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rustc: Enable LTO and multiple codegen units

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This commit is a refactoring of the LTO backend in Rust to support compilations
with multiple codegen units. The immediate result of this PR is to remove the
artificial error emitted by rustc about `-C lto -C codegen-units-8`, but longer
term this is intended to lay the groundwork for LTO with incremental compilation
and ultimately be the underpinning of ThinLTO support.

The problem here that needed solving is that when rustc is producing multiple
codegen units in one compilation LTO needs to merge them all together.
Previously only upstream dependencies were merged and it was inherently relied
on that there was only one local codegen unit. Supporting this involved
refactoring the optimization backend architecture for rustc, namely splitting
the `optimize_and_codegen` function into `optimize` and `codegen`. After an LLVM
module has been optimized it may be blocked and queued up for LTO, and only
after LTO are modules code generated.

Non-LTO compilations should look the same as they do today backend-wise, we'll
spin up a thread for each codegen unit and optimize/codegen in that thread. LTO
compilations will, however, send the LLVM module back to the coordinator thread
once optimizations have finished. When all LLVM modules have finished optimizing
the coordinator will invoke the LTO backend, producing a further list of LLVM
modules. Currently this is always a list of one LLVM module. The coordinator
then spawns further work to run LTO and code generation passes over each module.

In the course of this refactoring a number of other pieces were refactored:

* Management of the bytecode encoding in rlibs was centralized into one module
  instead of being scattered across LTO and linking.
* Some internal refactorings on the link stage of the compiler was done to work
  directly from `CompiledModule` structures instead of lists of paths.
* The trans time-graph output was tweaked a little to include a name on each
  bar and inflate the size of the bars a little
This commit is contained in:
Alex Crichton 2017-07-23 08:14:38 -07:00
parent d514263ce3
commit ded38dbfc2
13 changed files with 804 additions and 431 deletions
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18
src/librustc/session/config.rs
View file

@ -1552,24 +1552,6 @@ pub fn build_session_options_and_crate_config(matches: &getopts::Matches)
early_error(error_format, "Value for codegen units must be a positive nonzero integer");
}
// It's possible that we have `codegen_units > 1` but only one item in
// `trans.modules`. We could theoretically proceed and do LTO in that
// case, but it would be confusing to have the validity of
// `-Z lto -C codegen-units=2` depend on details of the crate being
// compiled, so we complain regardless.
if cg.lto {
if let Some(n) = codegen_units {
if n > 1 {
// This case is impossible to handle because LTO expects to be able
// to combine the entire crate and all its dependencies into a
// single compilation unit, but each codegen unit is in a separate
// LLVM context, so they can't easily be combined.
early_error(error_format, "can't perform LTO when using multiple codegen units");
}
}
codegen_units = Some(1);
}
if cg.lto && debugging_opts.incremental.is_some() {
early_error(error_format, "can't perform LTO when compiling incrementally");
}