Why does the compiler assume that these seemingly equal pointers differ?

Question

Looks like GCC with some optimization thinks two pointers from different translation units can never be same even if they are actually the same.

Code:

main.c

#include <stdint.h>
#include <stdio.h>

int a __attribute__((section("test")));
extern int b;

void check(int cond) { puts(cond ? "TRUE" : "FALSE"); }

int main() {
    int * p = &a + 1;
    check(
        (p == &b)
        ==
        ((uintptr_t)p == (uintptr_t)&b)
    );
    check(p == &b);
    check((uintptr_t)p == (uintptr_t)&b);
    return 0;
}

b.c

int b __attribute__((section("test")));

If I compile it with -O0, it prints

TRUE
TRUE
TRUE

But with -O1

FALSE
FALSE
TRUE

So p and &b are actually the same value, but the compiler optimized out their comparison assuming they can never be equal.

I can't figure out, which optimization made this.

It doesn't look like strict aliasing, because pointers are of one type, and -fstrict-aliasing option doesn't make this effect.

Is this the documented behavour? Or is this a bug?

Answer 1

There are three aspects in your code which result in general problems:

1. Conversion of a pointer to an integer is implementation defined. There is no guarantee conversion of two pointers to have all bits identical.

2. uintptr_t is guaranteed to convert from a pointer to the same type then back unchanged (i.e. compare equal to the original pointer). But nothing more. The integer values themselves are not guaranteed to compare equal. E.g. there could be unused bits with arbitrary value. See the standard, 7.20.1.4.

3. And (briefly) two pointers can only compare equal if they point into the same array or right behind it (last entry plus one) or at least one is a null pointer. For any other constellation, they compare unequal. For the exact details, see the standard, 6.5.9p6.

Finally, there is no guarantee how variables are placed in memory by the toolchain (typically the linker for static variables, the compiler for automatic variables). Only an array or a struct (i.e. composite types) guarantee the ordering of its elements.

For your example, 6.5.9p7 also applies. It basically treats a pointer to a non-array object for comparision like on to the first entry of an array of size 1. This does not cover an incremented pointer past the object like &a + 1. Relevant is the object the pointer is based on. That is object a for pointer p and b for pointer &b. The rest can be found in paragraph 6.

None of your variables is an array (last part of paragraph 6), so the pointers need not compare equal, even for &a + 1 == &b. The last "TRUE" might arise from gcc assuming the uintptr_t comparison returning true.

gcc is known to agressively optimise while strictly following the standard. Other compilers are more conservative, but that results in less optimised code. Please don't try "solving" this by disabling optimisation or other hacks, but fix it using well-defined behaviour. It is a bug in the code.

Answer 2

p == &b is a pointer comparison and is subject to the following rules from the C Standard (6.5.9 Equality operators, point 4):

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

(uintptr_t)p == (uintptr_t)&b is an arithmetic comparison and is subject to the following rules (6.5.9 Equality operators, point 6):

If both of the operands have arithmetic type, the usual arithmetic conversions are performed. Values of complex types are equal if and only if both their real parts are equal and also their imaginary parts are equal. Any two values of arithmetic types from different type domains are equal if and only if the results of their conversions to the (complex) result type determined by the usual arithmetic conversions are equal.

These two excerpts require very different things from the implementation. And it is clear that the C specification places no requirement on an implementation to mimic the behavior of the former kind of comparison in cases where the latter kind is invoked and vice versa. The implementation is only required to follow this rule (7.18.1.4 Integer types capable of holding object pointers in C99 or 7.20.1.4 in C11):

The [uintptr_t] type designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer.

(Addendum: The above quote isn't applicable in this case, because the conversion from int* to uintptr_t does not involve void* as an intermediate step. See Hadi's answer for an explanation and citation on this. Still, the conversion in question is implementation-defined and the two comparisons you are attempting are not required to exhibit the same behavior, which is the main takeaway here.)

As an example of the difference, consider two pointers that point at the same address of two different address spaces. Comparing them as pointers shouldn't return true, but comparing them as unsigned integers might.

