[LLVMbugs] [Bug 20907] New: Inlining corrupts debug locations if call-site's debug location is unknown

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20907

            Bug ID: 20907
           Summary: Inlining corrupts debug locations if call-site's debug
                    location is unknown
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: michaelwoerister at posteo.net
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In the Rust compiler we have consistently run into an assertion in
LexicalScopes::getOrCreateRegularScope() at:

assert(DISubprogram(Scope).describes(MF->getFunction()));

This would only occur when function inlining was activated. After debugging
into this, it turns out the inlining pass leaves the debug source location info
in a state the is not properly handled by the LexicalScopes class:

The fixupLineNumbers() function in lib/Transforms/Utils/InlineFunction.cpp will
not modify debug locations *at all* if the source location of the call-site is
unknown. Because of this the instructions copied from the callee into the
caller are still associated to the callee source location, but without having
the additional InlinedAt data. The LexicalScopes class will then not know that
this is actually valid code it is dealing with.

As a quick fix, I modified the fixupLineNumbers() function to clear all debug
location information from inlined instructions if the call-site has no debug
location info attached. This solved the problem but is a bit of a crude
solution. I'm sure you people can come up with something better!

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