[DEBUG_INFO, NVPTX] Do not emit .debug_loc section.

[ABataev]

Summary

.debug_loc section is not supported for NVPTX target. If there is an
object whose location can change during its lifetime, we do not generate
debug location info for this variable.

[probinson]

This seems rather drastic. Nearly all variables will be reported as "optimized away" at any optimization level above -O0.
Have you looked at whether you can identify a single preferred location for the variable?

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

1201 ↗

(On Diff #153349)

Typos: ... if .debug_loc section ...

[ABataev]

In D48730#1147100, @probinson wrote:

This seems rather drastic. Nearly all variables will be reported as "optimized away" at any optimization level above -O0.
Have you looked at whether you can identify a single preferred location for the variable?

Thanks for the review, Paul. I agree this is not the best solution. But this is how NVidia toolchain works. It does not allow debug info in optimized code. Debug info may be emitted only at -O0. For other optimization levels, only line number info emission is allowed.
I tried to do this kind of analysis and sometimes we can do it, but I don't think this is necessary as debug info is allowed only for non-optimized code.

[ABataev]

In D48730#1147109, @ABataev wrote:

In D48730#1147100, @probinson wrote:

This seems rather drastic. Nearly all variables will be reported as "optimized away" at any optimization level above -O0.
Have you looked at whether you can identify a single preferred location for the variable?

Thanks for the review, Paul. I agree this is not the best solution. But this is how NVidia toolchain works. It does not allow debug info in optimized code. Debug info may be emitted only at -O0. For other optimization levels, only line number info emission is allowed.
I tried to do this kind of analysis and sometimes we can do it, but I don't think this is necessary as debug info is allowed only for non-optimized code.

BTW, I think that because of support of such simple DWARF version the debug info is allowed only at O0. Or vice versa, they're using such simple debug info only because it is allowed at O0 only. I lean to the first, as I think the original problem is in the oversimplified PTX file format.

[ABataev]

Fixed comment.

[probinson]

Okay. Most compilers don't bother with location lists at -O0, and Clang is among them. So this should be fine for -O0 and won't break your tools at higher optimization. LGTM.

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

class DwarfDebug : public DebugHandlerBase {
 
   /// Allow emission of .debug_ranges section.
   bool UseRangesSection = true;
 
   /// True if the sections itself must be used as references and don't create
   /// temp symbols inside DWARF sections.
   bool UseSectionsAsReferences = false;
 
+  ///Allow emission of the .debug_loc section.
+  bool UseLocSection = true;
+
   /// DWARF5 Experimental Options
   /// @{
   AccelTableKind TheAccelTableKind;
   bool HasAppleExtensionAttributes;
   bool HasSplitDwarf;
 
   /// Whether to generate the DWARF v5 string offsets table.
   /// It consists of a series of contributions, each preceded by a header.
public:
 
   /// Returns whether to use the DWARF2 format for bitfields instyead of the
   /// DWARF4 format.
   bool useDWARF2Bitfields() const { return UseDWARF2Bitfields; }
 
   /// Returns whether to use inline strings.
   bool useInlineStrings() const { return UseInlineStrings; }
 
-  /// Returns whether GNU oub sections should be emitted.
+  /// Returns whether GNU pub sections should be emitted.
   bool usePubSections() const { return UsePubSections; }
 
   /// Returns whether ranges section should be emitted.
   bool useRangesSection() const { return UseRangesSection; }
 
   /// Returns whether to use sections as labels rather than temp symbols.
   bool useSectionsAsReferences() const {
     return UseSectionsAsReferences;
   }
 
+  /// Returns whether .debug_loc section should be emitted.
+  bool useLocSection() const { return UseLocSection; }
+
   // Experimental DWARF5 features.
 
   /// Returns what kind (if any) of accelerator tables to emit.
   AccelTableKind getAccelTableKind() const { return TheAccelTableKind; }
 
   bool useAppleExtensionAttributes() const {
     return HasAppleExtensionAttributes;
   }

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

DwarfDebug::DwarfDebug(AsmPrinter *A, Module *M)
   } else
     TheAccelTableKind = AccelTables;
 
   if (DwarfInlinedStrings == Default)
     UseInlineStrings = TT.isNVPTX();
   else
     UseInlineStrings = DwarfInlinedStrings == Enable;
 
+  UseLocSection = !TT.isNVPTX();
+
   HasAppleExtensionAttributes = tuneForLLDB();
 
   // Handle split DWARF.
   HasSplitDwarf = !Asm->TM.Options.MCOptions.SplitDwarfFile.empty();
 
