DWARF v5: emit DW_AT_addr_base if DW_AT_low_pc references .debug_addr

[MaskRay]

Summary

The condition !AddrPool.empty() is tested before attachRangesOrLowHighPC(), which may add an entry to AddrPool. We emit DW_AT_low_pc (DW_FORM_addrx) but may incorrectly omit DW_AT_addr_base for LineTablesOnly. This can be easily reproduced:

clang -gdwarf-5 -gmlt -c a.cc

Fix this by moving !AddrPool.empty() below.

This was discovered while investigating an lld crash (fixed by D61889) on such object files: ld.lld --gdb-index a.o

[MaskRay]

Mention this can be easily reproduced by clang -gdwarf-5 -gmlt

[probinson]

LGTM once you rephrase the comment in the test.

test/DebugInfo/Generic/line-table-addrx.ll

3 ↗

(On Diff #199431)

Comments should describe things as they exist with the patch, not how things used to be.

[MaskRay]

incorporate probinson's suggestion

llvm/trunk/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

for (const auto &P : CUMap) {
     // If we have code split among multiple sections or non-contiguous
     // ranges of code then emit a DW_AT_ranges attribute on the unit that will
     // remain in the .o file, otherwise add a DW_AT_low_pc.
     // FIXME: We should use ranges allow reordering of code ala
     // .subsections_via_symbols in mach-o. This would mean turning on
     // ranges for all subprogram DIEs for mach-o.
     DwarfCompileUnit &U = SkCU ? *SkCU : TheCU;
 
-    // We don't keep track of which addresses are used in which CU so this
-    // is a bit pessimistic under LTO.
-    if (!AddrPool.isEmpty() &&
-        (getDwarfVersion() >= 5 ||
-         (SkCU && !empty(TheCU.getUnitDie().children()))))
-      U.addAddrTableBase();
-
     if (unsigned NumRanges = TheCU.getRanges().size()) {
       if (NumRanges > 1 && useRangesSection())
         // A DW_AT_low_pc attribute may also be specified in combination with
         // DW_AT_ranges to specify the default base address for use in
         // location lists (see Section 2.6.2) and range lists (see Section
         // 2.17.3).
         U.addUInt(U.getUnitDie(), dwarf::DW_AT_low_pc, dwarf::DW_FORM_addr, 0);
       else
         U.setBaseAddress(TheCU.getRanges().front().getStart());
       U.attachRangesOrLowHighPC(U.getUnitDie(), TheCU.takeRanges());
     }
 
+    // We don't keep track of which addresses are used in which CU so this
+    // is a bit pessimistic under LTO.
+    if (!AddrPool.isEmpty() &&
+        (getDwarfVersion() >= 5 ||
+         (SkCU && !empty(TheCU.getUnitDie().children()))))
+      U.addAddrTableBase();
+
     if (getDwarfVersion() >= 5) {
       if (U.hasRangeLists())
         U.addRnglistsBase();
 
       if (!DebugLocs.getLists().empty() && !useSplitDwarf())
         U.addLoclistsBase();
     }
 

llvm/trunk/test/DebugInfo/Generic/line-table-addrx.ll

+; RUN: %llc_dwarf -filetype=obj %s -o - | llvm-dwarfdump -v -debug-info - | FileCheck %s
+
+;; In DWARF v5, emit DW_AT_addr_base as DW_AT_addr_base is used for DW_AT_low_pc.
+; CHECK: DW_AT_low_pc [DW_FORM_addrx]
+; CHECK: DW_AT_addr_base
+
+define i64 @foo() !dbg !7 {
+entry:
+  ret i64 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, emissionKind: LineTablesOnly, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "a.cc", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 5}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!7 = distinct !DISubprogram(name: "a", scope: !1, file: !1, line: 22, type: !8, scopeLine: 22, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !2)

llvm/trunk/test/DebugInfo/X86/convert-debugloc.ll

 ; RUN: llc -dwarf-version=5 -filetype=obj -O0 < %s | llvm-dwarfdump - | FileCheck %s --match-full-lines --check-prefix=DW5-CHECK
 ; RUN: llc -dwarf-version=4 -filetype=obj -O0 < %s | llvm-dwarfdump - | FileCheck %s --match-full-lines --check-prefix=DW4-CHECK
 
