rustc_codegen_llvm: add support for writing summary bitcode #125345
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r? @Nadrieril rustbot has assigned @Nadrieril. Use |
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Some changes occurred in compiler/rustc_codegen_cranelift cc @bjorn3 |
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r? compiler |
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r? compiler |
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Some changes occurred in compiler/rustc_codegen_gcc |
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This is enabled with The code in the PR looks ok to me, though LTO experts might have opinions -- @bjorn3? @mw? But I think that adding a new output type is a significant enough change that it should be done via the Major Change Proposal mechanism. |
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Ok, thanks for the extra info. @bjorn3 do you want to give the final approval here? You know what's going on here much better than I do. |
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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.
This was needed in an older version of this patch, but never got edited out when it became obsolete.
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I have confirmed that, somehow, the llvm-17 regression here is real and seemingly caused by my change. I'll investigate soon. |
If we don't do this, some versions of LLVM (at least 17, experimentally) will double-emit some error messages, which is how I noticed this. Given that it seems to be costing some extra work, let's only request the summary bitcode production if we'll actually bother writing it down, otherwise skip it.
I did this in the user-facing logic, but I noticed while fixing a minor defect that I had missed it in a few places in the internal details.
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Fixed the llvm 17 failure. Sorry for the force-push, that ended up being hard to avoid once I got into the weeds debugging. I also cleaned up a couple of variable names so things are generally consistent in using |
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@bors r+ |
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…llaumeGomez Rollup of 6 pull requests Successful merges: - rust-lang#125263 (rust-lld: fallback to rustc's sysroot if there's no path to the linker in the target sysroot) - rust-lang#125345 (rustc_codegen_llvm: add support for writing summary bitcode) - rust-lang#125362 (Actually use TAIT instead of emulating it) - rust-lang#125412 (Don't suggest adding the unexpected cfgs to the build-script it-self) - rust-lang#125445 (Migrate `run-make/rustdoc-with-short-out-dir-option` to `rmake.rs`) - rust-lang#125452 (Cleanup check-cfg handling in core and std) r? `@ghost` `@rustbot` modify labels: rollup
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…llaumeGomez Rollup of 6 pull requests Successful merges: - rust-lang#125263 (rust-lld: fallback to rustc's sysroot if there's no path to the linker in the target sysroot) - rust-lang#125345 (rustc_codegen_llvm: add support for writing summary bitcode) - rust-lang#125362 (Actually use TAIT instead of emulating it) - rust-lang#125412 (Don't suggest adding the unexpected cfgs to the build-script it-self) - rust-lang#125445 (Migrate `run-make/rustdoc-with-short-out-dir-option` to `rmake.rs`) - rust-lang#125452 (Cleanup check-cfg handling in core and std) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#125345 - durin42:thin-link-bitcode, r=bjorn3 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.
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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.cand 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-onlyto 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.