Hello All,
Consider the below toy example (I'm using Boehm GC 7.2d as provided on
Debian/Sid/x86-64):
// filetestclogc.cc
#include <algorithm>
#include <memory>
#include <functional>
#include <cstdio>
#include <gc/gc_allocator.h>
gc_allocator<int> mygcalloc;
extern "C" void apply_to_each(int lo, int hi,
const std::function<void(int)>&f)
{
for (int i=lo; i<hi; i++) f(i);
}
extern "C" void test_gc_closure(int mx)
{
std::function<void(int)> f (
#if 0 /* we should accept this but we don't yet */
std::allocator_arg, mygcalloc,
#endif
[&](int i) {printf("i=%d\n", i);});
apply_to_each(0, mx, f);
}
First, my understanding of the C++11 standard thru
http://en.cppreference.com/w/cpp/utility/functional/function/function
(which I know is imperfect, but I find it much more readable than
n3337.pdf draft of C++11 ....)
is that in principle we should compile this code with the #if 0 replaced
by #if 1 (but we don't yet, our <functional> header says "TODO: needs
allocator_arg_t"). Perhaps the type of mygcalloc is wrong...
(I don't understand at all the issues of accepting allocators for
std::function)
Then, I am surprised (and a bit disappointed) that (using GCC 4.8.2 from
Debian) with
g++-4.8 -Wall -std=c++11 -O3 -S \
-fdump-tree-phiopt -fverbose-asm testclogc.cc
we are not inlining all the lambda stuff. I was expecting we'll do the
inlining, since similar experiments with std::array etc... are very well
optimized, like
#include <cstdio>
#include <array>
#include <functional>
#include <algorithm>
int sum (const std::array<int,10>&a)
{
int s=0;
std::for_each(a.begin(), a.end(),
[&](int i) {printf("i=%d\n", i); s += i;});
return s;
}
I am not at all expert on C++ optimization, so probably I am very naive.
If someone had time to explain what passes are optimizing this sum
function with g++ -std=c++11 -O3 I would be very happy (the handling of
lambda-s is really mysterious to me)...
Cheers.
--
Basile STARYNKEVITCH http://starynkevitch.net/Basile/
email: basile<at>starynkevitch<dot>net mobile: +33 6 8501 2359
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*** opinions {are only mine, sont seulement les miennes} ***
