[PATCH] D138992: [DebugInfo][SROA] Correct debug info for global variables spanning multiple fragments in case of SROA

[Alok Kumar Sharma via Phabricator via llvm-commits]

alok created this revision.
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The function transferSRADebugInfo is modified to include missing cases.

The existing handling produced crash for test case (attached with patch).

Please consider below IR

  %struct.BSS3 = type <{ [24 x i8] }>
  @.BSS3 = internal unnamed_addr global %struct.BSS3 zeroinitializer, align 32, !dbg !0, !dbg !7, !dbg !29
  
  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
  !1 = distinct !DIGlobalVariable(name: "bar1", scope: !2, file: !4, type: !12, isLocal: true, isDefinition: true)
  !12 = !DICompositeType(tag: DW_TAG_array_type, baseType: !13, size: 96, align: 32, elements: !14)

In SROA @.BSS3 is broken into @.BSS3.0, @.BSS3.1, @.BSS3.2

  @.BSS3.0 = internal unnamed_addr global double 0.000000e+00, align 32, !dbg !0, !dbg !24
  @.BSS3.1 = internal unnamed_addr global double 0.000000e+00, align 32, !dbg !26, !dbg !27
  @.BSS3.2 = internal unnamed_addr global double 0.000000e+00, align 16, !dbg !28

Since original memory chunk is broken into three (each of size 64bits), the variable "bar1" (of size 96=64+32) spreads across @.BSS3.0 (completely) and @.BSS3.1 (partially). So below DebugInfo should be generated.

  @.BSS3.0 = internal unnamed_addr global double 0.000000e+00, align 32, !dbg !0, !dbg !24
  @.BSS3.1 = internal unnamed_addr global double 0.000000e+00, align 32, !dbg !25, !dbg !26, !dbg !27
  @.BSS3.2 = internal unnamed_addr global double 0.000000e+00, align 16, !dbg !28
  
  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression(DW_OP_LLVM_fragment, 0, 64))
  !1 = distinct !DIGlobalVariable(name: "bar1", scope: !2, file: !4, type: !13, isLocal: true, isDefinition: true)
  !25 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression(DW_OP_LLVM_fragment, 64, 32))

But currently it was wrongly generating DW_OP_LLVM_fragment DIExpression with incorrect second argument (64 in place of 32).

  fragment is larger than or outside of variable
  !0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression(DW_OP_LLVM_fragment, 0, 64))
  !26 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression(DW_OP_LLVM_fragment, 64, 64))

Later the verification fails as variable size is less than fragment. Which leads a crash.

  Breakpoint 2, (anonymous namespace)::Verifier::verifyFragmentExpression<llvm::DIGlobalVariableExpression const> (this=0x7fffffffb790, V=..., Fragment=...,
      Desc=0x555555712450) at /tmp/llvm-project/llvm/lib/IR/Verifier.cpp:6135
  6135   CheckDI(FragSize + FragOffset <= *VarSize,
  6136           "fragment is larger than or outside of variable", Desc, &V);

Now the function transferSRADebugInfo is modified to handle such cases.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D138992

Files:
  llvm/lib/Transforms/IPO/GlobalOpt.cpp
  llvm/test/DebugInfo/X86/global-sra-struct-fit-segment.ll
  llvm/test/DebugInfo/X86/global-sra-struct-part-overlap-segment.ll

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