[llvm-dev] noinline changes between 3.8 and 4.0?

[Matthew O'Connor via llvm-dev]

All,

I'm in the process of upgrading an LLVM client from using 3.8 to using 4.0
and am running into the following issue:

// compile with:
// clang++ -std=c++11 -Wno-c++14-extensions -S -emit-llvm
//   ~/chrono.cpp -o chrono.ll
#include <chrono>
#include <ratio>
#include <thread>
using namespace std;
using namespace std::chrono;

using tick = ratio<1, 300>;
using tick_duration = duration<long, tick>;

class ticking_clock {
public:
  using duration = tick_duration;
  using rep = duration::rep;
  using period = duration::period;
  using time_point = time_point<ticking_clock, duration>;
  static constexpr bool is_steady = true;

  static time_point now() noexcept;
};

void f(long &elapsed_time) {
    auto start = ticking_clock::now();
    this_thread::sleep_for(tick_duration(71));
    auto finish = ticking_clock::now();
    auto dur = finish - start;
    elapsed_time = dur.count();
}

The functions defined in the resultant IR are marked `noinline` in 4.0 but
do not have that same attribute in 3.8. Running the unoptimized IR through
`opt -O2 <chrono.ll -S >chrono.opt.ll` produces the expected 6 line
function in 3.8 but does no inlining in 4.0 (respecting the attribute).

This appears to be a large change from where `clang++ -O2` was roughly
equal to `clang++ -O0 | opt -O2` in 3.8 but is not in 4.0.

Is there a way to obtain that previous functionality in 4.0?

Thanks,
Matthew
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