 ; DW5-CHECK: .debug_info contents:
 ; DW5-CHECK-NEXT: 0x00000000: Compile Unit: length = 0x0000005c version = 0x0005 unit_type = DW_UT_compile abbr_offset = 0x0000 addr_size = 0x08 (next unit at 0x00000060)
 ; DW5-CHECK-EMPTY:
 ; DW5-CHECK-NEXT: 0x0000000c: DW_TAG_compile_unit
 ; DW5-CHECK-NEXT:               DW_AT_producer  ("clang version 9.0.0 (trunk 353791) (llvm/trunk 353801)")
 ; DW5-CHECK-NEXT:               DW_AT_language  (DW_LANG_C99)
 ; DW5-CHECK-NEXT:               DW_AT_name  ("dbg.c")
 ; DW5-CHECK-NEXT:               DW_AT_str_offsets_base  (0x00000008)
 ; DW5-CHECK-NEXT:               DW_AT_stmt_list (0x00000000)
 ; DW5-CHECK-NEXT:               DW_AT_comp_dir {{.*}}
-; DW5-CHECK-NEXT:               DW_AT_addr_base (0x00000008)
 ; DW5-CHECK-NEXT:               DW_AT_low_pc  (0x0000000000000000)
 ; DW5-CHECK-NEXT:               DW_AT_high_pc (0x0000000000000007)
+; DW5-CHECK-NEXT:               DW_AT_addr_base (0x00000008)
 ; DW5-CHECK-NEXT:               DW_AT_loclists_base (0x0000000c)
 ; DW5-CHECK-EMPTY:
 ; DW5-CHECK-NEXT: 0x00000027:   DW_TAG_base_type
 ; DW5-CHECK-NEXT:                 DW_AT_name  ("DW_ATE_signed_8")
 ; DW5-CHECK-NEXT:                 DW_AT_encoding  (DW_ATE_signed)
 ; DW5-CHECK-NEXT:                 DW_AT_byte_size (0x01)
 ; DW5-CHECK-EMPTY:
 ; DW5-CHECK-NEXT: 0x0000002b:   DW_TAG_base_type

llvm/trunk/test/DebugInfo/X86/fission-ranges.ll

 ; RUN: llc -split-dwarf-file=foo.dwo -O0 %s -mtriple=x86_64-unknown-linux-gnu -filetype=obj -o %t
 ; RUN: llvm-dwarfdump -v %t | FileCheck %s
 ; RUN: llvm-objdump -h %t | FileCheck --check-prefix=HDR %s
 
 ; RUN: llc -dwarf-version=5 -O0 %s -mtriple=x86_64-unknown-linux-gnu -filetype=obj -o %t
 ; RUN: llvm-dwarfdump -v %t | FileCheck --check-prefix=V5RNGLISTS %s
 
 ; CHECK: .debug_info contents:
 ; CHECK: DW_TAG_compile_unit
 ; CHECK-NEXT: DW_AT_stmt_list
 ; CHECK-NEXT: DW_AT_comp_dir
 ; CHECK-NEXT: DW_AT_GNU_dwo_name
 ; CHECK-NEXT: DW_AT_GNU_dwo_id
 ; CHECK-NEXT: DW_AT_GNU_ranges_base
+; CHECK-NEXT: DW_AT_low_pc
+; CHECK-NEXT: DW_AT_high_pc
 ; CHECK-NEXT: DW_AT_GNU_addr_base [DW_FORM_sec_offset]                   (0x00000000)
 
 ; CHECK: .debug_info.dwo contents:
 ; CHECK: DW_AT_location [DW_FORM_sec_offset]   ([[A:0x[0-9a-z]*]]
 ; CHECK: DW_AT_location [DW_FORM_sec_offset]   ([[E:0x[0-9a-z]*]]
 ; CHECK: DW_AT_location [DW_FORM_sec_offset]   ([[B:0x[0-9a-z]*]]
 ; CHECK: DW_AT_location [DW_FORM_sec_offset]   ([[D:0x[0-9a-z]*]]
 ; CHECK: DW_AT_ranges [DW_FORM_sec_offset]   (0x00000000