   // SCE defaults to linkage names only for abstract subprograms.
   if (DwarfLinkageNames == DefaultLinkageNames)
     UseAllLinkageNames = !tuneForSCE();
for (const auto &I : DbgValues) {
     assert(MInsn->isDebugValue() && "History must begin with debug value");
 
     // Check if there is a single DBG_VALUE, valid throughout the var's scope.
     if (Ranges.size() == 1 &&
         validThroughout(LScopes, MInsn, Ranges.front().second)) {
       RegVar->initializeDbgValue(MInsn);
       continue;
     }
+    // Do not emit location lists if .debug_loc secton is disabled.
+    if (!useLocSection())
+      continue;
 
     // Handle multiple DBG_VALUE instructions describing one variable.
     DebugLocStream::ListBuilder List(DebugLocs, TheCU, *Asm, *RegVar, *MInsn);
 
     // Build the location list for this variable.
     SmallVector<DebugLocEntry, 8> Entries;
     buildLocationList(Entries, Ranges);
 

llvm/trunk/test/DebugInfo/NVPTX/debug-loc-offset.ll

 !28 = !DILocation(line: 8, scope: !29)
 !29 = distinct !DILexicalBlock(line: 8, column: 0, file: !10, scope: !14)
 !30 = !DILocation(line: 9, scope: !29)
 !31 = !DILocation(line: 10, scope: !32)
 !32 = distinct !DILexicalBlock(line: 10, column: 0, file: !10, scope: !14)
 !33 = !DILocation(line: 11, scope: !32)
 !34 = !DILocation(line: 12, scope: !14)
 
-
-; CHECK: // .section .debug_loc
-; CHECK: // {
-; CHECK: Ldebug_loc0:
-; CHECK: // .b64 Lfunc_begin0-Lfunc_begin0
-; CHECK: // .b64 Ltmp0-Lfunc_begin0
-; CHECK: // .b8 2                                // Loc expr size
-; CHECK: // .b8 0
-; CHECK: // .b8 17                               // DW_OP_consts
-; CHECK: // .b8 0                                // 0
-; CHECK: // .b64 Ltmp0-Lfunc_begin0
-; CHECK: // .b64 Lfunc_end0-Lfunc_begin0
-; CHECK: // .b8 2                                // Loc expr size
-; CHECK: // .b8 0
-; CHECK: // .b8 17                               // DW_OP_consts
-; CHECK: // .b8 1                                // 1
-; CHECK: // .b64 0
-; CHECK: // .b64 0
-; CHECK: Ldebug_loc1:
-; CHECK: // .b64 Ltmp2-Lfunc_begin1
-; CHECK: // .b64 Ltmp2-Lfunc_begin1
-; CHECK: // .b8 2                                // Loc expr size
-; CHECK: // .b8 0
-; CHECK: // .b8 17                               // DW_OP_consts
-; CHECK: // .b8 2                                // 2
-; CHECK: // .b64 Ltmp2-Lfunc_begin1
-; CHECK: // .b64 Lfunc_end1-Lfunc_begin1
-; CHECK: // .b8 2                                // Loc expr size
-; CHECK: // .b8 0
-; CHECK: // .b8 17                               // DW_OP_consts
-; CHECK: // .b8 3                                // 3
-; CHECK: // .b64 0
-; CHECK: // .b64 0
-; CHECK: // }
 ; CHECK: // .section .debug_abbrev
 ; CHECK: // {
 ; CHECK: // .b8 1                                // Abbreviation Code
 ; CHECK: // .b8 17                               // DW_TAG_compile_unit
 ; CHECK: // .b8 1                                // DW_CHILDREN_yes
 ; CHECK: // .b8 37                               // DW_AT_producer
 ; CHECK: // .b8 8                                // DW_FORM_string
 ; CHECK: // .b8 19                               // DW_AT_language

 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 63                               // DW_AT_external
 ; CHECK: // .b8 12                               // DW_FORM_flag
 ; CHECK: // .b8 0                                // EOM(1)
 ; CHECK: // .b8 0                                // EOM(2)
 ; CHECK: // .b8 4                                // Abbreviation Code
 ; CHECK: // .b8 52                               // DW_TAG_variable
 ; CHECK: // .b8 0                                // DW_CHILDREN_no
-; CHECK: // .b8 2                                // DW_AT_location
-; CHECK: // .b8 6                                // DW_FORM_data4
 ; CHECK: // .b8 3                                // DW_AT_name
 ; CHECK: // .b8 8                                // DW_FORM_string
 ; CHECK: // .b8 58                               // DW_AT_decl_file
 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 59                               // DW_AT_decl_line
 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 73                               // DW_AT_type
 ; CHECK: // .b8 16                               // DW_FORM_ref_addr

