Don't add DBG_VALUE instructions for static allocas in dbg.declare

[rnk]

Summary

An llvm.dbg.declare of a static alloca is always added to the
MachineFunction dbg variable map, so these values are entirely
redundant. They survive all the way through codegen to be ignored by
DWARF emission.

Effectively revert r113967

Two bugpoint-reduced test cases from 2012 broke as a result of this
change. Despite my best efforts, I haven't been able to rewrite the test
case using dbg.value. I'm not too concerned about the lost coverage
because these were reduced from the test-suite, which we still run.

[aprantl]

An llvm.dbg.declare of a static alloca is always added to the
MachineFunction dbg variable map, so these values are entirely
redundant.

Makes sense so far.

They survive all the way through codegen to be ignored by DWARF emission.

Why are they ignored? Shouldn't they turn the variable location into a location list that has the FI from the MMI side table *and* the DEBUG_VALUE?

[rnk]

In D32920#747642, @aprantl wrote:

They survive all the way through codegen to be ignored by DWARF emission.

Why are they ignored? Shouldn't they turn the variable location into a location list that has the FI from the MMI side table *and* the DEBUG_VALUE?

It doesn't really make sense to build a location list for a variable that lives in a static alloca described with dbg.declare. There's only ever one location in memory where the variable lives. Extra instructions describing the location are kind of superfluous.

The commit message for rL113967 actually acknowledges that this code shouldn't do anything. I think it's just that whatever Devang wanted to do with this code never came to pass: "Right now, this has no material impact because varible info also collected using offset table maintained in machine module info."

[aprantl]

I totally understand that it doesn't need to be tracked in the MMI table and in a DEBUG_VALUE, my question was that it sounded to my like you implied that the DWARF backend actively ignores DEBUG_VALUEs pointing to static allocas which I found really surprising (as there are legitimate reasons for allocas to be described by a DEBUG_VALUE) .

For example:

void f(int i) {
  int x = i;
   if (x == 0)
     // DEBUG_VALUE x, const 0
     return bar();
   // DEBUG_VALUE x, FI-1
   baz(&x);
}

[aprantl]

This does not really have anything to do with the code in this patch though. I just really want to understand it.

[rnk]

In D32920#747734, @aprantl wrote:

I totally understand that it doesn't need to be tracked in the MMI table and in a DEBUG_VALUE, my question was that it sounded to my like you implied that the DWARF backend actively ignores DEBUG_VALUEs pointing to static allocas which I found really surprising (as there are legitimate reasons for allocas to be described by a DEBUG_VALUE) .

The backend doesn't ignore DEBUG_VALUEs pointing to static allocas, it ignores DEBUG_VALUEs when there is an entry for the same variable in the dbg.declare side table. It's this code:

void DwarfDebug::collectVariableInfo(DwarfCompileUnit &TheCU,
                                     const DISubprogram *SP,
                                     DenseSet<InlinedVariable> &Processed) {
  // Grab the variable info that was squirreled away in the MMI side-table.
  collectVariableInfoFromMFTable(Processed);

  for (const auto &I : DbgValues) {
    InlinedVariable IV = I.first;
    if (Processed.count(IV))
      continue;

[aprantl]

Interesting, right!

• adrian

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

if (N.getNode()) {
                               true, 0, dl, SDNodeOrder);
       }
       DAG.AddDbgValue(SDV, N.getNode(), isParameter);
     } else {
       // If Address is an argument then try to emit its dbg value using
       // virtual register info from the FuncInfo.ValueMap.
       if (!EmitFuncArgumentDbgValue(Address, Variable, Expression, dl, 0, true,
                                     N)) {
-        // If variable is pinned by a alloca in dominating bb then
-        // use StaticAllocaMap.
-        if (const AllocaInst *AI = dyn_cast<AllocaInst>(Address)) {
-          if (AI->getParent() != DI.getParent()) {
-            DenseMap<const AllocaInst*, int>::iterator SI =
-              FuncInfo.StaticAllocaMap.find(AI);
-            if (SI != FuncInfo.StaticAllocaMap.end()) {
-              SDV = DAG.getFrameIndexDbgValue(Variable, Expression, SI->second,
-                                              0, dl, SDNodeOrder);
-              DAG.AddDbgValue(SDV, nullptr, false);
-              return nullptr;
-            }
-          }
-        }
         DEBUG(dbgs() << "Dropping debug info for " << DI << "\n");
       }
     }
     return nullptr;
   }
   case Intrinsic::dbg_value: {
     const DbgValueInst &DI = cast<DbgValueInst>(I);
     assert(DI.getVariable() && "Missing variable");

