Bootstrap: `llvm-bitcode-linker` adds unnecessary recompilation

[Kobzol]

The llvm-bitcode-linker, which is enabled by default in the compiler config, is recompiled everytime I try to run tests:

$ python3 x.py test tests/run-make/hir-tree/
Creating a sysroot for stage1 compiler (use `rustup toolchain link 'name' build/host/stage1`)
Building tool llvm-bitcode-linker (stage0 -> stage1, x86_64-unknown-linux-gnu)
   Compiling once_cell v1.19.0
   Compiling cfg-if v1.0.0
...
Building tool rustdoc (stage0 -> stage1, x86_64-unknown-linux-gnu)
   Compiling cfg-if v1.0.0
   Compiling once_cell v1.19.0
...
$ python3 x.py test tests/run-make/hir-tree/
Creating a sysroot for stage1 compiler (use `rustup toolchain link 'name' build/host/stage1`)
Building tool llvm-bitcode-linker (stage0 -> stage1, x86_64-unknown-linux-gnu)
   Compiling once_cell v1.19.0
   Compiling cfg-if v1.0.0
...
Building tool rustdoc (stage0 -> stage1, x86_64-unknown-linux-gnu)
   Compiling cfg-if v1.0.0
   Compiling once_cell v1.19.0
...

The recompilation also causes further recompilation of rustdoc (if I disable the bitcode linker, rustdoc is not recompiled).

[workingjubilee]

cc @kjetilkjeka

[kjetilkjeka]

Oh, that's not good. Thanks for reporting. I will look at it tomorrow.

[kjetilkjeka]

I have tracked it down to this ensure. If I comment out that code it wont trigger the recompilation.

rust/src/bootstrap/src/core/build_steps/compile.rs

Lines 1847 to 1855 in ee03c28

	if builder.config.llvm_bitcode_linker_enabled {
	let src_path = builder.ensure(crate::core::build_steps::tool::LlvmBitcodeLinker {
	compiler: build_compiler,
	target: target_compiler.host,
	extra_features: vec![],
	});
	let tool_exe = exe("llvm-bitcode-linker", target_compiler.host);
	builder.copy(&src_path, &libdir_bin.join(&tool_exe));
	}

From the documentation of ensure it say

This will cache the step, so it is safe (and good!) to call this as often as needed to ensure that all dependencies are built.

Also the definition of LlvmBitcodeLinker is done with the tool_extended macro. That makes me think that it's related to something like the build_compiler invalidating the caching. I'm currently exploring that a bit further.

[kjetilkjeka]

The stuff above seems to work fine as building the llvm-bitcode-linker (without rustdoc) works fine.

The problem seems to be that rustdoc and llvm-bitcode-linker breaks the cache for each other. I suspect that means something is off in feature resolution between those modules.

[kjetilkjeka]

I now know what the problem is. I will need some help to figure out the correct way to solve it.

The problem

impl Step for Rustdoc uses a very specifc cargo invocation which manually sets rustflags if a parallell compilation can be used.

rust/src/bootstrap/src/core/build_steps/tool.rs

Lines 483 to 485 in accc516

	if builder.config.rustc_parallel {
	cargo.rustflag("--cfg=parallel_compiler");
	}

This causes common dependencies between the LLVM bitcode linker and Rustdoc to be rebuilt when one is called after the other. Since Rustdoc builds the bitcode linker (presumably through the Assemble step) this will always happen when Rustdoc is built.

The questions I need resolved to fix it

Is it correct/ethical of rustdoc to set rustflags like this? Or should rustflags be handled more centrally so every cargo invocation will end up with the same set of rustflags and this cache invalidation cannot occur?

If it is important for rustdoc to use its own set of rustflags, should it then use a different build directory to other tools/steps using cargo?

[kjetilkjeka]

I see that @m-ou-se added this in c7f443a6e3320b4cb70da5af08810f1dd60e146c. Maybe she knows more about this?

[onur-ozkan]

Is it correct/ethical of rustdoc to set rustflags like this? Or should rustflags be handled more centrally so every cargo invocation will end up with the same set of rustflags and this cache invalidation cannot occur?

We can't have same set of rustflags for every Step, in most cases we want the opposite; setting step specific rustflags. What about adding --cfg=parallel_compiler to llvm-bitcode-linker tool?

[kjetilkjeka]

That seems to work well in the scenario described above and in other scenarios I tested for. I think I just imagined that this was more frequently occurring problem if all tools share build folder and are able to set rustflags separately and adding this to llvm-bitcode-linker would cause a conflict with something else (which it doesn't seem to do).

Anyway, as this at least is fixing the apparent issue I'm creating a MR with this solution now