Meta LLVM codegen regression tracking issue

[nikomatsakis]

What is this bug?

From time to time, we encounter regressions that seem to be due to an LLVM misoptimization or other bug (though sometimes it's a case of invalid unsafe code, or rustc generating bad IR). In such cases, we try to narrow the problem down to some LLVM IR that shows the bug independent of Rust; this can then be filed with LLVM and tracked separately. This metabug serves to track those sorts of bugs, and specifically cases where we have not yet managed to create a reproducible test case.

Indications that a bug may fit this category

• A common symptom is that enabling or disabling LTO causes the bug to appear or disappear.
• Similarly, bugs that appear shortly after LLVM upgades are common candidates.

Guide to diagnosing

If you'd like to help out, @pnkfelix and @nagisa plan to write a guide for how to extract LLVM IR from rustc. It has sadly not yet been written.

Tracked issues

• Segmentation fault on nightly and beta when running with --release Segmentation fault on nightly and beta when running with --release #49010
• aarch64 musl binaries panic since 2018-02-05 nightly aarch64 musl binaries panic since 2018-02-05 nightly #48967
• heisenbug: debug builds Integer::repr_discr has transient ICE on compile-fail/enum-discrim-too-small2.rs heisenbug: debug builds Integer::repr_discr has transient ICE on compile-fail/enum-discrim-too-small2.rs #47381

Priority

This bug is sometimes marked P-high when we find that there are a lot of such problems not yet diagnosed.