llvm-test-suite.cmake.example: fix optimization level selection

[atrosinenko]

When constructing compiler command lines, the arguments in *_FLAGS_RELEASE are appended after those in *_FLAGS by CMake. Since -O3 -DNDEBUG was left intact in *_FLAGS_RELEASE, this resulted in -O3 being used instead of any custom -O<level> flag passed via CUSTOM_FLAGS.

This commit overrides both set of variables (and passes FORCE flag to set) to prevent hard-to-spot errors.

[kovdan01]

The *_FLAGS_RELEASE options are added after *_FLAGS ones by CMake, resulting in -O3 -DNDEBUG being added unconditionally.

@atrosinenko Could you please describe the scenario which was broken in a bit more detail? My understanding of this phrase is "If we were compiling llvm-test-suite with BUILD_TYPE=Release, but custom -Ox option in CMAKE_XXX_FLAGS, we effectively had -O3 instead of -Ox". Is this interpretation correct?

[atrosinenko]

@kovdan01 I updated the description of this PR, hope the updated version is clearer.

The idea is that CUSTOM_FLAGS are intended to be a quick way to override "everything".

I use a script along these lines

opt_levels=(-O0 -O2 -Oz)
clang=/opt/llvm-pauth/bin/clang

for opt in "${opt_levels[@]}"; do
  echo "### Testing at $opt ###"
  mkdir -p /path/to/llvm-test-suite/build-tmp
  cd       /path/to/llvm-test-suite/build-tmp

  # recheck before uncommenting # rm -rf * .ninja_* .lit_test_times.txt

  cmake .. -G Ninja \
    -DCUSTOM_FLAGS="$opt" \
    -DTEST_SUITE_RUN_BENCHMARKS=ON \
    -DCMAKE_C_COMPILER="$clang" \
    -C ../cmake/caches/pauth.cmake

  ninja

  mkdir -p /tmp/llvmts-results

  /path/to/bin/llvm-lit -o "/tmp/llvmts-results/report$opt.json" . > "/tmp/llvmts-results/report$opt.log" 2>&1 || true
done

The test runs performed with different optimization levels turned out to take almost identical amount of time to complete, which seemed highly unlikely for -O0 and -O2, for example. Turned out, the reason was that I assumed it is enough to set CMAKE_C_FLAGS, CMAKE_CXX_FLAGS and so on, and then it is not actually that important whether CMAKE_BUILD_TYPE is Release, Debug or anything else, as I have already set all options explicitly. This was wrong, as the *_RELEASE (or whatever "build type" is selected) counterparts of these variables override the common options. Roughly speaking, CMake constructs command lines like this: ${CMAKE_C_COMPILER} ... ${CMAKE_C_FLAGS} ${CMAKE_C_FLAGS_RELEASE}.