[llvm-dev] [RFC] Setting the current debug loc when the insertion point changes

[Vedant Kumar via llvm-dev]

Hi everybody,

I'm investigating some correctness issues with debug line table information in optimized code. I've noticed something problematic in IRBuilder. Setting the insertion point of an IRBuilder sets the builder’s current debug loc to the one attached to the insertion point. IMO this isn't always a correct or convenient thing to do, and it shouldn't be the default behavior.

# Correctness

The IRBuilder shouldn't assume that the debug loc at its insertion point applies to any instructions it emits. It doesn't have enough information to make that connection. This assumption leads to bugs like llvm.org/PR25630 <http://llvm.org/PR25630>. I.e it creates situations in which we silently push around incorrect DILocations without realizing it, because the debug locs in the IR look nice and contiguous.

To fix the issue, clients of IRBuilder should be required to either:

1. Explicitly set the correct debug location before inserting instructions, or
2. Opt-in to the current behavior of propagating the debug loc from the insertion point.

# Convenience

The current behavior of IRBuilder can be convenient when it's correct. But it can be *really* inconvenient when it's incorrect.

If a client sets an IRBuilder's debug loc correctly and makes a call that changes the builder's insertion point, all generated instructions will pick up a bogus debug loc from the IP. E.g this is what happens with SCEVExpander::expandCodeFor(). There's no way to fix this bug without changing IRBuilder's behavior.

# Solutions

1. Add a method called SetInsertPointAndDebugLoc() which behaves identically to the current SetInsertPoint(). Change SetInsertPoint() so that it doesn't change the current debug loc.
2. Add a boolean argument to SetInsertPoint() called UpdateDbgLoc which defaults to false. Only change the current debug loc if UpdateDbgLoc is true.

I have a mild preference for idea #1 because it's easier to read, IMO.

Whichever solution we pick, my plan is to first migrate all in-tree users of SetInsertPoint() to the explicit API, and to then gradually fix passes which rely on the current incorrect behavior.

I'd appreciate your thoughts on this!

thanks,
vedant
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