为什么不优化这个未使用的变量呢?-Whyisn'tthisunusedvariableoptimisedaway?

29  

std::vector is a fairly complicated class that involves dynamic allocation. While clang++ is sometimes able to elide heap allocations, it is a fairly tricky optimization and you should not rely on it. Example:

向量 是一个比较复杂的类，涉及动态分配。虽然clang++ +有时可以省略堆分配，但这是一个相当棘手的优化，您不应该依赖它。例子:

int foo() {
    int* p = new int{5};
    return *p;
}

foo():                                # @foo()
        mov     eax, 5
        ret

As an example, using std::array (which does not dynamically allocate) produces fully-inlined code:

例如，使用std::array (不动态分配)会生成完全内联的代码:

#include 

int foo() {
    std::array v{1, 2, 3, 4, 5};
    return v[4];
}

foo():                                # @foo()
        mov     eax, 5
        ret

As Marc Glisse noted in the other answer's comments, this is what the Standard says in [expr.new] #10:

正如Marc Glisse在另一个答案的评论中所指出的，这是该标准在[expr]中所说的。新)# 10:

An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]). When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression. The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended: [...]

实现可以忽略对可替换的全局分配函数的调用([new.delete.single]， [new.delete.array])。当它这样做时，存储由实现提供，或者通过扩展另一个新表达式的分配来提供。如果分配没有扩展，则实现可以扩展新表达式e1的分配来为新表达式e2提供存储:[…]

7  

As the comments note, operator new can be replaced. This can happen in any Translation Unit. Optimizing a program for the case it's not replaced therefore requires Whole-Program Analysis. And if it is replaced, you have to call it of course.

如注释所示，操作符new可以被替换。这在任何翻译单位都可能发生。因此，优化一个没有被替换的程序需要全程序分析。如果它被替换了，你当然要叫它。

Whether the default operator new is a library I/O call is unspecified. That matters, because library I/O calls are observable and therefore they can't be optimized out either.

默认操作符new是一个library I/O调用，但未指定。这很重要，因为库I/O调用是可观察的，因此它们也不能优化。

4  

N3664's change to [expr.new], cited in one answer and one comment, permits new-expressions to not call a replaceable global allocation function. But vector allocates memory using std::allocator::allocate, which calls ::operator new directly, not via a new-expression. So that special permission doesn't apply, and generally compilers cannot elide such direct calls to ::operator new.

N3664改变[expr。在一个回答和一个评论中提到，允许新表达式不调用可替换的全局分配函数。但是vector使用std::分配器 :: allocation内存，它直接调用:::operator new，而不是通过一个新的表达式。因此，该特殊权限不适用，而且编译器通常无法避免对::operator new的这种直接调用。

All hope is not lost, however, for std::allocator::allocate's specification has this to say:

然而，对于std:::分配器 ::: distribution 's specification have this to say:

Remarks: the storage is obtained by calling ​::​operator new, but it is unspecified when or how often this function is called.

备注:存储是通过调用::::操作符new获得的，但是在调用该函数的时间或频率上是不指定的。

Leveraging this permission, libc++'s std::allocator uses special clang built-ins to indicate to the compiler that elision is permitted. With -stdlib=libc++, clang compiles your code down to

利用这个权限，libc+ 's std::分配器使用特殊的clang内置元素向编译器表明可以省略。使用-stdlib=libc++， clang将代码编译为

foo():                                # @foo()
        mov     eax, 5
        ret