From 715f2264a9cfc79bab3e6c5bc4f2f7608c4a7af4 Mon Sep 17 00:00:00 2001 From: Augie Fackler Date: Fri, 19 Jan 2024 14:42:43 -0500 Subject: [PATCH] rustc_codegen_llvm: add support for writing summary bitcode Typical uses of ThinLTO don't have any use for this as a standalone file, but distributed ThinLTO uses this to make the linker phase more efficient. With clang you'd do something like `clang -flto=thin -fthin-link-bitcode=foo.indexing.o -c foo.c` and then get both foo.o (full of bitcode) and foo.indexing.o (just the summary or index part of the bitcode). That's then usable by a two-stage linking process that's more friendly to distributed build systems like bazel, which is why I'm working on this area. I talked some to @teresajohnson about naming in this area, as things seem to be a little confused between various blog posts and build systems. "bitcode index" and "bitcode summary" tend to be a little too ambiguous, and she tends to use "thin link bitcode" and "minimized bitcode" (which matches the descriptions in LLVM). Since the clang option is thin-link-bitcode, I went with that to try and not add a new spelling in the world. Per @dtolnay, you can work around the lack of this by using `lld --thinlto-index-only` to do the indexing on regular .o files of bitcode, but that is a bit wasteful on actions when we already have all the information in rustc and could just write out the matching minimized bitcode. I didn't test that at all in our infrastructure, because by the time I learned that I already had this patch largely written. --- src/driver/aot.rs | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/driver/aot.rs b/src/driver/aot.rs index fce4690f97d..394c810176a 100644 --- a/src/driver/aot.rs +++ b/src/driver/aot.rs @@ -200,7 +200,7 @@ fn produce_final_output_artifacts( // to get rid of it. for output_type in crate_output.outputs.keys() { match *output_type { - OutputType::Bitcode => { + OutputType::Bitcode | OutputType::ThinLinkBitcode => { // Cranelift doesn't have bitcode // user_wants_bitcode = true; // // Copy to .bc, but always keep the .0.bc. There is a later