llvm/test/CodeGen/X86/2012-11-30-misched-dbg.ll

-; RUN: llc < %s -mtriple=x86_64-apple-macosx -enable-misched \
-; RUN:          -verify-machineinstrs | FileCheck %s
-;
-; Test MachineScheduler handling of DBG_VALUE.
-; rdar://12776937.
-;
-; CHECK: %if.else581
-; CHECK: DEBUG_VALUE: num1
-; CHECK: call
-
-%union.rec = type {}
-
-@.str15 = external hidden unnamed_addr constant [6 x i8], align 1
-
-declare void @llvm.dbg.declare(metadata, metadata, metadata) nounwind readnone
-
-define i32 @AttachGalley(%union.rec** nocapture %suspend_pt) nounwind uwtable ssp !dbg !21 {
-entry:
-  %num14075 = alloca [20 x i8], align 16
-  br label %if.end33
-
-if.end33:                                         ; preds = %entry
-  %cmp1733 = icmp eq i32 undef, 0
-  br label %if.else581
-
-if.else581:                                       ; preds = %if.end33
-  %cmp586 = icmp eq i8 undef, -123
-  br i1 %cmp586, label %if.then588, label %if.else594
-
-if.then588:                                       ; preds = %if.else581
-  br label %for.cond1710.preheader
-
-if.else594:                                       ; preds = %if.else581
-  unreachable
-
-for.cond1710.preheader:                           ; preds = %if.then588
-  br label %for.cond1710
-
-for.cond1710:                                     ; preds = %for.cond1710, %for.cond1710.preheader
-  br i1 undef, label %for.cond1710, label %if.then3344
-
-if.then3344:
-  br label %if.then4073
-
-if.then4073:                                      ; preds = %if.then3344
-  call void @llvm.dbg.declare(metadata [20 x i8]* %num14075, metadata !4, metadata !DIExpression()), !dbg !DILocation(scope: !5)
-  %arraydecay4078 = getelementptr inbounds [20 x i8], [20 x i8]* %num14075, i64 0, i64 0
-  %0 = load i32, i32* undef, align 4
-  %add4093 = add nsw i32 %0, 0
-  %conv4094 = sitofp i32 %add4093 to float
-  %div4095 = fdiv float %conv4094, 5.670000e+02
-  %conv4096 = fpext float %div4095 to double
-  %call4097 = call i32 (i8*, i32, i64, i8*, ...) @__sprintf_chk(i8* %arraydecay4078, i32 0, i64 20, i8* getelementptr inbounds ([6 x i8], [6 x i8]* @.str15, i64 0, i64 0), double %conv4096) nounwind
-  br i1 %cmp1733, label %if.then4107, label %if.else4114
-
-if.then4107:                                      ; preds = %if.then4073
-  unreachable
-
-if.else4114:                                      ; preds = %if.then4073
-  unreachable
-}
-
-declare i32 @__sprintf_chk(i8*, i32, i64, i8*, ...)
-
-!llvm.dbg.cu = !{!0}
-!llvm.module.flags = !{!35}
-
-!0 = distinct !DICompileUnit(language: DW_LANG_C99, producer: "clang version 3.3 (trunk 168918) (llvm/trunk 168920)", isOptimized: true, emissionKind: FullDebug, file: !19, enums: !2, retainedTypes: !2, globals: !2)
-!1 = !{!2}
-!2 = !{}
-!4 = !DILocalVariable(name: "num1", line: 815, scope: !5, file: !14, type: !15)
-!5 = distinct !DILexicalBlock(line: 815, column: 0, file: !14, scope: !6)
-!6 = distinct !DILexicalBlock(line: 812, column: 0, file: !14, scope: !7)
-!7 = distinct !DILexicalBlock(line: 807, column: 0, file: !14, scope: !8)
-!8 = distinct !DILexicalBlock(line: 440, column: 0, file: !14, scope: !9)
-!9 = distinct !DILexicalBlock(line: 435, column: 0, file: !14, scope: !10)
-!10 = distinct !DILexicalBlock(line: 434, column: 0, file: !14, scope: !11)
-!11 = distinct !DILexicalBlock(line: 250, column: 0, file: !14, scope: !12)
