Changes between Version 1 and Version 2 of SoC2017

Timestamp:

Jan 25, 2017, 8:31:04 AM (6 years ago)

Author:

Niall Douglas

Comment:

--

SoC2017

@@ -145 +145 @@
 // Easy: generates no runtime code, this is as if literal "apple"
 constexpr const char *what_is_5 = cmap[5];
+// Easy: generates no runtime code, this is as if literal "apple"
+const char *what_is_0 = std::get<0>(cmap);
 
 // Challenging: needs to only generate code loading immediately from a memory location.
@@ -154 +156 @@
  executed by the compiler: the compiler only ''has'' to constexpr execute when all the inputs are constant
  expressions '''and''' all the outputs will be used in constant expressions, and it only ''may'' constexpr execute
- when all the inputs are constant expressions. Indeed empirical testing of clang 3.7 and VS2015 found both currently
- do '''not''' constexpr execute when the result is not stored constexpr, so in a naive implementation the last
- statement '''does''' generate a runtime lookup on current compiler technology despite the constant expression inputs.
- If you'd like to see more detail about this specific problem, have a look at the assembler generated for a toy
- static_map implementation at https://goo.gl/eO7ooa.
+ when all the inputs are constant expressions. The outcome therefore depends entirely on which compiler version
+ you are using and a fair bit of luck - sometimes GCC but not clang will work, sometimes it's the opposite. And
+ Visual Studio is always a surprise!
 
 2. To implement a `static_map` class which runs on at least two major C++ compilers.
@@ -180 +180 @@
 
 ==== Programming competency test ====
-Write a function which uses one of the compile time string hashing techniques at https://stackoverflow.com/questions/2111667/compile-time-string-hashing
-(or a constexpr hash function of your preference) to generate a constexpr `std::array<unsigned>` of its input strings. The function ought to have the following
-prototype:
-
-{{{
-#!c++
-template<class... Strings> constexpr std::array<unsigned, sizeof...(Strings)> hash_strings(Strings&&... strings);
-
-// Examples of usage
-constexpr std::array<unsigned, 0> string_hashes0 = hash_strings();
-constexpr std::array<unsigned, 2> string_hashes2 = hash_strings("niall", "douglas");
-constexpr std::array<unsigned, 4> string_hashes4 = hash_strings("google", "summer", "of", "code");
-// This should fail elegantly and usefully ...
-//constexpr std::array<unsigned, 0> string_hashes_fail = hash_strings(5);
-}}}
-
-In your submission you should:
-
-1. Explain why you chose the constexpr string hash function you did and the strengths of your choice over other choices.
-2. Test that the output is identical whether executed by the compiler at compile time, or at runtime.
-3. '''Prove''' that the compile time constexpr implementation generates no runtime code whatsoever.
+1. Persuade the toy static_map implementation at https://goo.gl/eO7ooa to generate the same minimum output
+under -O3 optimisation with GCC as it does with clang. Extra bonus points if you can persuade VS2017 to
+do the same. You may use C++ 17 features if you find that helps.
 
 Submission of the programming test should be via copying and pasting the code you wrote into the end of the proposal you submit to Google Melange.