[LifetimeSafety] Building LLVM with clang@head hangs when using -fexperimental-lifetime-safety

[usx95]

Build configuration

cmake -G Ninja ../src/llvm/llvm-project/llvm  \
  -DCMAKE_BUILD_TYPE=Release \
  -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;" \
  -DLLVM_ENABLE_ASSERTIONS=On \
  -DCMAKE_C_FLAGS_RELEASE="-O3 -gmlt -UNDEBUG" \
  -DCMAKE_CXX_FLAGS_RELEASE="-O3 -gmlt -UNDEBUG -Xclang -fexperimental-lifetime-safety -Wexperimental-lifetime-safety" \
  -DCMAKE_C_COMPILER=~/build/bin/clang \
  -DCMAKE_CXX_COMPILER=~/build/bin/clang++ \
  -DLLVM_USE_LINKER=lld \
  -DLLVM_CCACHE_BUILD=1

The analysis is expected to have rough edges and slow down on specially large functions. This needs to be dealt with soonish and would be a priority after we have good enough C++ coverage (after milestone 2).

[llvmbot]

@llvm/issue-subscribers-clang-temporal-safety

Author: Utkarsh Saxena (usx95)

Build configuration ``` cmake -G Ninja ../src/llvm/llvm-project/llvm \ -DCMAKE_BUILD_TYPE=Release \ -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra;" \ -DLLVM_ENABLE_ASSERTIONS=On \ -DCMAKE_C_FLAGS_RELEASE="-O3 -gmlt -UNDEBUG" \ -DCMAKE_CXX_FLAGS_RELEASE="-O3 -gmlt -UNDEBUG -Xclang -fexperimental-lifetime-safety -Wexperimental-lifetime-safety" \ -DCMAKE_C_COMPILER=~/build/bin/clang \ -DCMAKE_CXX_COMPILER=~/build/bin/clang++ \ -DLLVM_USE_LINKER=lld \ -DLLVM_CCACHE_BUILD=1 ```

The analysis is expected to have rough edges and slow down on specially large functions. This needs to be dealt with soonish and would be a priority after we have good enough C++ coverage (after milestone 2).

[usx95]

Using perf record -g for compiling llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp shows significant compile-time hit.
Even if bailout on CFG with 3000 blocks, we see a 10% increase in compile times.
With this limit raised to 6000 blocks, we see 70% increase in compile times.

perf record suggests all the increase goes into clang::lifetimes::internal::LoanPropagationAnalysis::join/clang::lifetimes::internal::utils::join.

This seems to be becoming even more important for rollout.

Closest representative benchmark test we have is https://github.com/llvm/llvm-project/blob/main/clang/test/Analysis/LifetimeSafety/benchmark.py#L65-L83

struct MyObj { int id; ~MyObj() {} };

void conditional_merges_4(bool condition) {
  MyObj v1, v2, v3, v4;
  MyObj *p1 = nullptr, *p2 = nullptr, *p3 = nullptr, *p4 = nullptr;

  if(condition) { p1 = &v1; }
  if(condition) { p2 = &v2; }
  if(condition) { p3 = &v3; }
  if(condition) { p4 = &v4; }
}