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Thu, 13 Feb 2025 01:51:45 -0500 (EST) Date: Thu, 13 Feb 2025 07:51:29 +0100 From: Fabian =?utf-8?Q?Gr=C3=BCnbichler?= <debian@fabian.gruenbichler.email> To: Debian Bug Tracking System <submit@bugs.debian.org> Subject: rustc unsoundness/miscompilations on i386 Message-ID: <k54tsscfqnxf2hww6uvq7bsxmaq5lh62wqh65x2ce4jh4xkzco@dekfmkld5peu> X-Reportbug-Version: 13.0.2 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Greylist: delayed 562 seconds by postgrey-1.36 at buxtehude; Thu, 13 Feb 2025 07:01:13 UTC Delivered-To: submit@bugs.debian.org Source: rustc Version: 1.84.0+dfsg1-1 Severity: serious Tags: upstream Justification: Policy 2.2.1 X-Debbugs-Cc: debian-rust@lists.debian.org, sylvestre@debian.org Filing this bug to have a record in BTS - I'll file a separate one for release.debian.org about the broader baseline question. This issue came up back in November on debian-devel/-release: https://lists.debian.org/debian-release/2024/11/msg00443.html My original mail, partially wrongly summarizing the issue. https://lists.debian.org/debian-release/2024/11/msg00445.html My reponse linking to more upstream details, including why downgrading to i586 in LLVM/rustc doesn't work. It boils down to the following: - Debian's i386 baseline is currently 32-bit x86 without MMX or SSE (i686) - Debian's LLVM and rustc packages accordingly patch their i686 targets to remove SSE support, which would be part of that target's baseline upstream otherwise [0,1] - Upstream LLVM and rustc consider this combination unsound and unfixable (for IMHO valid reasons) because it can cause subtle miscompilations leading to runtime crashes, in addition to the (usual, expected) different semantics of x87 and SSE2 floating point implementations [2,3] I see the following options: A) bump i386 baseline for Trixie to match LLVM/rustc's upstream i686 target, i.e. Pentium 4 *with* MMX and SSE2 (separate RT bug will be filed shortly) B) RC-ignore this bug and implications for Trixie, bump baseline for Trixie+1 C) remove LLVM and rustc and all their transitive rdeps (this includes Firefox and Chromium, most major Desktop environments, sequoia, and probably a bit more) on i386 from Trixie, further reducing the usefulness of this arch Personally (with my rustc maintainer hat on) I have a strong preference for A). Please keep responses on details of A) to the corresponding release.debian.org bug! Fabian 0: https://salsa.debian.org/rust-team/rust/-/blob/debian/sid/debian/patches/behaviour/d-rustc-i686-baseline.patch?ref_type=heads 1: https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/blob/19/debian/patches/clang-baseline-fix-i386.patch 2: https://github.com/rust-lang/compiler-team/issues/808 3: https://github.com/rust-lang/rust/issues/114479#issuecomment-2072052116 ]