 ; CHECK: // .b8 19                               // DW_FORM_ref4
 ; CHECK: // .b8 63                               // DW_AT_external
 ; CHECK: // .b8 12                               // DW_FORM_flag
 ; CHECK: // .b8 0                                // EOM(1)
 ; CHECK: // .b8 0                                // EOM(2)
 ; CHECK: // .b8 6                                // Abbreviation Code
 ; CHECK: // .b8 5                                // DW_TAG_formal_parameter
 ; CHECK: // .b8 0                                // DW_CHILDREN_no
-; CHECK: // .b8 2                                // DW_AT_location
-; CHECK: // .b8 6                                // DW_FORM_data4
 ; CHECK: // .b8 3                                // DW_AT_name
 ; CHECK: // .b8 8                                // DW_FORM_string
 ; CHECK: // .b8 58                               // DW_AT_decl_file
 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 59                               // DW_AT_decl_line
 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 73                               // DW_AT_type
 ; CHECK: // .b8 19                               // DW_FORM_ref4

 ; CHECK: // .b8 11                               // DW_AT_byte_size
 ; CHECK: // .b8 11                               // DW_FORM_data1
 ; CHECK: // .b8 0                                // EOM(1)
 ; CHECK: // .b8 0                                // EOM(2)
 ; CHECK: // .b8 0                                // EOM(3)
 ; CHECK: // }
 ; CHECK: // .section .debug_info
 ; CHECK: // {
-; CHECK: // .b32 152                             // Length of Unit
+; CHECK: // .b32 148                             // Length of Unit
 ; CHECK: // .b8 2                                // DWARF version number
 ; CHECK: // .b8 0
 ; CHECK: // .b32 .debug_abbrev                   // Offset Into Abbrev. Section
 ; CHECK: // .b8 8                                // Address Size (in bytes)
-; CHECK: // .b8 1                                // Abbrev [1] 0xb:0x91 DW_TAG_compile_unit
+; CHECK: // .b8 1                                // Abbrev [1] 0xb:0x8d DW_TAG_compile_unit
 ; CHECK: // .b8 99                               // DW_AT_producer
 ; CHECK: // .b8 108
 ; CHECK: // .b8 97
 ; CHECK: // .b8 110
 ; CHECK: // .b8 103
 ; CHECK: // .b8 32
 ; CHECK: // .b8 118
 ; CHECK: // .b8 101

 ; CHECK: // .b8 99
 ; CHECK: // .b8 0
 ; CHECK: // .b64 Lfunc_begin1                    // DW_AT_low_pc
 ; CHECK: // .b64 Lfunc_end1                      // DW_AT_high_pc
 ; CHECK: // .b8 2                                // Abbrev [2] 0x64:0x4 DW_TAG_structure_type
 ; CHECK: // .b8 65                               // DW_AT_name
 ; CHECK: // .b8 0
 ; CHECK: // .b8 1                                // DW_AT_declaration
-; CHECK: // .b8 3                                // Abbrev [3] 0x68:0x33 DW_TAG_subprogram
+; CHECK: // .b8 3                                // Abbrev [3] 0x68:0x2f DW_TAG_subprogram
 ; CHECK: // .b64 Lfunc_begin1                    // DW_AT_low_pc
 ; CHECK: // .b64 Lfunc_end1                      // DW_AT_high_pc
 ; CHECK: // .b8 95                               // DW_AT_MIPS_linkage_name
 ; CHECK: // .b8 90
 ; CHECK: // .b8 51
 ; CHECK: // .b8 98
 ; CHECK: // .b8 97
 ; CHECK: // .b8 122
 ; CHECK: // .b8 49
 ; CHECK: // .b8 65
 ; CHECK: // .b8 0
 ; CHECK: // .b8 98                               // DW_AT_name
 ; CHECK: // .b8 97
 ; CHECK: // .b8 122
 ; CHECK: // .b8 0
 ; CHECK: // .b8 2                                // DW_AT_decl_file
 ; CHECK: // .b8 6                                // DW_AT_decl_line
 ; CHECK: // .b8 1                                // DW_AT_external
-; CHECK: // .b8 4                                // Abbrev [4] 0x89:0x11 DW_TAG_variable
-; CHECK: // .b32 Ldebug_loc1                     // DW_AT_location
+; CHECK: // .b8 4                                // Abbrev [4] 0x89:0xd DW_TAG_variable
 ; CHECK: // .b8 122                              // DW_AT_name
 ; CHECK: // .b8 0
 ; CHECK: // .b8 2                                // DW_AT_decl_file
 ; CHECK: // .b8 7                                // DW_AT_decl_line
-; CHECK: // .b64 .debug_info+306                 // DW_AT_type
+; CHECK: // .b64 .debug_info+298                 // DW_AT_type
 ; CHECK: // .b8 0                                // End Of Children Mark
 ; CHECK: // .b8 0                                // End Of Children Mark
-; CHECK: // .b32 154                             // Length of Unit
+; CHECK: // .b32 150                             // Length of Unit
 ; CHECK: // .b8 2                                // DWARF version number
 ; CHECK: // .b8 0
 ; CHECK: // .b32 .debug_abbrev                   // Offset Into Abbrev. Section
 ; CHECK: // .b8 8                                // Address Size (in bytes)
-; CHECK: // .b8 1                                // Abbrev [1] 0xb:0x93 DW_TAG_compile_unit
+; CHECK: // .b8 1                                // Abbrev [1] 0xb:0x8f DW_TAG_compile_unit
 ; CHECK: // .b8 99                               // DW_AT_producer
 ; CHECK: // .b8 108
 ; CHECK: // .b8 97
 ; CHECK: // .b8 110
 ; CHECK: // .b8 103
 ; CHECK: // .b8 32
 ; CHECK: // .b8 118
 ; CHECK: // .b8 101

