[DebugInfo] Flang should not emit DW_AT_main_subprogram for DWARF version lower than 4

[alok]

Summary

Summary:

DWARF attribute DW_AT_main_subprogram should be emitted with DWARF version 4 or greater.

Testing:

• check-llvm
• check-debuginfo

[aprantl]

Is this causing real problems? Usually we emit newer attributes even for older DWARF versions because most consumers just ignore unknown attributes.

llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp

addUInt(SPDie, dwarf::DW_AT_accessibility, dwarf::DW_FORM_data1,
             dwarf::DW_ACCESS_private);
   else if (SP->isPublic())
     addUInt(SPDie, dwarf::DW_AT_accessibility, dwarf::DW_FORM_data1,
             dwarf::DW_ACCESS_public);
 
   if (SP->isExplicit())
     addFlag(SPDie, dwarf::DW_AT_explicit);
 
-  if (SP->isMainSubprogram())
+  if ((DD->getDwarfVersion() >= 4) && SP->isMainSubprogram())
     addFlag(SPDie, dwarf::DW_AT_main_subprogram);
   if (SP->isPure())
     addFlag(SPDie, dwarf::DW_AT_pure);
   if (SP->isElemental())
     addFlag(SPDie, dwarf::DW_AT_elemental);
   if (SP->isRecursive())
     addFlag(SPDie, dwarf::DW_AT_recursive);
 

llvm/test/DebugInfo/X86/dwarfdump-main-subprogram.ll

+;; check emission of DW_AT_main_subprogram is done for DWARF version >= 4
+; RUN: %llc_dwarf -dwarf-version 2 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF23
+; RUN: %llc_dwarf -dwarf-version 3 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF23
+; RUN: %llc_dwarf -dwarf-version 4 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF45
+; RUN: %llc_dwarf -dwarf-version 5 -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s  --check-prefix=DWARF45
+
+; DWARF23-NOT: DW_AT_main_subprogram
+; DWARF45: DW_AT_main_subprogram   (true)
+
+; ModuleID = 'dwarfdump_main_subprogram.ll'
+source_filename = "dwarfdump_main_subprogram.ll"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @MAIN_() #0 !dbg !5 {
+L.entry:
+  ret void, !dbg !8
+}
+
+!llvm.module.flags = !{!0, !1}
+!llvm.dbg.cu = !{!2}
+
+!0 = !{i32 2, !"Dwarf Version", i32 2}
+!1 = !{i32 2, !"Debug Info Version", i32 3}
+!2 = distinct !DICompileUnit(language: DW_LANG_Fortran90, file: !3, producer: " F90 Flang - 1.5 2017-05-01", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, retainedTypes: !4, globals: !4, imports: !4)
+!3 = !DIFile(filename: "dwarfdump_main_subprogram.f90", directory: "/home/alok/amdllvm/llvm-project")
+!4 = !{}
+!5 = distinct !DISubprogram(name: "main", scope: !2, file: !3, line: 1, type: !6, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagMainSubprogram, unit: !2)
+!6 = !DISubroutineType(cc: DW_CC_program, types: !7)
+!7 = !{null}
+!8 = !DILocation(line: 2, column: 1, scope: !5)