From dfbf61029fec121026eb7a77731cfd52e88a3f4c Mon Sep 17 00:00:00 2001 From: Ian Douglas Scott Date: Sun, 19 Mar 2023 19:00:17 -0700 Subject: [PATCH] Set LLVM `LLVM_UNREACHABLE_OPTIMIZE` to `OFF` This option was added to LLVM in https://reviews.llvm.org/D121750?id=416339. It makes `llvm_unreachable` in builds without assertions compile to an `LLVM_BUILTIN_TRAP` instead of `LLVM_BUILTIN_UNREACHABLE` (which causes undefined behavior and is equivalent to `std::hint::unreachable_unchecked`). Having compiler bugs triggering undefined behavior generally seems undesirable and inconsistent with Rust's goals. There is a check in `src/tools/tidy/src/style.rs` to reject code using `llvm_unreachable`. But it is used a lot within LLVM itself. For instance, this changes a failure I get compiling `libcore` for m68k from a `SIGSEGV` to `SIGILL`, which seems better though it still doesn't provide a useful message without switching to an LLVM build with asserts. It may be best not to do this if it noticeably degrades compiler performance, but worthwhile if it doesn't do so in any significant way. I haven't looked into what benchmarks there are for Rustc. That should be considered before merging. --- src/bootstrap/download-ci-llvm-stamp | 2 +- src/bootstrap/native.rs | 1 + 2 files changed, 2 insertions(+), 1 deletion(-) diff --git a/src/bootstrap/download-ci-llvm-stamp b/src/bootstrap/download-ci-llvm-stamp index 94630e40f3c4c..36f9aaa595d0f 100644 --- a/src/bootstrap/download-ci-llvm-stamp +++ b/src/bootstrap/download-ci-llvm-stamp @@ -1,4 +1,4 @@ Change this file to make users of the `download-ci-llvm` configuration download a new version of LLVM from CI, even if the LLVM submodule hasn’t changed. -Last change is for: https://github.com/rust-lang/rust/pull/104748 +Last change is for: https://github.com/rust-lang/rust/pull/109373 diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs index 41ee509655326..6f09c8307fca4 100644 --- a/src/bootstrap/native.rs +++ b/src/bootstrap/native.rs @@ -309,6 +309,7 @@ impl Step for Llvm { cfg.out_dir(&out_dir) .profile(profile) .define("LLVM_ENABLE_ASSERTIONS", assertions) + .define("LLVM_UNREACHABLE_OPTIMIZE", "OFF") .define("LLVM_ENABLE_PLUGINS", plugins) .define("LLVM_TARGETS_TO_BUILD", llvm_targets) .define("LLVM_EXPERIMENTAL_TARGETS_TO_BUILD", llvm_exp_targets)