[DWARF] Allow empty address range tables.

[ikudrin]

Summary

Empty address range tables are allowed by the DWARF standard; moreover, generating them is recommended as a best practice.

[dblaikie]

llvm/test/DebugInfo/X86/dwarfdump-debug-aranges.s

21

Might be better to hardcode the value here (since you're hardcoding the '7' anyway) & include a comment about what this is padding out to?

(& just for amusement I looked at the DWARF spec & I find no mention of this padding - though clearly GCC and LLVM emit & parse/expect such padding)

[ikudrin]

• Add a macro and an explanation comment for the padding wizardry in the test. Hope it is more clear now.

[dblaikie]

Padding stuff seems like overkill to me, hardcoding the actual value seems like it'd probably be fine, but I don't mind so much.

[ikudrin]

• Use hardcoded padding.

It looks like I was not very convincing about the macro for calculating the padding. Moreover, as I am adding gtest-based unit tests in other patches, and they anyway use hardcoded paddings, it makes less sense to have that intricate calculation anymore.

[aprantl]

llvm/lib/DebugInfo/DWARF/DWARFDebugArangeSet.cpp

93

FYI, the LLVM coding style prefers putting the comment on a line of its own before the statement.

[ikudrin]

llvm/lib/DebugInfo/DWARF/DWARFDebugArangeSet.cpp

93

I am unable to find such a recommendation in https://llvm.org/docs/CodingStandards.html. Could you point that out?

Anyway, these lines are temporary and are going to be removed in the next patch of the stack, D71875.

llvm/lib/DebugInfo/DWARF/DWARFDebugArangeSet.cpp

while (data.isValidOffset(*offset_ptr)) {
       arangeDescriptor.Address = data.getUnsigned(offset_ptr, HeaderData.AddrSize);
       arangeDescriptor.Length = data.getUnsigned(offset_ptr, HeaderData.AddrSize);
 
       // Each set of tuples is terminated by a 0 for the address and 0
       // for the length.
       if (arangeDescriptor.Address || arangeDescriptor.Length)
         ArangeDescriptors.push_back(arangeDescriptor);
       else
-        break; // We are done if we get a zero address and length
+        return true; // We are done if we get a zero address and length
     }
 
-    return !ArangeDescriptors.empty();
+    return false; // No termination tuple is found.
   }
   return false;
 }
 
 void DWARFDebugArangeSet::dump(raw_ostream &OS) const {
   OS << format("Address Range Header: length = 0x%8.8x, version = 0x%4.4x, ",
                HeaderData.Length, HeaderData.Version)
      << format("cu_offset = 0x%8.8x, addr_size = 0x%2.2x, seg_size = 0x%2.2x\n",
                HeaderData.CuOffset, HeaderData.AddrSize, HeaderData.SegSize);
 
   for (const auto &Desc : ArangeDescriptors) {
     Desc.dump(OS, HeaderData.AddrSize);
     OS << '\n';
   }
 }

llvm/test/DebugInfo/X86/dwarfdump-debug-aranges.s

+# RUN: llvm-mc -triple x86_64-unknown-linux %s -filetype=obj -o - | \
+# RUN:  llvm-dwarfdump -debug-aranges - 2>&1 | \
+# RUN:  FileCheck %s
+
+    .section .debug_aranges,"",@progbits
+# CHECK: .debug_aranges contents:
+
+## Check that an empty set of ranges is supported.
+.L1:
+    .long   .L1end - .L1version     # Length
+# CHECK: Address Range Header: length = 0x00000014,
+.L1version:
+    .short  2                       # Version
+    .long   0x3456789a              # Debug Info Offset
+    .byte   4                       # Address Size
+    .byte   0                       # Segment Selector Size
+# CHECK-SAME: version = 0x0002,
+# CHECK-SAME: cu_offset = 0x3456789a,
+# CHECK-SAME: addr_size = 0x04,
+# CHECK-SAME: seg_size = 0x00
+    .space  7 - (. - .L1 + 7) % 8   # Padding
+.L1tuples:
+    .long   0, 0                    # Termination tuple
+# CHECK-NOT: [0x
+.L1end: