[llvm-dwarfdump][Statistics] Ignore DW_TAG_subroutine_type in statistics

[krisb]

Summary

DW_TAG_subroutine_type is not really useful for statistics purposes, as it never has location information. But it may contain DW_TAG_formal_parameter children that generate a number of parameters w/o location and decrease 'availability' metric significantly.

[djtodoro]

Looks good to me.

llvm/test/tools/llvm-dwarfdump/X86/statistics.ll

 ; RUN: llc -O0 %s -o - -filetype=obj \
 ; RUN:   | llvm-dwarfdump -statistics - | FileCheck %s
 ; CHECK: "version":4
 
 ; int GlobalConst = 42;
 ; int Global;
 ;
 ; struct S {
 ;   static const int constant = 24;
+;   int (*fn)(int);
 ; } s;
 ;
 ; int __attribute__((always_inline)) square(int i) { return i * i; }
 ; int cube(int i) {
 ;   int squared = square(i);
 ;   return squared*i;
 ; }
 
-; GlobalConst,Global,s,s.constant,square::i,cube::i,cube::squared
+; Following variables/arguments/members should be counted:
+;     - GlobalConst,
+;     - Global,
+;     - s, s.constant ('fn' and its arguments should be skipped),
+;     - square::i,
+;     - cube::i, cube::squared
+
 ; CHECK: "unique source variables":7
 ; +1 extra inline i.
 ; CHECK: "source variables":8
 ; -1 square::i
 ; CHECK: "variables with location":7
 ; CHECK: "scope bytes total":[[BYTES:[0-9]+]]
 ; Because of the dbg.value in the middle of the function, the pc range coverage
 ; must be below 100%.
 ; CHECK-NOT: "scope bytes covered":0
 ; CHECK-NOT "scope bytes covered":[[BYTES]]
 ; CHECK: "scope bytes covered":
 ; CHECK: "total function size":[[FUNCSIZE:[0-9]+]]
 ; CHECK: "total inlined function size":[[INLINESIZE:[0-9]+]]
 
 
 ; ModuleID = '/tmp/quality.cpp'
 source_filename = "/tmp/quality.cpp"
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-apple-macosx10.12.0"
 
-%struct.S = type { i8 }
+%struct.S = type { i8 (i32)* }
 
 @GlobalConst = global i32 42, align 4, !dbg !0
 @Global = global i32 0, align 4, !dbg !6
 @s = global %struct.S zeroinitializer, align 1, !dbg !9
 
 ; Function Attrs: alwaysinline nounwind ssp uwtable
 define i32 @_Z6squarei(i32 %i) #0 !dbg !20 {
 entry:

 !4 = !{}
 !5 = !{!0, !6, !9}
 !6 = !DIGlobalVariableExpression(var: !7, expr: !DIExpression())
 !7 = distinct !DIGlobalVariable(name: "Global", scope: !2, file: !3, line: 2, type: !8, isLocal: false, isDefinition: true)
 !8 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
 !9 = !DIGlobalVariableExpression(var: !10, expr: !DIExpression())
 !10 = distinct !DIGlobalVariable(name: "s", scope: !2, file: !3, line: 6, type: !11, isLocal: false, isDefinition: true)
 !11 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !3, line: 4, size: 8, elements: !12, identifier: "_ZTS1S")
-!12 = !{!13}
+!12 = !{!13, !45}
 !13 = !DIDerivedType(tag: DW_TAG_member, name: "constant", scope: !11, file: !3, line: 5, baseType: !14, flags: DIFlagStaticMember, extraData: i32 24)
 !14 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !8)
 !15 = !{i32 2, !"Dwarf Version", i32 4}
 !16 = !{i32 2, !"Debug Info Version", i32 3}
 !17 = !{i32 1, !"wchar_size", i32 4}
 !18 = !{i32 7, !"PIC Level", i32 2}
 !19 = !{!"clang version 6.0.0 (trunk 310529) (llvm/trunk 310534)"}
 !20 = distinct !DISubprogram(name: "square", linkageName: "_Z6squarei", scope: !3, file: !3, line: 8, type: !21, isLocal: false, isDefinition: true, scopeLine: 8, flags: DIFlagPrototyped, isOptimized: false, unit: !2, retainedNodes: !4)

 !37 = !DILocation(line: 10, column: 24, scope: !30)
 !38 = !DILocation(line: 8, column: 59, scope: !20, inlinedAt: !32)
 !39 = !DILocation(line: 8, column: 63, scope: !20, inlinedAt: !32)
 !40 = !DILocation(line: 8, column: 61, scope: !20, inlinedAt: !32)
 !41 = !DILocation(line: 11, column: 10, scope: !30)
 !42 = !DILocation(line: 11, column: 18, scope: !30)
 !43 = !DILocation(line: 11, column: 17, scope: !30)
 !44 = !DILocation(line: 11, column: 3, scope: !30)
+!45 = !DIDerivedType(tag: DW_TAG_member, name: "fn", scope: !11, file: !3, line: 5, baseType: !46, size: 64)
+!46 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !47, size: 64)
+!47 = !DISubroutineType(types: !22)

llvm/tools/llvm-dwarfdump/Statistics.cpp

 
 /// Recursively collect debug info quality metrics.
 static void collectStatsRecursive(DWARFDie Die, std::string FnPrefix,
                                   std::string VarPrefix, uint64_t BytesInScope,
                                   uint32_t InlineDepth,
                                   StringMap<PerFunctionStats> &FnStatMap,
                                   GlobalStats &GlobalStats,
                                   LocationStats &LocStats) {
-  // Handle any kind of lexical scope.
   const dwarf::Tag Tag = Die.getTag();
+  // Skip function types.
+  if (Tag == dwarf::DW_TAG_subroutine_type)
+    return;
+
+  // Handle any kind of lexical scope.
   const bool IsFunction = Tag == dwarf::DW_TAG_subprogram;
   const bool IsBlock = Tag == dwarf::DW_TAG_lexical_block;
   const bool IsInlinedFunction = Tag == dwarf::DW_TAG_inlined_subroutine;
   if (IsFunction || IsInlinedFunction || IsBlock) {
 
     // Reset VarPrefix when entering a new function.
     if (Die.getTag() == dwarf::DW_TAG_subprogram ||
         Die.getTag() == dwarf::DW_TAG_inlined_subroutine)