cmd/compile: emit DWARF info about dictionary entries

When emitting the DIE of the instantiation of a generic function also
emit one DW_TAG_typedef_type entry for each dictionary entry in use,
referencing the shape type and having a custom attribute containing the
index inside the dictionary.

When emitting the DIE of variables that have an instantiated parametric
type, instead of referencing the shape type directly go through the
DW_TAG_typedef_type entry emitted for the dictionary entry describing
the real type of the variable.

Change-Id: Ia45d157ecd4c25e2b60300469e43bbb27a663582
Reviewed-on: https://go-review.googlesource.com/c/go/+/344929
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Jeremy Faller <jeremy@golang.org>

1 files changed, 66 insertions, 1 deletions

diff --git a/src/cmd/internal/dwarf/dwarf.go b/src/cmd/internal/dwarf/dwarf.go
index 69aafaf986..be37641706 100644
--- a/src/cmd/internal/dwarf/dwarf.go
+++ b/src/cmd/internal/dwarf/dwarf.go
@@ -50,6 +50,7 @@ type Var struct {
 	Abbrev        int // Either DW_ABRV_AUTO[_LOCLIST] or DW_ABRV_PARAM[_LOCLIST]
 	IsReturnValue bool
 	IsInlFormal   bool
+	DictIndex     uint16 // index of the dictionary entry describing the type of this variable
 	StackOffset   int32
 	// This package can't use the ssa package, so it can't mention ssa.FuncDebug,
 	// so indirect through a closure.
@@ -97,6 +98,8 @@ type FnState struct {
 	Scopes        []Scope
 	InlCalls      InlCalls
 	UseBASEntries bool
+
+	dictIndexToOffset []int64
 }
 
 func EnableLogging(doit bool) {
@@ -315,6 +318,7 @@ const (
 	DW_AT_go_runtime_type   = 0x2904
 
 	DW_AT_go_package_name = 0x2905 // Attribute for DW_TAG_compile_unit
+	DW_AT_go_dict_index   = 0x2906 // Attribute for DW_TAG_typedef_type, index of the dictionary entry describing the real type of this type shape
 
 	DW_AT_internal_location = 253 // params and locals; not emitted
 )
@@ -362,6 +366,7 @@ const (
 	DW_ABRV_STRINGTYPE
 	DW_ABRV_STRUCTTYPE
 	DW_ABRV_TYPEDECL
+	DW_ABRV_DICT_INDEX
 	DW_NABRV
 )
 
@@ -882,6 +887,17 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 			{DW_AT_type, DW_FORM_ref_addr},
 		},
 	},
+
+	/* DICT_INDEX */
+	{
+		DW_TAG_typedef,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_go_dict_index, DW_FORM_udata},
+		},
+	},
 }
 
 // GetAbbrev returns the contents of the .debug_abbrev section.
@@ -1196,6 +1212,9 @@ func putPrunedScopes(ctxt Context, s *FnState, fnabbrev int) error {
 		sort.Sort(byChildIndex(pruned.Vars))
 		scopes[k] = pruned
 	}
+
+	s.dictIndexToOffset = putparamtypes(ctxt, s, scopes, fnabbrev)
+
 	var encbuf [20]byte
 	if putscope(ctxt, s, scopes, 0, fnabbrev, encbuf[:0]) < int32(len(scopes)) {
 		return errors.New("multiple toplevel scopes")
@@ -1451,6 +1470,47 @@ func PutDefaultFunc(ctxt Context, s *FnState, isWrapper bool) error {
 	return nil
 }
 
+// putparamtypes writes typedef DIEs for any parametric types that are used by this function.
+func putparamtypes(ctxt Context, s *FnState, scopes []Scope, fnabbrev int) []int64 {
+	if fnabbrev == DW_ABRV_FUNCTION_CONCRETE {
+		return nil
+	}
+
+	maxDictIndex := uint16(0)
+
+	for i := range scopes {
+		for _, v := range scopes[i].Vars {
+			if v.DictIndex > maxDictIndex {
+				maxDictIndex = v.DictIndex
+			}
+		}
+	}
+
+	if maxDictIndex == 0 {
+		return nil
+	}
+
+	dictIndexToOffset := make([]int64, maxDictIndex)
+
+	for i := range scopes {
+		for _, v := range scopes[i].Vars {
+			if v.DictIndex == 0 || dictIndexToOffset[v.DictIndex-1] != 0 {
+				continue
+			}
+
+			dictIndexToOffset[v.DictIndex-1] = ctxt.CurrentOffset(s.Info)
+
+			Uleb128put(ctxt, s.Info, int64(DW_ABRV_DICT_INDEX))
+			n := fmt.Sprintf(".param%d", v.DictIndex-1)
+			putattr(ctxt, s.Info, DW_ABRV_DICT_INDEX, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
+			putattr(ctxt, s.Info, DW_ABRV_DICT_INDEX, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
+			putattr(ctxt, s.Info, DW_ABRV_DICT_INDEX, DW_FORM_udata, DW_CLS_CONSTANT, int64(v.DictIndex-1), nil)
+		}
+	}
+
+	return dictIndexToOffset
+}
+
 func putscope(ctxt Context, s *FnState, scopes []Scope, curscope int32, fnabbrev int, encbuf []byte) int32 {
 
 	if logDwarf {
@@ -1624,7 +1684,12 @@ func putvar(ctxt Context, s *FnState, v *Var, absfn Sym, fnabbrev, inlIndex int,
 			putattr(ctxt, s.Info, abbrev, DW_FORM_flag, DW_CLS_FLAG, isReturn, nil)
 		}
 		putattr(ctxt, s.Info, abbrev, DW_FORM_udata, DW_CLS_CONSTANT, int64(v.DeclLine), nil)
-		putattr(ctxt, s.Info, abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
+		if v.DictIndex > 0 && s.dictIndexToOffset != nil && s.dictIndexToOffset[v.DictIndex-1] != 0 {
+			// If the type of this variable is parametric use the entry emitted by putparamtypes
+			putattr(ctxt, s.Info, abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, s.dictIndexToOffset[v.DictIndex-1], s.Info)
+		} else {
+			putattr(ctxt, s.Info, abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
+		}
 	}
 
 	if abbrevUsesLoclist(abbrev) {