Apply path remapping to DW_AT_GNU_dwo_name when producing split DWARF

[cbeuw]

--remap-path-prefix doesn't apply to paths to .o (in case of packed) or .dwo (in case of unpacked) files in DW_AT_GNU_dwo_name. GCC also has this bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91888

[rust-highfive]

r? @oli-obk

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[oli-obk]

cc @davidtwco I saw you mention dwp on zulip recently

[davidtwco]

I don't have time to do a full review right now (will take a closer look tomorrow), but we could probably do this without breaking packaging:

As rustc invokes the dwarf packaging utility (rather than that being part of a separate build process), we can just always provide the paths to the object files directly rather than just rely on the path computed in the packaging utility from DW_AT_comp_dir and DW_AT_GNU_dwo_name (and for dependencies, we look inside the rlib).

[davidtwco]

r? @davidtwco

[davidtwco]

Implementation looks good to me, are there other changes you want to make (the PR is marked WIP)?

[cbeuw]

I marked it as draft because I didn't really know if this will work and I wanted to see the Linux CI run results (I'm on Windows so can't really test DWARF properly).

I'll remove WIP and I'm happy to merge as-is once bors is happy too

[cbeuw]

Turns out my test actually wasn't doing anything and DW_AT_GNU_dwo_name still isn't properly remapped.

This acutally came up before #82074. Will look into it later

[cbeuw]

I misunderstood how MCOptions.SplitDwarfFile is used in LLVM. It does control what will be emitted into DW_AT_[GNU_]dwo_name, but it can be remapped because it's not used as a real path if pass in a separate a filestream when we create an emit file pass, which we do here:

rust/compiler/rustc_codegen_llvm/src/back/write.rs

Lines 913 to 924 in 4a66a70

	with_codegen(tm, llmod, config.no_builtins, |cpm| {
	write_output_file(
	diag_handler,
	tm,
	cpm,
	llmod,
	&obj_out,
	dwo_out,
	llvm::FileType::ObjectFile,
	&cgcx.prof,
	)
	})?;

And eventually

rust/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

Lines 644 to 656 in 4a66a70

	if (DwoPath) {
	raw_fd_ostream DOS(DwoPath, EC, sys::fs::OF_None);
	EC.clear();
	if (EC)
	ErrorInfo = EC.message();
	if (ErrorInfo != "") {
	LLVMRustSetLastError(ErrorInfo.c_str());
	return LLVMRustResult::Failure;
	}
	buffer_ostream DBOS(DOS);
	unwrap(Target)->addPassesToEmitFile(*PM, BOS, &DBOS, FileType, false);
	PM->run(*unwrap(M));
	} else {

.

So the LLVM change is not needed and the current solution works

[davidtwco]

Looks good to me, comment or two about some changes but broadly what I'd expect.

[davidtwco]

@bors r+

[bors]

📌 Commit 5e481d0 has been approved by davidtwco