-!12 = distinct !DILexicalBlock(line: 249, column: 0, file: !14, scope: !13)
-!13 = distinct !DILexicalBlock(line: 221, column: 0, file: !14, scope: !21)
-!14 = !DIFile(filename: "MultiSource/Benchmarks/MiBench/consumer-typeset/z19.c", directory: "MultiSource/Benchmarks/MiBench/consumer-typeset")
-!15 = !DICompositeType(tag: DW_TAG_array_type, size: 160, align: 8, baseType: !16, elements: !17)
-!16 = !DIBasicType(tag: DW_TAG_base_type, name: "char", size: 8, align: 8, encoding: DW_ATE_signed_char)
-!17 = !{!18}
-!18 = !DISubrange(count: 20)
-!19 = !DIFile(filename: "MultiSource/Benchmarks/MiBench/consumer-typeset/z19.c", directory: "MultiSource/Benchmarks/MiBench/consumer-typeset")
-
-!21 = distinct !DISubprogram(name: "AttachGalley", isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !0, scopeLine: 1, file: !19, scope: !14, type: !22)
-!22 = !DISubroutineType(types: !23)
-!23 = !{null}
-
-; Test DebugValue uses visited by RegisterPressureTracker findUseBetween().
-;
-; CHECK: @main
-; CHECK: DEBUG_VALUE: main:X
-; CHECK: call
-
-%"class.__gnu_cxx::hash_map" = type { %"class.__gnu_cxx::hashtable" }
-%"class.__gnu_cxx::hashtable" = type { i64, i64, i64, i64, i64, i64 }
-
-define void @main() uwtable ssp personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*) !dbg !37 {
-entry:
-  %X = alloca %"class.__gnu_cxx::hash_map", align 8
-  br i1 undef, label %cond.true, label %cond.end
-
-cond.true:                                        ; preds = %entry
-  unreachable
-
-cond.end:                                         ; preds = %entry
-  call void @llvm.dbg.declare(metadata %"class.__gnu_cxx::hash_map"* %X, metadata !31, metadata !DIExpression()), !dbg !DILocation(scope: !37)
-  %_M_num_elements.i.i.i.i = getelementptr inbounds %"class.__gnu_cxx::hash_map", %"class.__gnu_cxx::hash_map"* %X, i64 0, i32 0, i32 5
-  invoke void @_Znwm()
-          to label %exit.i unwind label %lpad2.i.i.i.i
-
-exit.i:                                           ; preds = %cond.end
-  unreachable
-
-lpad2.i.i.i.i:                                    ; preds = %cond.end
-  %0 = landingpad { i8*, i32 }
-          cleanup
-  br i1 undef, label %lpad.body.i.i, label %if.then.i.i.i.i.i.i.i.i
-
-if.then.i.i.i.i.i.i.i.i:                          ; preds = %lpad2.i.i.i.i
-  unreachable
-
-lpad.body.i.i:                                    ; preds = %lpad2.i.i.i.i
-  resume { i8*, i32 } %0
-}
-
-declare i32 @__gxx_personality_v0(...)
-
-declare void @_Znwm()
-
-!llvm.dbg.cu = !{!30}
-
-!30 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.3 (trunk 169129) (llvm/trunk 169135)", isOptimized: true, emissionKind: FullDebug, file: !34, enums: !2, retainedTypes: !2)
-!31 = !DILocalVariable(name: "X", line: 29, scope: !37, type: !32)
-!32 = !DIDerivedType(tag: DW_TAG_typedef, name: "HM", line: 28, file: !34, baseType: null)
-!33 = !DIFile(filename: "SingleSource/Benchmarks/Shootout-C++/hash.cpp", directory: "SingleSource/Benchmarks/Shootout-C++")
-!34 = !DIFile(filename: "SingleSource/Benchmarks/Shootout-C++/hash.cpp", directory: "SingleSource/Benchmarks/Shootout-C++")
-!35 = !{i32 1, !"Debug Info Version", i32 3}
-!37 = distinct !DISubprogram(name: "main", isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !30, scopeLine: 1, file: !19, scope: !14, type: !22)

