llvmPackages_{20,21,git}.compiler-rt: no fmv on aarch64 without libc #409265
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Upstream? |
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Already tried going upstream but the PR was closed llvm/llvm-project#125922 |
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Hmm, unfortunate… @Ericson2314 what do you think about applying this? Seems a real shame to be adding a new LLVM patch. |
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@Ericson2314 just published a blog post adjacent to this: https://blog.obsidian.systems/compiler-bootstrapping-in-nixpkgs/ As patch author, my opinion is that given the timescales involved for a proper fix (e.g. llvm/llvm-project#127227), it would be good to unbreak this today in nixpkgs. |
Wrong issue linked? |
Right issue number, wrong repo. Fixed. |
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Can we make the patch less linux-specific? Conceptually, we're talking about about generally building a "freestanding" compiler-rt on a non-freestanding platform, of which Linux is merely one example. |
It looks like setting |
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The commit message and conditioning aren't quite right. We want FMV (Function MultiVersioning) to work on aarch64. Where it doesn't work is in the no-libc compiler-rt, where linux features are unavailable. The original patch meant that the linux build of compiler-rt does have FMV switched on; only if For a bit of background, FMV is machinery which enables having multiple versions of functions targeting different architectural features, and to have the best version selected at runtime. But to do this it needs libc available so it can interrogate the kernel to discover which features are available. The problem is that we're trying to build a compiler-rt without libc available, so we need to condition on this -- it should only apply for |
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(Also btw @RossComputerGuy, we'll need to introduce a second GCC NG libgcc build to match LLVM for similar reasons.) |
aarch64 FMV (Function MultiVersioning) is not available before the libc is available. This causes issues while building with compiler-rt on aarch64. By disabling FMV before the libc is available resolves missing symbol errors.
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@peterwaller-arm Thanks for the explanation, I've updated things and this should be gtg now. |
| @@ -244,6 +244,10 @@ stdenv.mkDerivation (finalAttrs: { | |||
| ++ lib.optionals (noSanitizers && lib.versionAtLeast release_version "19") [ | |||
| (lib.cmakeBool "COMPILER_RT_BUILD_CTX_PROFILE" false) | |||
| ] | |||
| ++ | |||
| lib.optional (useLLVM && stdenv.hostPlatform.isAarch64 && !haveLibc) | |||
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Based on the explanation in the commit message, wouldn't this also be required for !useLLVM?
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No because the error only pops ups with pkgsLLVM.
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So how does it work in the non-pkgsLLVM case?
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My guess is that it's using something from GCC so the symbol isn't missing.
…patch
Things done
Fixes #393603
Reverts #380213
Required for #407738
We're sending this patch to master and backporting it so
pkgsLLVMactually works correctly with LLVM 20 and git.nix.conf? (See Nix manual)sandbox = relaxedsandbox = truenix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage./result/bin/)Add a 👍 reaction to pull requests you find important.