Before replacing a lot of my "old" for loops with range based for loops, I ran some test with visual studio 2013:
std::vector<int> numbers;
for (int i = 0; i < 50; ++i) numbers.push_back(i);
int sum = 0;
//vectorization
for (auto number = numbers.begin(); number != numbers.end(); ++number) sum += *number;
//vectorization
for (auto number = numbers.begin(); number != numbers.end(); ++number) {
auto && ref = *number;
sum += ref;
}
//definition of range based for loops from http://en.cppreference.com/w/cpp/language/range-for
//vectorization
for (auto __begin = numbers.begin(),
__end = numbers.end();
__begin != __end; ++__begin) {
auto && ref = *__begin;
sum += ref;
}
//no vectorization :(
for (auto number : numbers) sum += number;
//no vectorization :(
for (auto& number : numbers) sum += number;
//no vectorization :(
for (const auto& number : numbers) sum += number;
//no vectorization :(
for (auto&& number : numbers) sum += number;
printf("%f\n", sum);
looking at the disassembly, standard for loops were all vectorized:
00BFE9B0 vpaddd xmm1,xmm1,xmmword ptr [eax]
00BFE9B4 add ecx,4
00BFE9B7 add eax,10h
00BFE9BA cmp ecx,edx
00BFE9BC jne main+140h (0BFE9B0h)
but range based for loops were not :
00BFEAC6 add esi,dword ptr [eax]
00BFEAC8 lea eax,[eax+4]
00BFEACB inc ecx
00BFEACC cmp ecx,edi
00BFEACE jne main+256h (0BFEAC6h)
Is there any reason why the compiler couldn't vectorize these loops ?
I really would like to use the new syntax, but loosing vectorization is too bad.
I just saw this question, so I tried the /Qvec-report:2 flag, giving another reason:
loop not vectorized due to reason '1200'
that is:
Loop contains loop-carried data dependences that prevent vectorization. Different iterations of the loop interfere with each other such that vectorizing the loop would produce wrong answers, and the auto-vectorizer cannot prove to itself that there are no such data dependences.
Is this the same bug ? (I also tried with the last vc++ compiler "Nov 2013 CTP")
Should I report it on MS connect too ?
edit
Du to comments, I did the same test with a raw int array instead of a vector, so no iterator class is involved, just raw pointers.
Now all loops are vectorized except the two "simulated range-based" loops.
Compiler says this is due to reason '501':
Induction variable is not local; or upper bound is not loop-invariant.
I don't get what's going on...
const size_t size = 50;
int numbers[size];
for (size_t i = 0; i < size; ++i) numbers[i] = i;
int sum = 0;
//vectorization
for (auto number = &numbers[0]; number != &numbers[0] + size; ++number) sum += *number;
//vectorization
for (auto number = &numbers[0]; number != &numbers[0] + size; ++number) {
auto && ref = *number;
sum += ref;
}
//definition of range based for loops from http://en.cppreference.com/w/cpp/language/range-for
//NO vectorization ?!
for (auto __begin = &numbers[0],
__end = &numbers[0] + size;
__begin != __end; ++__begin) {
auto && ref = *__begin;
sum += ref;
}
//NO vectorization ?!
for (auto __begin = &numbers[0],
__end = &numbers[0] + size;
__begin != __end; ++__begin) {
auto && ref = *__begin;
sum += ref;
}
//vectorization ?!
for (auto number : numbers) sum += number;
//vectorization ?!
for (auto& number : numbers) sum += number;
//vectorization ?!
for (const auto& number : numbers) sum += number;
//vectorization ?!
for (auto&& number : numbers) sum += number;
printf("%f\n", sum);
