DebugInfo: Make -no-dwarf-ranges-section just emit no address range rather than a bogus/partial one

[dblaikie]

Summary

First implemented in r328030 / 858a7dd6d70bfed500e0e2a5137f25dd71b90f19 and I
speculated in this review about what exactly we should be doing here - and now
I've come across some changes I'm making that trip over the somewhat arbitrary
behavior we chose here & deciding to revisit it for more correctness/less
arbitrariness.

With that previous commit, we made a single contiguous range (high/low_pc) from
the first and last entry in the range list - that's not correct, especially at
the CU level where this address range might then span multiple sections (maybe
even not sections that would all be unified by the linker, even - if you have
functions in special sections). Instead, just don't emit any addresses (ranges
or low/high) for such an entity.

I made the change for nested constructs (not just the CU) as well, though there
doesn't seem to be test coverage for that - I can add that if folks think it's
worthwhile. (probably should, I'm just being lazy) This could still hit the
multiple different sections issue in cases like -fbasic-block-sections.

Alexey - a question I had on the orginal patch: Any idea what other compilers
have done in this case? Is it the original behavior implemented here (beginning
of the first range to the end of the last, sort of paving over any address
holes).

[dblaikie]

Fix some obvious bug I left in here - and fix up one test case

[ABataev]

What I saw with the nvcc that it generated the debug info just like in the tests before. So, I just tried to mimic this behavior, since ptxas is very fragile and can be broken very easily. I would try to test it using ptxas/cuda-gdb to check that it does not break anything.

[dblaikie]

In D93502#2463383, @ABataev wrote:

What I saw with the nvcc that it generated the debug info just like in the tests before. So, I just tried to mimic this behavior, since ptxas is very fragile and can be broken very easily. I would try to test it using ptxas/cuda-gdb to check that it does not break anything.

Oh, huh - interesting. Is it possible for me to reproduce/play around with this behavior in some way? (I've never used nvcc, not sure where to download/install it from, etc)

[ABataev]

In D93502#2463936, @dblaikie wrote:

In D93502#2463383, @ABataev wrote:

What I saw with the nvcc that it generated the debug info just like in the tests before. So, I just tried to mimic this behavior, since ptxas is very fragile and can be broken very easily. I would try to test it using ptxas/cuda-gdb to check that it does not break anything.

Oh, huh - interesting. Is it possible for me to reproduce/play around with this behavior in some way? (I've never used nvcc, not sure where to download/install it from, etc)

You need a system with NVidia GPU to test cuda-gdb. I think you can get nvcc here https://developer.nvidia.com/cuda-downloads. I suggest to try some older versions, like 9 or 10, since some of the users can still use it.

[dblaikie]

In D93502#2463937, @ABataev wrote:

In D93502#2463936, @dblaikie wrote:

In D93502#2463383, @ABataev wrote:

What I saw with the nvcc that it generated the debug info just like in the tests before. So, I just tried to mimic this behavior, since ptxas is very fragile and can be broken very easily. I would try to test it using ptxas/cuda-gdb to check that it does not break anything.

Oh, huh - interesting. Is it possible for me to reproduce/play around with this behavior in some way? (I've never used nvcc, not sure where to download/install it from, etc)

You need a system with NVidia GPU to test cuda-gdb. I think you can get nvcc here https://developer.nvidia.com/cuda-downloads. I suggest to try some older versions, like 9 or 10, since some of the users can still use it.

Hmm - that might be beyond me, but if you had a chance to provide some more specific details/a recipe for this, I'd love to poke it further.