llvm/test/CodeGen/X86/2012-11-30-regpres-dbg.ll

-; RUN: llc < %s -mtriple=x86_64-apple-macosx -enable-misched \
-; RUN:          -verify-machineinstrs | FileCheck %s
-;
-; Test RegisterPressure handling of DBG_VALUE.
-;
-; CHECK: %entry
-; CHECK: DEBUG_VALUE: test:callback
-; CHECK: ret
-
-%struct.btCompoundLeafCallback = type { i32, i32 }
-
-declare void @llvm.dbg.declare(metadata, metadata, metadata) nounwind readnone
-
-define void @test() unnamed_addr uwtable ssp align 2 !dbg !2 {
-entry:
-  %callback = alloca %struct.btCompoundLeafCallback, align 8
-  br i1 undef, label %if.end, label %if.then
-
-if.then:                                          ; preds = %entry
-  unreachable
-
-if.end:                                           ; preds = %entry
-  call void @llvm.dbg.declare(metadata %struct.btCompoundLeafCallback* %callback, metadata !3, metadata !DIExpression()), !dbg !DILocation(scope: !2)
-  %m = getelementptr inbounds %struct.btCompoundLeafCallback, %struct.btCompoundLeafCallback* %callback, i64 0, i32 1
-  store i32 0, i32* undef, align 8
-  %cmp12447 = icmp sgt i32 undef, 0
-  br i1 %cmp12447, label %for.body.lr.ph, label %invoke.cont44
-
-for.body.lr.ph:                                   ; preds = %if.end
-  unreachable
-
-invoke.cont44:                                    ; preds = %if.end
-  ret void
-}
-
-!llvm.dbg.cu = !{!0}
-!llvm.module.flags = !{!8}
-
-!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.3 (trunk 168984) (llvm/trunk 168983)", isOptimized: true, emissionKind: FullDebug, file: !6)
-!2 = distinct !DISubprogram(name: "test", isLocal: false, isDefinition: true, virtualIndex: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !0, scopeLine: 1, file: !6, scope: !5, type: !7)
-!3 = !DILocalVariable(name: "callback", line: 214, scope: !2, type: !4)
-!4 = !DICompositeType(tag: DW_TAG_structure_type, name: "btCompoundLeafCallback", line: 90, size: 64, align: 64, file: !6)
-!5 = !DIFile(filename: "MultiSource/Benchmarks/Bullet/btCompoundCollisionAlgorithm.cpp", directory: "MultiSource/Benchmarks/Bullet")
-!6 = !DIFile(filename: "MultiSource/Benchmarks/Bullet/btCompoundCollisionAlgorithm.cpp", directory: "MultiSource/Benchmarks/Bullet")
-!7 = !DISubroutineType(types: !9)
-!8 = !{i32 1, !"Debug Info Version", i32 3}
-!9 = !{null}

llvm/test/DebugInfo/COFF/local-variables.ll

 
 ; ASM: f:                                      # @f
 ; ASM:         .cv_func_id 0
 ; ASM:         .cv_file        1 "D:\\src\\llvm\\build\\t.cpp"
 ; ASM:         .cv_loc 0 1 7 0 is_stmt 0       # t.cpp:7:0
 ; ASM: .seh_proc f
 ; ASM: # BB#0:                                 # %entry
 ; ASM:         subq    $56, %rsp
-; ASM:         #DEBUG_VALUE: f:param <- [%RSP+52]
 ; ASM:         movl    %ecx, 52(%rsp)
 ; ASM: [[prologue_end:\.Ltmp.*]]:
 ; ASM:         .cv_loc 0 1 8 7                 # t.cpp:8:7
 ; ASM:         testl   %ecx, %ecx
 ; ASM:         je      .LBB0_2
 ; ASM: [[if_start:\.Ltmp.*]]:
 ; ASM: # BB#1:                                 # %if.then
-; ASM:         #DEBUG_VALUE: f:param <- [%RSP+52]
-; ASM:         #DEBUG_VALUE: a <- [%RSP+40]
 ; ASM:         .cv_loc 0 1 9 9                 # t.cpp:9:9
 ; ASM:         movl    $42, 40(%rsp)
 ; ASM: [[inline_site1:\.Ltmp.*]]:
 ; ASM:         .cv_inline_site_id 1 within 0 inlined_at 1 10 5
 ; ASM:         .cv_loc 1 1 4 7                 # t.cpp:4:7
 ; ASM:         movl    $3, 44(%rsp)
 ; ASM:         leaq    44(%rsp), %rcx
 ; ASM:         .cv_loc 1 1 5 3                 # t.cpp:5:3
 ; ASM:         callq   capture
 ; ASM:         leaq    40(%rsp), %rcx
 ; ASM:         jmp     .LBB0_3
 ; ASM: [[else_start:\.Ltmp.*]]:
 ; ASM: .LBB0_2:                                # %if.else
-; ASM:         #DEBUG_VALUE: f:param <- [%RSP+52]
-; ASM:         #DEBUG_VALUE: b <- [%RSP+36]
 ; ASM:         .cv_loc 0 1 13 9                # t.cpp:13:9
 ; ASM:         movl    $42, 36(%rsp)
 ; ASM: [[inline_site2:\.Ltmp.*]]:
 ; ASM:         .cv_inline_site_id 2 within 0 inlined_at 1 14 5
 ; ASM:         .cv_loc 2 1 4 7                 # t.cpp:4:7
 ; ASM:         movl    $3, 48(%rsp)
 ; ASM:         leaq    48(%rsp), %rcx
 ; ASM:         .cv_loc 2 1 5 3                 # t.cpp:5:3