LLVM assertion failure when building Servo release version with debug symbols on

[rzambre]

Platform: Mac OSX 10.11
Servo issue: servo/servo#11828

After commenting out lines 30 and 31 in the Cargo.toml under components/servo, ./mach build -r compiles all the way until layout_thread but returns this LLVM assertion failure:

Assertion failed: (std::all_of(Expr.begin(), Expr.end(), { return E && E->isBitPiece(); }) && "conflicting locations for variable"), function addMMIEntry, file /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/src/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.h, line 137.

This is most likely an existing rustc bug that got exposed after some recent commits (not precisely sure which one but possibly servo/servo#11804) in Servo. The release version with debug symbols on was compiling successfully a couple of days ago.

[rzambre]

Sadly, the error still occurs with the newer rustc version (2016-06-24)

cc @larsbergstrom

[larsbergstrom]

@aturon Is there any work going on with making debug info work better for release builds? We've had to leave it disabled in Servo due to per-platform breakage for a long time (March of 2015 - servo/servo@fda3791). Thanks!

[alexcrichton]

cc @rust-lang/compiler as well, thoughts on debug info in release builds? (@larsbergstrom's previous comment)

[larsbergstrom]

The big thing that this makes challenging is doing any sort of profiling. Since it's only meaningful on release builds, we need to turn it on. I often get lucky on linux or android, but macOS and Windows seem less reliable.

[michaelwoerister]

-Cdebuginfo=1 should provide enough debuginfo for profiling while sidestepping some of the error cases that might cause LLVM to fail. Have you given that a try?

[rzambre]

@michaelwoerister how do I use the -Cdebuginfo=1 flag? I tried ./mach build -r -Cdebuginfo=1 for Servo but I get an unknown flag complaint.

[eddyb]

@rzambre You'd have to find the code that adds to RUSTFLAGS in python/servo and add the flag there.

[glennw]

I tried this and get "error: -g and -C debuginfo both provided" reported from several of our (and upstream) crates. I'm not sure what's causing that yet.

[sanxiyn]

@glennw That error is #7493, which is truly ancient.

[metajack]

I think release+debuginfo should be a tested Rust compiler code path. Do we need to file a separate bug from this to propose that Rust CI start doing this?

[eddyb]

@metajack We have debug-opt but it doesn't run tests.

[metajack]

@eddyb Then it sounds like my proposal is to enable tests on the existing debug-opt. I assume they are disabled because these code paths don't currently pass?