For now I think I'll abandon this change - I'd hoped it'd make another change a bit tidier, but it's not a huge deal/just a rather complicated if condition, not worth too much work on this side.

llvm/lib/CodeGen/AsmPrinter/DwarfCompileUnit.cpp

else {
     else
       addSectionLabel(ScopeDIE, dwarf::DW_AT_ranges, List.Label,
                       RangeSectionSym);
   }
 }
 
 void DwarfCompileUnit::attachRangesOrLowHighPC(
     DIE &Die, SmallVector<RangeSpan, 2> Ranges) {
-  if (Ranges.size() == 1 || !DD->useRangesSection()) {
+  if (Ranges.size() == 1) {
     const RangeSpan &Front = Ranges.front();
-    const RangeSpan &Back = Ranges.back();
-    attachLowHighPC(Die, Front.Begin, Back.End);
-  } else
+    attachLowHighPC(Die, Front.Begin, Front.End);
+  } else if (DD->useRangesSection())
     addScopeRangeList(Die, std::move(Ranges));
 }
 
 void DwarfCompileUnit::attachRangesOrLowHighPC(
     DIE &Die, const SmallVectorImpl<InsnRange> &Ranges) {
   SmallVector<RangeSpan, 2> List;
   List.reserve(Ranges.size());
   for (const InsnRange &R : Ranges) {

llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp

for (const auto &P : CUMap) {
     // 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;
 
     if (unsigned NumRanges = TheCU.getRanges().size()) {
-      if (NumRanges > 1 && useRangesSection())
+      if (NumRanges > 1) {
         // 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().Begin);
+        if (useRangesSection())
           U.attachRangesOrLowHighPC(U.getUnitDie(), TheCU.takeRanges());
+      } else {
+        const auto &E = TheCU.getRanges().front();
+        U.setBaseAddress(E.Begin);
+        U.attachLowHighPC(U.getUnitDie(), E.Begin, E.End);
+      }
     }
 
     // We don't keep track of which addresses are used in which CU so this
     // is a bit pessimistic under LTO.
     if ((HasSplitUnit || getDwarfVersion() >= 5) && !AddrPool.isEmpty())
       U.addAddrTableBase();
 
     if (getDwarfVersion() >= 5) {

llvm/test/DebugInfo/NVPTX/cu-range-hole.ll

 ; CHECK-NEXT: .b8 3                                // DW_AT_name
 ; CHECK-NEXT: .b8 8                                // DW_FORM_string
 ; CHECK-NEXT: .b8 16                               // DW_AT_stmt_list
 ; CHECK-NEXT: .b8 6                                // DW_FORM_data4
 ; CHECK-NEXT: .b8 27                               // DW_AT_comp_dir
 ; CHECK-NEXT: .b8 8                                // DW_FORM_string
 ; CHECK-NEXT: .b8 17                               // DW_AT_low_pc
 ; CHECK-NEXT: .b8 1                                // DW_FORM_addr
-; CHECK-NEXT: .b8 18                               // DW_AT_high_pc
-; CHECK-NEXT: .b8 1                                // DW_FORM_addr
 ; CHECK-NEXT: .b8 0                                // EOM(1)
 ; CHECK-NEXT: .b8 0                                // EOM(2)
 ; CHECK-NEXT: .b8 2                                // Abbreviation Code
 ; CHECK-NEXT: .b8 46                               // DW_TAG_subprogram
 ; CHECK-NEXT: .b8 1                                // DW_CHILDREN_yes
 ; CHECK-NEXT: .b8 17                               // DW_AT_low_pc
 ; CHECK-NEXT: .b8 1                                // DW_FORM_addr
 ; CHECK-NEXT: .b8 18                               // DW_AT_high_pc

