Which is exactly what this project is now able to do. Your parallel make jobs don't help if you have one gigantic compilation unit, as they originally did.
LTO can be parallelized, both implicitly from the Makefile, and also within the compiler. GCC's -flto itself takes an optional argument to control the number of parallel threads/jobs. See also the -flto-partition option for selecting symbol partitioning strategies.
Rust/Cargo does this automagically, except the only control you have are the crate and module boundaries. The analogous approach for C is to (optionally) manually group compilation units into a smaller set of object files in your Makefile, LTO'ing each object file in parallel (make -j), and then (optionally) telling the compiler to partition and parallelize a second time on the backend. Which is what Rust does, basically, IIUC--a crate is nominally the LTO codegen unit, except to speed up compilation Rust has heuristics for partitioning crates internally for parallel LTO.
wahern|10 months ago
Rust/Cargo does this automagically, except the only control you have are the crate and module boundaries. The analogous approach for C is to (optionally) manually group compilation units into a smaller set of object files in your Makefile, LTO'ing each object file in parallel (make -j), and then (optionally) telling the compiler to partition and parallelize a second time on the backend. Which is what Rust does, basically, IIUC--a crate is nominally the LTO codegen unit, except to speed up compilation Rust has heuristics for partitioning crates internally for parallel LTO.