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Why doesn't the compiler optimize this
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Bonita Montero
Apr 27, 2022, 12:58:36 PM4/27/22
to
#include <iostream>
using namespace std;
int main()
{
int i = 0, j = 0;
auto fn = [&]() { ++i, ++j; };
cout << sizeof fn << endl;
}
g++ 11.1.0, clang 13 and MSVC 2019 print "16", i.e. there are two
pointers to i and j. But that's really not necessary, there should
be only one pointer inside the parent stack frame, i.e. all adressses
should be referred relatively to this pointer.
using namespace std;
int main()
{
int i = 0, j = 0;
auto fn = [&]() { ++i, ++j; };
cout << sizeof fn << endl;
}
g++ 11.1.0, clang 13 and MSVC 2019 print "16", i.e. there are two
pointers to i and j. But that's really not necessary, there should
be only one pointer inside the parent stack frame, i.e. all adressses
should be referred relatively to this pointer.
Andrey Tarasevich
Apr 27, 2022, 6:32:42 PM4/27/22
to
for lambda expressions _literally_ as mere syntactic sugar for local
class declarations.
They just don't care/don't dare to optimize the general approach.
Lambda expressions are not seen as nested functions. Nobody even
considered following the "save a pointer to the parent's stack frame"
approach in their implementation. Maybe one day...
--
Best regards,
Andrey Tarasevich
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