NRVO not performed in template function with `auto` return type

[wjakob]

Bugzilla Link	39326
Version	7.0
OS	All
CC	@dwblaikie,@gergol,@zygoloid

Extended Description

I've run into the following rather bizarre bug on Clang that appears to be a standard violation: when a function template has an auto return value, return value optimization (RVO) no longer works. Everything is okay for non-template functions and functions with an explicitly specified return value type.

Consider the following piece of code that reproduces this issue:

#include <stdio.h>

struct A {
    A() { printf("A::A()\n"); }
    A(const A&) { printf("A::A(const A&)\n"); }
    A(A&&) { printf("A::A(A&&)\n"); }
    ~A() { printf("A::~A()\n"); }
    A& operator=(const A&) { printf("A::operator=(const A&)\n"); return *this; }
    A& operator=(A&&) { printf("A::operator=(A&&)\n"); return *this;}
};

A test1(int z) { A x; return x; }

auto test2(int z) { A x; return x; }

template <typename T> A test3(T z) { A x; return x; }

template <typename T> auto test4(T z) { A x; return x; } /// <- problem!

int main(int argc, char *argv[]) {
    printf("\n-- Test 1:\n");
    test1(1);
    printf("\n-- Test 2:\n");
    test2(1);
    printf("\n-- Test 3:\n");
    test3(1);
    printf("\n-- Test 4:\n");
    test4(1);
    return 0;
}

This produces the following output:

$ clang++ test.cpp -o test -std=c++14
$ ./test
-- Test 1:
A::A()
A::~A()

-- Test 2:
A::A()
A::~A()

-- Test 3:
A::A()
A::~A()

-- Test 4:
A::A()
A::A(A&&)
A::~A()
A::~A()

Comparison (GCC 8):

-- Test 1:
A::A()
A::~A()

-- Test 2:
A::A()
A::~A()

-- Test 3:
A::A()
A::~A()

-- Test 4:
A::A()
A::~A()

[gergol]

I can confirm this bug.

It seems only named RVO is affected, when the return value is constructed directly in the return statement, it works as expected.

It also affects generic lambdas. Thus, this has an impact on a huge part of modern generic programming.

[zygoloid]

NRVO is not mandatory, so Clang's behavior here is conforming, albeit suboptimal. In general, NRVO is done on a best-effort basis.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (eaecbb65-864a-4d9b-ab01-204400607207)

| | | | --- | --- | | Bugzilla Link | [39326](https://llvm.org/bz39326) | | Version | 7.0 | | OS | All | | CC | @dwblaikie,@gergol,@zygoloid |

Extended Description

I've run into the following rather bizarre bug on Clang that appears to be a standard violation: when a function template has an auto return value, return value optimization (RVO) no longer works. Everything is okay for non-template functions and functions with an explicitly specified return value type.

Consider the following piece of code that reproduces this issue:

#include <stdio.h>

struct A {
    A() { printf("A::A()\n"); }
    A(const A&) { printf("A::A(const A&)\n"); }
    A(A&&) { printf("A::A(A&&)\n"); }
    ~A() { printf("A::~A()\n"); }
    A& operator=(const A&) { printf("A::operator=(const A&)\n"); return *this; }
    A& operator=(A&&) { printf("A::operator=(A&&)\n"); return *this;}
};

A test1(int z) { A x; return x; }

auto test2(int z) { A x; return x; }

template <typename T> A test3(T z) { A x; return x; }

template <typename T> auto test4(T z) { A x; return x; } /// <- problem!

int main(int argc, char *argv[]) {
    printf("\n-- Test 1:\n");
    test1(1);
    printf("\n-- Test 2:\n");
    test2(1);
    printf("\n-- Test 3:\n");
    test3(1);
    printf("\n-- Test 4:\n");
    test4(1);
    return 0;
}

This produces the following output:

$ clang++ test.cpp -o test -std=c++14
$ ./test
-- Test 1:
A::A()
A::~A()

-- Test 2:
A::A()
A::~A()

-- Test 3:
A::A()
A::~A()

-- Test 4:
A::A()
A::A(A&&)
A::~A()
A::~A()

Comparison (GCC 8):

-- Test 1:
A::A()
A::~A()

-- Test 2:
A::A()
A::~A()

-- Test 3:
A::A()
A::~A()

-- Test 4:
A::A()
A::~A()

[theuni]

+1. I saw this today while testing the new -Wnrvo option. It's one of the more common detected cases, and is fixed by specifying a return type:

auto foo() {...} // warning
auto foo() {...} -> bar // ok