[llvm-bugs] [Bug 36410] New: Merging inlined llvm.dbg.value creates broken metadata

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36410

            Bug ID: 36410
           Summary: Merging inlined llvm.dbg.value creates broken metadata
           Product: libraries
           Version: trunk
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: uweigand at de.ibm.com
                CC: aprantl at apple.com, llvm-bugs at lists.llvm.org,
                    vsk at apple.com

Created attachment 19882
  --> https://bugs.llvm.org/attachment.cgi?id=19882&action=edit
Test case - run with "opt -simplifycfg"

Running the attached test case through "opt -simplifycfg" results in:

mismatched subprogram between llvm.dbg.value variable and !dbg attachment
LLVM ERROR: Broken module found, compilation aborted!

The problem seems to be that (after inlining) the test case contains two
llvm.dbg.value statements that were inlined from a subroutine:

  call void @llvm.dbg.value(metadata i64 %vala, metadata !8, metadata
!DIExpression()), !dbg !12

  call void @llvm.dbg.value(metadata i64 %valb, metadata !8, metadata
!DIExpression()), !dbg !13

!4 = distinct !DISubprogram(name: "callee", scope: !2, file: !2, line: 1, type:
!5, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: false, unit:
!1, variables: !7)      

!10 = distinct !DISubprogram(name: "caller", scope: !2, file: !2, line: 5,
type: !5, isLocal: false, isDefinition: true, scopeLine: 5, isOptimized: false,
unit: !1, variables: !3)
!11 = distinct !DILocation(line: 6, scope: !10)
!12 = !DILocation(line: 2, scope: !4, inlinedAt: !11)
!13 = !DILocation(line: 3, scope: !4, inlinedAt: !11)

These two statements are now being merged as part of a SimplifyCFG operation. 
The resulting llvm.dbg.value instruction gets a new !dbg metadata as part of an
applyMergedLocation call.  This is now:

  call void @llvm.dbg.value(metadata i64 %vala, metadata !7, metadata
!DIExpression()), !dbg !11

!11 = !DILocation(line: 0, scope: !4)

The numbers are off since all the metadata was renamed.  But the important
change is that this is now no longer an "inline" location, but a location that
points directly to some line in "caller".  But the *variable* identified by the
llvm.dbg.value of course remains a local variable of the inlined "callee"
function.

The module verifier now aborts because a llvm.dbg.value statement for a
variable in "callee" has a location in "caller".

Reading the code in applyMergedLocation / getMergedLocation and the comments in 
https://reviews.llvm.org/D39628
this behavior seems to have been deliberate?  But at least for llvm.dbg.value
statements this doesn't look appropriate.

I'm not sure exactly how to fix that, so any advice would be appreciated.

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