http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53097
Bug #: 53097
Summary: [c++0x] Missed optimization: lambda closure object
could be smaller
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: l...@mit.edu
This code:
include <stdio.h>
int main(int argc, char **argv)
{
int a, b;
auto foo = [&](){return a + b;};
printf("%d\n", (int)sizeof(foo));
return 0;
}
prints 16 (on x86-64) on gcc 4.6 and something quite close to 4.7 at -O2 and
-Ofast. This is as expected if the closure object is implemented as imagined
in the spec.
In this particular case, accessing a from the lambda is defined behavior iff
accessing b is defined (because either a and b are both in scope or both out of
scope, so the lambda could be optimized based on the knowledge that a and b are
at a fixed offset from each other. This would give size 8.
(It sounds like this could be rather difficult. clang++ 2.9 works the same way
as g++. I don't really expect to see this optimization implemented anytime
soon.)