 ; CHECK: // .b8 101
 ; CHECK: // .b8 95
 ; CHECK: // .b8 103
 ; CHECK: // .b8 99
 ; CHECK: // .b8 99
 ; CHECK: // .b8 0
 ; CHECK: // .b64 Lfunc_begin0                    // DW_AT_low_pc
 ; CHECK: // .b64 Lfunc_end0                      // DW_AT_high_pc
-; CHECK: // .b8 5                                // Abbrev [5] 0x64:0x32 DW_TAG_subprogram
+; CHECK: // .b8 5                                // Abbrev [5] 0x64:0x2e DW_TAG_subprogram
 ; CHECK: // .b64 Lfunc_begin0                    // DW_AT_low_pc
 ; CHECK: // .b64 Lfunc_end0                      // DW_AT_high_pc
 ; CHECK: // .b8 95                               // DW_AT_MIPS_linkage_name
 ; CHECK: // .b8 90
 ; CHECK: // .b8 51
 ; CHECK: // .b8 98
 ; CHECK: // .b8 97
 ; CHECK: // .b8 114
 ; CHECK: // .b8 105
 ; CHECK: // .b8 0
 ; CHECK: // .b8 98                               // DW_AT_name
 ; CHECK: // .b8 97
 ; CHECK: // .b8 114
 ; CHECK: // .b8 0
 ; CHECK: // .b8 1                                // DW_AT_decl_file
 ; CHECK: // .b8 1                                // DW_AT_decl_line
-; CHECK: // .b32 150                             // DW_AT_type
+; CHECK: // .b32 146                             // DW_AT_type
 ; CHECK: // .b8 1                                // DW_AT_external
-; CHECK: // .b8 6                                // Abbrev [6] 0x88:0xd DW_TAG_formal_parameter
-; CHECK: // .b32 Ldebug_loc0                     // DW_AT_location
+; CHECK: // .b8 6                                // Abbrev [6] 0x88:0x9 DW_TAG_formal_parameter
 ; CHECK: // .b8 98                               // DW_AT_name
 ; CHECK: // .b8 0
 ; CHECK: // .b8 1                                // DW_AT_decl_file
 ; CHECK: // .b8 1                                // DW_AT_decl_line
-; CHECK: // .b32 150                             // DW_AT_type
+; CHECK: // .b32 146                             // DW_AT_type
 ; CHECK: // .b8 0                                // End Of Children Mark
-; CHECK: // .b8 7                                // Abbrev [7] 0x96:0x7 DW_TAG_base_type
+; CHECK: // .b8 7                                // Abbrev [7] 0x92:0x7 DW_TAG_base_type
 ; CHECK: // .b8 105                              // DW_AT_name
 ; CHECK: // .b8 110
 ; CHECK: // .b8 116
 ; CHECK: // .b8 0
 ; CHECK: // .b8 5                                // DW_AT_encoding
 ; CHECK: // .b8 4                                // DW_AT_byte_size
 ; CHECK: // .b8 0                                // End Of Children Mark
 ; CHECK: // }
 ; CHECK: // .section .debug_macinfo
 ; CHECK: // {
 ; CHECK: // .b8 0                                // End Of Macro List Mark
 ; CHECK: // }