 ; CHECK-NEXT: .b8 11                               // DW_AT_byte_size
 ; CHECK-NEXT: .b8 11                               // DW_FORM_data1
 ; CHECK-NEXT: .b8 0                                // EOM(1)
 ; CHECK-NEXT: .b8 0                                // EOM(2)
 ; CHECK-NEXT: .b8 0                                // EOM(3)
 ; CHECK-NEXT: }
 ; CHECK-NEXT: .section .debug_info
 ; CHECK-NEXT: {
-; CHECK-NEXT: .b32 183                             // Length of Unit
+; CHECK-NEXT: .b32 175                             // Length of Unit
 ; CHECK-NEXT: .b8 2                                // DWARF version number
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b32 .debug_abbrev                   // Offset Into Abbrev. Section
 ; CHECK-NEXT: .b8 8                                // Address Size (in bytes)
-; CHECK-NEXT: .b8 1                                // Abbrev [1] 0xb:0xb0 DW_TAG_compile_unit
+; CHECK-NEXT: .b8 1                                // Abbrev [1] 0xb:0xa8 DW_TAG_compile_unit
 ; CHECK-NEXT: .b8 99                               // DW_AT_producer
 ; CHECK-NEXT: .b8 108
 ; CHECK-NEXT: .b8 97
 ; CHECK-NEXT: .b8 110
 ; CHECK-NEXT: .b8 103
 ; CHECK-NEXT: .b8 32
 ; CHECK-NEXT: .b8 118
 ; CHECK-NEXT: .b8 101

 ; CHECK-NEXT: .b8 47                               // DW_AT_comp_dir
 ; CHECK-NEXT: .b8 115
 ; CHECK-NEXT: .b8 111
 ; CHECK-NEXT: .b8 117
 ; CHECK-NEXT: .b8 114
 ; CHECK-NEXT: .b8 99
 ; CHECK-NEXT: .b8 101
 ; CHECK-NEXT: .b8 0
-; CHECK-NEXT: .b64 Lfunc_begin0                    // DW_AT_low_pc
-; CHECK-NEXT: .b64 Lfunc_end2                      // DW_AT_high_pc
-; CHECK-NEXT: .b8 2                                // Abbrev [2] 0x65:0x27 DW_TAG_subprogram
+; CHECK-NEXT: .b64 0                               // DW_AT_low_pc
+; CHECK-NEXT: .b8 2                                // Abbrev [2] 0x5d:0x27 DW_TAG_subprogram
 ; CHECK-NEXT: .b64 Lfunc_begin0                    // DW_AT_low_pc
 ; CHECK-NEXT: .b64 Lfunc_end0                      // DW_AT_high_pc
 ; CHECK-NEXT: .b8 1                                // DW_AT_frame_base
 ; CHECK-NEXT: .b8 156
 ; CHECK-NEXT: .b8 98                               // DW_AT_name
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_file
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_line
 ; CHECK-NEXT: .b8 1                                // DW_AT_prototyped
-; CHECK-NEXT: .b32 179                             // DW_AT_type
+; CHECK-NEXT: .b32 171                             // DW_AT_type
 ; CHECK-NEXT: .b8 1                                // DW_AT_external
-; CHECK-NEXT: .b8 3                                // Abbrev [3] 0x82:0x9 DW_TAG_formal_parameter
+; CHECK-NEXT: .b8 3                                // Abbrev [3] 0x7a:0x9 DW_TAG_formal_parameter
 ; CHECK-NEXT: .b8 99                               // DW_AT_name
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_file
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_line
-; CHECK-NEXT: .b32 179                             // DW_AT_type
+; CHECK-NEXT: .b32 171                             // DW_AT_type
 ; CHECK-NEXT: .b8 0                                // End Of Children Mark
-; CHECK-NEXT: .b8 2                                // Abbrev [2] 0x8c:0x27 DW_TAG_subprogram
+; CHECK-NEXT: .b8 2                                // Abbrev [2] 0x84:0x27 DW_TAG_subprogram
 ; CHECK-NEXT: .b64 Lfunc_begin2                    // DW_AT_low_pc
 ; CHECK-NEXT: .b64 Lfunc_end2                      // DW_AT_high_pc
 ; CHECK-NEXT: .b8 1                                // DW_AT_frame_base
 ; CHECK-NEXT: .b8 156
 ; CHECK-NEXT: .b8 100                              // DW_AT_name
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_file
 ; CHECK-NEXT: .b8 3                                // DW_AT_decl_line
 ; CHECK-NEXT: .b8 1                                // DW_AT_prototyped
-; CHECK-NEXT: .b32 179                             // DW_AT_type
+; CHECK-NEXT: .b32 171                             // DW_AT_type
 ; CHECK-NEXT: .b8 1                                // DW_AT_external
-; CHECK-NEXT: .b8 3                                // Abbrev [3] 0xa9:0x9 DW_TAG_formal_parameter
+; CHECK-NEXT: .b8 3                                // Abbrev [3] 0xa1:0x9 DW_TAG_formal_parameter
 ; CHECK-NEXT: .b8 101                              // DW_AT_name
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b8 1                                // DW_AT_decl_file
 ; CHECK-NEXT: .b8 3                                // DW_AT_decl_line
-; CHECK-NEXT: .b32 179                             // DW_AT_type
+; CHECK-NEXT: .b32 171                             // DW_AT_type
 ; CHECK-NEXT: .b8 0                                // End Of Children Mark
-; CHECK-NEXT: .b8 4                                // Abbrev [4] 0xb3:0x7 DW_TAG_base_type
+; CHECK-NEXT: .b8 4                                // Abbrev [4] 0xab:0x7 DW_TAG_base_type
 ; CHECK-NEXT: .b8 105                              // DW_AT_name
 ; CHECK-NEXT: .b8 110
 ; CHECK-NEXT: .b8 116
 ; CHECK-NEXT: .b8 0
 ; CHECK-NEXT: .b8 5                                // DW_AT_encoding
 ; CHECK-NEXT: .b8 4                                // DW_AT_byte_size
 ; CHECK-NEXT: .b8 0                                // End Of Children Mark
 ; CHECK-NEXT: }

