[Issue]: Scan test failure in rocThrust caused by changes in LLVM for the ROCm 6.2.1 release

[Beanavil]

Problem Description

One of rocThrust's tests for the scan algorithm is failing right after the ROCm 6.2.1 release. During debugging, it was noticed that modifying the addition operator of the FixedVector type used in the test like so:

    __host__ __device__
    FixedVector operator+(const FixedVector& bs) const
    {
        FixedVector output;
+     #pragma unroll 1
        for(unsigned int i = 0; i < N; i++)
            output.data[i] = data[i] + bs.data[i];
        return output;
    }

fixes the test.

Further investigation tracked the origin of the issue to this particular commit. Manually building this repo checked out at said commit and compiling with the clang++ executable generated reproduces the issue, while checking out to a previous commit and repeating the same process yields a successful execution.

This test failure is confirmed to happen on Vega20 (gfx906), MI100 (gfx908), V620 (gfx1030), and rx7900xtx (gfx1100).

Operating System

Ubuntu 20.04.6 LTS

CPU

AMD EPYC 7713P 64-Core Processor

GPU

AMD Instinct MI100

ROCm Version

ROCm 6.2.0

ROCm Component

llvm-project

Steps to Reproduce

1. Clone this repo
2. Checkout to commit 6598eee547c545449daaf57323d55c63d7718106
3. Configure, build and install

cmake -G Ninja -S llvm -B build -DLLVM_INSTALL_UTILS=ON -DCMAKE_INSTALL_PREFIX=/path/to/llvm/install -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_TESTS=False  -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt"

ninja -C build install

4. Clone rocThrust
5. Configure & build rocThrust scan test

cmake -G Ninja -D CMAKE_CXX_FLAGS=" -D_GLIBCXX_ASSERTIONS=ON" -D CMAKE_CXX_COMPILER=/path/to/llvm/install/bin/clang++ -D CMAKE_BUILD_TYPE=Release -D BUILD_TEST=ON -D BUILD_EXAMPLES=OFF -D BUILD_BENCHMARKS=OFF -D AMDGPU_TARGETS="gfx900;gfx906;gfx908;gfx1030;gfx90a" -D AMDGPU_TEST_TARGETS="gfx900;gfx906;gfx908;gfx1030;gfx90a" -D RNG_SEED_COUNT=3 -D PRNG_SEEDS="0, 1000" -S ~/rocThrust/ -B ~/rocThrust/build  -DROCPRIM_ROOT=./rocPRIM/ -D DOWNLOAD_ROCPRIM=ON

cmake --build build --target test_thrust_scan

6. Run test

./build/testing/test_thrust_scan # this should fail

7. Checkout to commit 164a5a0d9533300baf49c22cf36d1609a8cfe446
8. Re-configure, build and install

cmake -G Ninja -S llvm -B build -DLLVM_INSTALL_UTILS=ON -DCMAKE_INSTALL_PREFIX=/path/to/llvm/install -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_TESTS=False  -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt"

ninja -C build install

9. Re-configure & build rocThrust scan test

rm -rf build

cmake -G Ninja -D CMAKE_CXX_FLAGS=" -D_GLIBCXX_ASSERTIONS=ON" -D CMAKE_CXX_COMPILER=/path/to/llvm/install/bin/clang++ -D CMAKE_BUILD_TYPE=Release -D BUILD_TEST=ON -D BUILD_EXAMPLES=OFF -D BUILD_BENCHMARKS=OFF -D AMDGPU_TARGETS="gfx900;gfx906;gfx908;gfx1030;gfx90a" -D AMDGPU_TEST_TARGETS="gfx900;gfx906;gfx908;gfx1030;gfx90a" -D RNG_SEED_COUNT=3 -D PRNG_SEEDS="0, 1000" -S ~/rocThrust/ -B ~/rocThrust/build  -DROCPRIM_ROOT=./rocPRIM/ -D DOWNLOAD_ROCPRIM=ON && 

cmake --build build --target test_thrust_scan

10. Run test

./build/testing/test_thrust_scan # this should pass

(Optional for Linux users) Output of /opt/rocm/bin/rocminfo --support

No response

Additional Information

No response

[lamb-j]

Thanks for the detailed report! @alex-t is taking a look

[alex-t]

The change was an attempt to redesign the whole control flow lowering in the AMDGPU backend. Further, I found a simpler and cleaner way to solve the problem that the change originated from.
The culprit commit is going to be reverted and for sure won't go to the next release.
So, it doesn't make sense to investigate the exact reason and make any changes to it now.