[llvm-bugs] [Bug 42375] New: UpgradeDebugInfo doesn't process function-level debug info in lazy load path

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42375

            Bug ID: 42375
           Summary: UpgradeDebugInfo doesn't process function-level debug
                    info in lazy load path
           Product: new-bugs
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: JCTremoulet at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

I'm hitting an error (failure to correctly auto-upgrade) processing a .bc file
generated by an older clang with a newer llvm-link.  I'm not sure if LTO can
hit the same error or not.

The issue with the older clang is the issue with loop metadata maintenance
during inlining that was fixed with r361149.

***To reproduce:

Start with the cpp code that the testcase which r361149 added
(llvm/test/Transforms/Inline/inlined-loop-metadata.ll) is reduced from:

----------inlined.cpp------------
extern int *Array;
static int bar(unsigned x)
{
  int Ret = 0;
  for (unsigned i = 0; i < x; ++i)
  {
    Ret += Array[i] * i;
  }
  return Ret;
}

int foo(unsigned x)
{
  int Bar = bar(x);
  return Bar;
}
----------end file---------------

Compile it to a .bc file with clang (older than r361149) and appropriate
optimization flags:

> clang-6.0 -c -emit-llvm -gmlt -O2 inlined.cpp -o inlined.bc

Then use llvm-link (newer than r363725) to link that .bc file (don't need to
link it to anything else):

> llvm-link inlined.bc -o linked.bc


***Observed results:

llvm-link fails with a verifier error on the linked module (i.e. the output of
llvm-link):

!dbg attachment points at wrong subprogram for function
!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 12, type:
!8, scopeLine: 13, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition |
DISPFlagOptimized, unit: !0, retainedNodes: !2)
i32 (i32)* @_Z3fooj
  br i1 %49, label %50, label %16, !dbg !17, !llvm.loop !23
!24 = !DILocation(line: 5, column: 3, scope: !10)
!10 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type:
!8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit |
DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
!10 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type:
!8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit |
DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
llvm-link: error: linked module is broken!
 ^ the error above is the issue


***Expected results:

llvm-link should have stripped the broken debug info when it read in the input
module.  Note that this is the behavior of tools such as llvm-dis (as seen in
the testcase added with r363725, which uses llvm-as):

> llvm-dis inlined.bc -o inlined.ll

!dbg attachment points at wrong subprogram for function
!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 12, type:
!8, scopeLine: 13, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition |
DISPFlagOptimized, unit: !0, retainedNodes: !2)
i32 (i32)* @_Z3fooj
  br i1 %49, label %50, label %16, !dbg !17, !llvm.loop !23
!24 = !DILocation(line: 5, column: 3, scope: !10)
!10 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type:
!8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit |
DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
!10 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type:
!8, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit |
DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
llvm-dis: warning: ignoring invalid debug info in inlined.bc
 ^ the warning above is ok


***Other info:

Looking at this in the debugger, I can see that when UpgradeDebugInfo gets
called in the non-lazy case (the one that llvm-dis always uses, and the one
that llvm-link uses with an explicit -disable-lazy-loading), the verifier
reports the error to it as expected, the warning is emitted, and the debug info
is stripped.  But in the lazy case (llvm-link's default), when the verifier
processes the function with the bad debug info, since it hasn't been
materialized, the verifier doesn't visit the debug locs, so the bad debug info
doesn't get stripped but instead gets propagated to the output module, where
the verifier eventually notices it and an error gets reported.

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