llvm/test/DebugInfo/X86/no_debug_ranges.ll

-; RUN: llc -filetype=asm -mtriple=x86_64-pc-linux-gnu < %s -o - -dwarf-version=2 -no-dwarf-ranges-section | FileCheck %s --check-prefix=DISABLED
+; RUN: llc -filetype=asm -mtriple=x86_64-pc-linux-gnu < %s -o - -dwarf-version=2 -no-dwarf-ranges-section \
+; RUN:     | FileCheck %s --check-prefix=DISABLED "--implicit-check-not={{DW_AT_ranges|.debug_ranges}}"
 ; RUN: llc -filetype=asm -mtriple=x86_64-pc-linux-gnu < %s -o - -dwarf-version=2 | FileCheck %s
 
-; DISABLED-NOT:  {{DW_AT_ranges|.debug_ranges}}
-; DISABLED:      .section .debug_info
-; DISABLED-NOT:  {{DW_AT_ranges|.section}}
-; DISABLED:      .quad .Lfunc_begin0 # DW_AT_low_pc
+; DISABLED: .section .debug_info
+; DISABLED-NOT: {{DW_AT_low_pc|DW_AT_high_pc}}
+; DISABLED: .quad 0 # DW_AT_low_pc
+; DISABLED-NOT: {{DW_AT_low_pc|DW_AT_high_pc}}
+; DISABLED: .quad .Lfunc_begin0 # DW_AT_low_pc
+; DISABLED-NEXT: .quad .Lfunc_end0   # DW_AT_high_pc
+; DISABLED-NOT: {{DW_AT_low_pc|DW_AT_high_pc}}
+; DISABLED: .quad .Lfunc_begin1 # DW_AT_low_pc
 ; DISABLED-NEXT: .quad .Lfunc_end1   # DW_AT_high_pc
-; DISABLED-NOT:  {{DW_AT_ranges|.debug_ranges}}
+; DISABLED-NOT: {{DW_AT_low_pc|DW_AT_high_pc}}
 
 ; .debug_ranges section must be emitted by default
 ; CHECK: .section .debug_info
 ; CHECK: quad 0 # DW_AT_low_pc
 ; CHECK-NEXT: long [[RANGE0:[.]Ldebug_ranges[0-9]+]] # DW_AT_ranges
 ; CHECK: .debug_ranges
 ; CHECK-NEXT: [[RANGE0]]:
 ; CHECK-NEXT: .quad .Lfunc_begin0