--- |
  ; RUN: llc %s -march=x86-64 -run-pass=livedebugvalues -o - -experimental-debug-variable-locations | FileCheck %s -implicit-check-not=DBG_VALUE

  ; The MIR below represents a pathalogical case for value-tracking
  ; LiveDebugValues. The code structure is eight nested loops, with loop heads
  ; from bb.1 to bb.8, a central block bb.9 that does nothing, and loop ends
  ; from bb.10 to bb.17. The CMP's and jumps might be broken; the only
  ; important part is that it looks like nested loops to LiveDebugValues.
  ;
  ; The variable location is always $rsi, which enters the function live.
  ; There's also a def of $rsi in bb.14, in a loop tail, half way into the
  ; loop nest.s.
  ;
  ; This presents a serious problem: the outer four loops each implicitly have
  ; a PHI value for $rsi, because the block could be entered on a path straight
  ; from entry, or from bb.14 where $rsi is def'd. While the innermost four
  ; loops have a value of $rsi that is live-through each loop from bb.5
  ; onwards.
  ;
  ; Value-tracking LiveDebugValues _must_ correctly identify each PHI value.
  ; Observe the DBG_VALUE in bb.2: this variable location musn't be propagated
  ; any further, because there's a path to either successor that goes through
  ; bb.14 where the value is overwritten.Value tracking needs to identify the
  ; PHI value on entry to the block; and that each successor has a different
  ; PHI value in that register.
  ;
  ; Likewise, we mustn't identify values as PHIs which aren't. Entering bb.5
  ; has a PHI value (from bb.4) in $rsi. There are no paths to bb.5 that pass
  ; through the clobbering bb.14, which don't also pass through bb.4: thus
  ; that value is live-through the innermost four loops. If we
  ; over-approximated where PHIs happened, we would lose variable location
  ; coverage here, by not propagating the variable location through the inner
  ; loops.
  ;
  ; Getting this right requires the lattice descent (described in the
  ; implementation) to search loop head PHI values, until one is found that is
  ; live-through a loop.

  ; This location in bb.2 should not be propagated further,
  ; CHECK-LABEL: bb.2:
  ; CHECK:  DBG_VALUE $rsi, $noreg

  ; This location should be live through the inner loops, til bb.14
  ; CHECK-LABEL: bb.5:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.6:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.7:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.8:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.9:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.10:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.11:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.12:
  ; CHECK:  DBG_VALUE $rsi, $noreg
  ; CHECK-LABEL: bb.13:
  ; CHECK:  DBG_VALUE $rsi, $noreg

  declare i64 @bees(i64 %arg);

  define i32 @chiasm(i64 %arg) local_unnamed_addr !dbg !12 {
  entry:
    br label %bb1, !dbg !17
  bb1:
    br label %bb2, !dbg !17
  bb2:
    br label %bb3, !dbg !17
  bb3:
    ret i32 0, !dbg !17
  }

  !llvm.dbg.cu = !{!0}
  !llvm.module.flags = !{!7, !8, !9, !10}
  !llvm.ident = !{!11}
  !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !3, debugInfoForProfiling: true, nameTableKind: None)
  !1 = !DIFile(filename: "main.cpp", directory: "F:\")
  !2 = !{}
  !3 = !{!4}
  !4 = !DIGlobalVariableExpression(var: !5, expr: !DIExpression())
  !5 = distinct !DIGlobalVariable(name: "start", scope: !0, file: !1, line: 4, type: !6, isLocal: false, isDefinition: true)
  !6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
  !7 = !{i32 2, !"Dwarf Version", i32 4}
  !8 = !{i32 2, !"Debug Info Version", i32 3}
  !9 = !{i32 1, !"wchar_size", i32 2}
  !10 = !{i32 7, !"PIC Level", i32 2}
  !11 = !{!"clang version 10.0.0"}
  !12 = distinct !DISubprogram(name: "bb_to_bb", linkageName: "bb_to_bb", scope: !1, file: !1, line: 6, type: !13, scopeLine: 6, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !15)
  !13 = !DISubroutineType(types: !14)
  !14 = !{!6, !6}
  !15 = !{!16}
  !16 = !DILocalVariable(name: "myVar", scope: !12, file: !1, line: 7, type: !6)
  !17 = !DILocation(line: 10, scope: !12)

...
---
name: chiasm
tracksRegLiveness: true
liveins:
  - { reg: '$rdi', virtual-reg: '' }
stack:
  - { id: 0, name: '', type: spill-slot, offset: -8, size: 8, alignment: 8,
      stack-id: default, callee-saved-register: '', callee-saved-restored: true,
      debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
body:  |
  bb.0.entry:
  liveins: $rdi, $rsi

  bb.1:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 1, implicit-def $eflags, debug-location !17
    JCC_1 %bb.17, 4, implicit $eflags, debug-location !17

  bb.2:
  liveins: $rsi, $rdi
    DBG_VALUE $rsi, $noreg, !16, !DIExpression(), debug-location !17
    CMP64ri8 renamable $rdi, 2, implicit-def $eflags, debug-location !17
    JCC_1 %bb.16, 4, implicit $eflags, debug-location !17

  bb.3:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 3, implicit-def $eflags, debug-location !17
    JCC_1 %bb.15, 4, implicit $eflags, debug-location !17

  bb.4:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 4, implicit-def $eflags, debug-location !17
    JCC_1 %bb.14, 4, implicit $eflags, debug-location !17

  bb.5:
  liveins: $rsi, $rdi
    DBG_VALUE $rsi, $noreg, !16, !DIExpression(), debug-location !17
    CMP64ri8 renamable $rdi, 4, implicit-def $eflags, debug-location !17
    JCC_1 %bb.13, 4, implicit $eflags, debug-location !17

  bb.6:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 4, implicit-def $eflags, debug-location !17
    JCC_1 %bb.12, 4, implicit $eflags, debug-location !17

  bb.7:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 4, implicit-def $eflags, debug-location !17
    JCC_1 %bb.11, 4, implicit $eflags, debug-location !17

  bb.8:
  liveins: $rsi, $rdi
    CMP64ri8 renamable $rdi, 4, implicit-def $eflags, debug-location !17
    JCC_1 %bb.10, 4, implicit $eflags, debug-location !17

  bb.9:
  liveins: $rsi, $rdi, $eflags
    ;$rsi = MOV64ri 0, debug-location !17
    ;JMP_1 %bb.1, debug-location !17

  bb.10:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.8, 4, implicit $eflags, debug-location !17
    
  bb.11:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.7, 4, implicit $eflags, debug-location !17
 
  bb.12:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.6, 4, implicit $eflags, debug-location !17
 
  bb.13:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.5, 4, implicit $eflags, debug-location !17
 
  bb.14:
  liveins: $rsi, $rdi, $eflags
    $rsi = MOV64ri 0, debug-location !17
    JCC_1 %bb.4, 4, implicit $eflags, debug-location !17

  bb.15:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.3, 4, implicit $eflags, debug-location !17

  bb.16:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.2, 4, implicit $eflags, debug-location !17

  bb.17:
  liveins: $rsi, $rdi, $eflags
    JCC_1 %bb.1, 4, implicit $eflags, debug-location !17
 
  bb.18:
  liveins: $rsi, $rdi, $eflags
    RETQ

...