Why do (only) some compilers use the same address for identical string literals?

94
votes

https://godbolt.org/z/cyBiWY

I can see two 'some' literals in assembler code generated by MSVC, but only one with clang and gcc. This leads to totally different results of code execution.

static const char *A = "some";
static const char *B = "some";

void f() {
    if (A == B) {
        throw "Hello, string merging!";
    }
}

Can anyone explain the difference and similarities between those compilation outputs? Why does clang/gcc optimize something even when no optimizations are requested? Is this some kind of undefined behaviour?

I also notice that if I change the declarations to those shown below, clang/gcc/msvc do not leave any "some" in the assembler code at all. Why is the behaviour different?

static const char A[] = "some";
static const char B[] = "some";
4
c++language-lawyerstring-literalsstring-interning
stackoverflow.com/a/52424271/1133179 Some nice relevant answer to a closely related question, with standard quotes. – luk32
@luk32 I discuss compiler flags that effect this here – Shafik Yaghmour
For MSVC, the /GF compiler option controls this behavior. See docs.microsoft.com/en-us/cpp/build/reference/… – Sjoerd
FYI, this can happen for functions too. – user541686
How Do C++ Compilers Merge Identical String Literals – phuclv

4 Answers

111
votes

This is not undefined behavior, but unspecified behavior. For string literals,

The compiler is allowed, but not required, to combine storage for equal or overlapping string literals. That means that identical string literals may or may not compare equal when compared by pointer.

That means the result of A == B might be true or false, on which you shouldn't depend.

From the standard, [lex.string]/16:

Whether all string literals are distinct (that is, are stored in nonoverlapping objects) and whether successive evaluations of a string-literal yield the same or a different object is unspecified.

36
votes

The other answers explained why you cannot expect the pointer addresses to be different. Yet you can easily rewrite this in a way that guarantees that A and B don't compare equal:

static const char A[] = "same";
static const char B[] = "same";// but different

void f() {
    if (A == B) {
        throw "Hello, string merging!";
    }
}

The difference being that A and B are now arrays of characters. This means that they aren't pointers and their addresses have to be distinct just like those of two integer variables would have to be. C++ confuses this because it makes pointers and arrays seem interchangeable (operator* and operator[] seem to behave the same), but they are really different. E.g. something like const char *A = "foo"; A++; is perfectly legal, but const char A[] = "bar"; A++; isn't.

One way to think about the difference is that char A[] = "..." says "give me a block of memory and fill it with the characters ... followed by \0", whereas char *A= "..." says "give me an address at which I can find the characters ... followed by \0".

23
votes

Whether or not a compiler chooses to use the same string location for A and B is up to the implementation. Formally you can say that the behaviour of your code is unspecified.

Both choices implement the C++ standard correctly.

4
votes

It is an optimization to save space, often called "string pooling". Here is the docs for MSVC:

https://msdn.microsoft.com/en-us/library/s0s0asdt.aspx

Therefore if you add /GF to the command line you should see the same behavior with MSVC.

By the way you probably shouldn't be comparing strings via pointers like that, any decent static analysis tool will flag that code as defective. You need to compare what they point to, not the actual pointer values.