[flang-dev] Build Time Comparision

[Michael Kruse via flang-dev]

At yesterday's flang technical call, we wanted to have an additional data
point for the build time of flang itself, by also getting the compilation
time using msvc.

Methodology:
Using my personal notebook with Intel Core i7-7700HQ CPU @ 2.80GHz, 16GB
RAM, Windows 10 1909. I could not find a reliable way to get the
user/system time for a process tree under Windows, so I am using wall clock
time and single threads (ninja -j1). To keep the build time reasonable, I
only timed compilation flang itself and its f18 executable (ninja says 130
tasks after touching every file in the flang subdirectory, consistent with
every compiler). -DCMAKE_BUILD_TYPE=Release. I was using the computer while
taking the time. Peak resident/working set size was determined using python
psutil.

Msvc has parallelized its code generation and uses 4 threads by default. To
make numbers more comparable, I have another run where I switched this off
(/cgthreads1).

Results:

GCC 7.5 (Ubuntu 18.04.4, WSL)
Wall clock duration: 0:45:47
Peak compiler resident size: 4042.9 MiB (by cc1plus PFTBuilder.cpp)
Peak linker resident size: 151.8 MiB (by ld.lld f18)

Clang 11.0.0 (release build 3e94943d, WSL)
Wall clock duration: 1:42:22
Peak compiler resident size: 3266.0 MiB (by clang++ PFTBuilder.cpp)
Peak linker resident size: 260.7 MiB (by ld.lld f18)

MSVC 19.26.28806 (Visual Studio 2019 16.6.5 -- current latest)
Wall clock duration: 0:59:26
Peak compiler working set: 7915.9 MiB (by cl.exe f18.cpp)
Peak linker working set: 747.9 MiB (by link.exe f18.exe)

MSVC /cgthreads1
Wall clock duration: 1:32:19
Peak compiler working set: 7918.3 MiB (by cl.exe f18.cpp)
Peak linker working set: 747.9 MiB (by link.exe f18.exe)

I also tried Intel icc, which unfortunately failed: tablegen does recognize
its command line argument. I assume the cl::opt command line registration
static initializer does not work.

Michael
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