, however, almost every action that accessed a region of storage would simultaneously access the stored value of objects of every type that would fit, meaning that all non-character-type accesses to anything would violate the constraint. This could be resolved by interpreting the rule as saying:

An object which is accessed within a particular context must not have its stored value accessed in conflicting fashion within that same context by any lvalue which isn't visibly associated with an lvalue of one of the following types...

The C99 Effective Type rules were supposed to clarify things, but they instead gave the false impression that the Standard was intended to specify everything programmers should need to accomplish the things they would need to do, and that the Standard was intended to forbid actions over which it waived jurisdiction.

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