It does sound like a disappointing MSVC bug. Perhaps C++14 variable
templates generate better MSVC output? Something like:
template <typename T> constexpr T unit {1};
constexpr BitCount BITS = unit<BitCount>;
If so, that could be part of a solution. I'll test when I can.
Harris
On Mon, Feb 27, 2017 at 11:36 AM, Kenton Varda <[email protected]> wrote:
> Hi Vitaliy,
>
> Hmm, this seems like a pretty big bug in MSVC. It should be de-duplicating
> constants across object files, and it should be eliding the ones that
> aren't used.
>
> Note that these constants are only simple integers if you aren't defining
> CAPNP_DEBUG_TYPES. If CAPNP_DEBUG_TYPES is defined then these constants
> have special types that perform unit analysis, and your example code would
> not compile.
>
> I don't see how we can avoid defining the constants in a release build.
> They are used all over in the Cap'n Proto implementation. But it's very
> surprising to me that MSVC can't handle elide simple integer constants.
>
> Harris, do you have any thoughts?
>
> -Kenton
>
> On Mon, Feb 27, 2017 at 3:07 AM, Vitaliy Bondarchuk <
> [email protected]> wrote:
>
>> hi
>>
>> VS2015 (update pack 3) compiler produced own copy of these constants in
>> each object file:
>> constexpr BitCount BITS = kj::unit<BitCount>();
>> constexpr ByteCount BYTES = kj::unit<ByteCount>();
>> constexpr WordCount WORDS = kj::unit<WordCount>();
>> constexpr ElementCount ELEMENTS = kj::unit<ElementCount>();
>> constexpr WirePointerCount POINTERS = kj::unit<WirePointerCount>();
>>
>> As example map file of resulted binary tell me that I have 22 copies of
>> capnp::BITS in it. 88 bytes is not a big deal, but... it's a bit inaccurate
>> )
>> And this happens when the header visible to just 17 my cpp files. And a
>> cpp file doesn't need even use a constant - include the header enough for
>> produce variable in the object file.
>>
>> As I understand you use it for special types in DEBUG build.
>> But can it be a bit more trivial for RELEASE?
>>
>> I tried simple project with the header:
>>
>> namespace kj
>> {
>> template <typename T>
>> inline constexpr T unit() { return T(1); }
>>
>> inline constexpr unsigned int unit3() { return 1; }
>> }
>>
>> namespace capnp
>> {
>> typedef unsigned int uint;
>> typedef uint BitCount;
>>
>> constexpr BitCount BITS1 = kj::unit<BitCount>();
>> constexpr BitCount BITS2 = 1;
>> constexpr BitCount BITS3 = kj::unit3();
>> }
>>
>> all 3 constants have value 1. and all 3 properly can be used as size of
>> an array
>> #include "Header.h"
>>
>> void f();
>>
>> int main()
>> {
>> int b1[capnp::BITS1] = {};
>> int b2[capnp::BITS2] = {};
>> int b3[capnp::BITS3] = {};
>> f();
>>
>> printf("1=%d 2=%d 3=%d\n", capnp::BITS1, capnp::BITS2, capnp::BITS3);
>>
>> return 0;
>> }
>>
>> but the map file is:
>> 3 .bss 62c 4 unsigned int const capnp::BITS1 ConsoleApplication1.obj
>> ?BITS1@capnp@@3IB
>> 3 .bss 630 4 unsigned int const capnp::BITS3 ConsoleApplication1.obj
>> ?BITS3@capnp@@3IB
>> 3 .bss 634 4 unsigned int const capnp::BITS1 Source.obj ?BITS1@capnp@@3IB
>> 3 .bss 638 8 unsigned int const capnp::BITS3 Source.obj ?BITS1@capnp@@3IB
>>
>>
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>
>
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