Clang/LLVM 5.0 optnone attribute with -O0


We have a research LLVM-based domain-specific code generator that we want to upgrade form LLVM 4.0 to 5.0. The code generator is written as an out-of-tree loadable module for opt.

Till Clang 4.0 we were compiling the front-end code (annotated C++) using -O0. The generated bitcode was further processed using opt with our module loaded. In Clang 5.0 we see that using -O0 adds the optnone attribute to all functions, making it impossible to use any existing LLVM passes in our code generator.

We also tried using -
"clang -Xclang -load -Xclang <module>.so -mllvm <custom options>"

, but this caused a segfault at llvm::legacy::FunctionPassManagerImpl::doInitialization(llvm::Module&).

What is the right way to get around this issue?


To prevent optnone from being added you can replace -O0 with “-O1 -Xclang -disable-llvm-passes”. The -O1 will prevent the optnone, and the rest will stop clang from performing any optimizations.

You can also add the -Xclang -disable-O0-optnone flag to your command line. This will disable the implicit optnone when compiling with O0.