Add an implementation for CPU_TIME using the POSIX function
clock_gettime. I think on most POSIX systems this will be included for
free via <ctime>, which corresponds to "time.h" (YMMV, we can fix the
code if the need arises).
Detecting that clock_gettime is available is tricky. For instance, commit
827407a86a used the following incantation in f18-parse-demo.cpp:
#if _POSIX_C_SOURCE >= 199309L && _POSIX_TIMERS > 0 && _POSIX_CPUTIME && \
defined CLOCK_PROCESS_CPUTIME_ID
This doesn't work on my AArch64 Ubuntu system, which provides
clock_gettime but doesn't define _POSIX_TIMERS. Since finding the right
combination of macros requires infinite time, patience and access to
sundry POSIX systems, we should probably try a different approach.
This patch attempts to use SFINAE instead of the preprocessor to choose
an implementation for CPU_TIME. We define a helper function template
which helps us check if clock_gettime is available (and has the
interface we expect). I hope the comments explain it well enough.
This approach has the advantage that it keeps the detection of
clock_gettime close to the code that uses it. An alternative would be to
use CMake to check for the symbol (I personally haven't used this before
so I don't know if there are any quirks).
Differential Revision: https://reviews.llvm.org/D104020
Add an implementation for CPU_TIME based on std::clock(), which should
be available on all the platforms that we support.
Also add a test that's basically just a sanity check to make sure we
return positive values and that the value returned at the start of some
amount of work is larger than the one returned after the end.
Differential Revision: https://reviews.llvm.org/D104019
Diana Picus