&a + 1 is an integer added to a pointer, which is subject to the following rules (6.5.6 Additive operators, point 8):

When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i−n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

I believe that this excerpt shows that pointer addition (and subtraction) is defined only for pointers within the same array object or one past the last element. And because (1) a is not an array and (2) a and b aren't members of the same array object, it seems to me that your pointer math operation invokes undefined behavior and your compiler takes advantage of it to assume that the pointer comparison returns false. Again as pointed out in Hadi's answer (and in contrast to what my original answer assumed at this point), pointers to non-array objects can be considered pointers to array objects of length one, and thus adding one to your pointer to the scalar does qualify as pointing to one past the end of the array.

Therefore your case seems to fall under the last part of the first excerpt mentioned in this answer, making your comparison well-defined to evaluate to true if and only if the two variables are linked in sequence and in ascending order. Whether this is true for your program is left unspecified by the standard and it's up to the implementation.

Answer 3

While one of the answers has already been accepted, the accepted answer (and all other answers for that matter) are critically wrong as I'll explain and then answer the question. I'll be quoting from the same C standard, namely n1570.

Let's start with &a + 1. In contrast to what @Theodoros and @Peter has stated, this expression has defined behavior. To see this, consider section 6.5.6 paragraph 7 "Additive operators" which states:

For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

and paragraph 8 (in particular, the emphasized part):

When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression. In other words, if the expression P points to the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and (P)-N (where N has the value n) point to, respectively, the i+n-th and i−n-th elements of the array object, provided they exist. Moreover, if the expression P points to the last element of an array object, the expression (P)+1 points one past the last element of the array object, and if the expression Q points one past the last element of an array object, the expression (Q)-1 points to the last element of the array object. If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

The expression (uintptr_t)p == (uintptr_t)&b has two parts. The conversion from a pointer to an uintptr_t is NOT defined by section 7.20.1.4 (in contrast to what @Olaf and @Theodoros have said):

The following type designates an unsigned integer type with the property that any valid pointer to void can be converted to this type, then converted back to pointer to void, and the result will compare equal to the original pointer:

uintptr_t

It's important to recognize that this rule applies only to valid pointers to void. However, in this case, we have a valid pointer to int. A relevant paragraph can be found in section 6.3.2.3 paragraph 1:

A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

This means that (uintptr_t)(void*)p is allowed according to this paragraph and 7.20.1.4. But (uintptr_t)p and (uintptr_t)&b are ruled by section 6.3.2.3 paragraph 6:

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.

Note that uintptr_t is an integer type as stated in section 7.20.1.4 mentioned above and therefore this rule applies.

The second part of (uintptr_t)p == (uintptr_t)&b is comparing for equality. As previously discussed, since the result of conversion is implementation-defined, the result of equality is also implementation defined. This applies irrespective of whether the pointers themselves are equal or not.

Now I'll discuss p == &b. The third point in @Olaf's answer is wrong and @Theodoros's answer is incomplete regarding this expression. Section 6.5.9 "Equality operators" paragraph 7:

For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

and paragraph 6:

Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.)

In contrast what @Olaf have said, comparing pointers using the == operator never results in undefined behavior (which may occur only when using relational operators such as <= according to section 6.5.8 paragraph 5 which I'll omit here for brevity). Now since p points to the next int relative to a, it will be equal to &b only when the linker has placed b in that location in the binary. Otherwise, there are unequal. So this is implementation-dependent (the relative order of a and b is unspecified by the standard). Since the declarations of a and b use a language extension, namely __attribute__((section("test"))), the relative locations is indeed implementation-dependent by J.5 and 3.4.2 (omitted for brevity).

We conclude that the results of check(p == &b) and check((uintptr_t)p == (uintptr_t)&b) are implementation-dependent. So the answer depends on which version of which compiler you are using. I'm using gcc 4.8 and by compiling with default options except for the level of optimization, the output I get in both -O0 and -O1 cases is all TRUE.