cmd/{compile,link}: relocate generation of DWARF for global vars

Move DWARF generation for global variables from the linker to the
compiler. This effectively parallelizes this part of DWARF generation,
speeds up the linker minutely, and gives us a slightly more rational
implementation (there was really no compelling reason to do DWARF gen
for globals in the linker).

Change-Id: I0c1c98d3a647258697e90eb91d1d8a9f6f7f376a
Reviewed-on: https://go-review.googlesource.com/c/go/+/295011
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

2 files changed, 24 insertions, 35 deletions

diff --git a/src/cmd/link/internal/ld/dwarf.go b/src/cmd/link/internal/ld/dwarf.go
index 561f6f1475..3c3f0e5b85 100644
--- a/src/cmd/link/internal/ld/dwarf.go
+++ b/src/cmd/link/internal/ld/dwarf.go
@@ -458,12 +458,6 @@ func newmemberoffsetattr(die *dwarf.DWDie, offs int32) {
 	newattr(die, dwarf.DW_AT_data_member_location, dwarf.DW_CLS_CONSTANT, int64(offs), nil)
 }
 
-// GDB doesn't like FORM_addr for AT_location, so emit a
-// location expression that evals to a const.
-func (d *dwctxt) newabslocexprattr(die *dwarf.DWDie, addr int64, symIdx loader.Sym) {
-	newattr(die, dwarf.DW_AT_location, dwarf.DW_CLS_ADDRESS, addr, dwSym(symIdx))
-}
-
 func (d *dwctxt) lookupOrDiag(n string) loader.Sym {
 	symIdx := d.ldr.Lookup(n, 0)
 	if symIdx == 0 {
@@ -1020,25 +1014,6 @@ func (d *dwctxt) synthesizechantypes(ctxt *Link, die *dwarf.DWDie) {
 	}
 }
 
-func (d *dwctxt) dwarfDefineGlobal(ctxt *Link, symIdx loader.Sym, str string, v int64, gotype loader.Sym) {
-	// Find a suitable CU DIE to include the global.
-	// One would think it's as simple as just looking at the unit, but that might
-	// not have any reachable code. So, we go to the runtime's CU if our unit
-	// isn't otherwise reachable.
-	unit := d.ldr.SymUnit(symIdx)
-	if unit == nil {
-		unit = ctxt.runtimeCU
-	}
-	ver := d.ldr.SymVersion(symIdx)
-	dv := d.newdie(unit.DWInfo, dwarf.DW_ABRV_VARIABLE, str, int(ver))
-	d.newabslocexprattr(dv, v, symIdx)
-	if d.ldr.SymVersion(symIdx) < sym.SymVerStatic {
-		newattr(dv, dwarf.DW_AT_external, dwarf.DW_CLS_FLAG, 1, 0)
-	}
-	dt := d.defgotype(gotype)
-	d.newrefattr(dv, dwarf.DW_AT_type, dt)
-}
-
 // createUnitLength creates the initial length field with value v and update
 // offset of unit_length if needed.
 func (d *dwctxt) createUnitLength(su *loader.SymbolBuilder, v uint64) {
@@ -1552,7 +1527,7 @@ func appendSyms(syms []loader.Sym, src []sym.LoaderSym) []loader.Sym {
 
 func (d *dwctxt) writeUnitInfo(u *sym.CompilationUnit, abbrevsym loader.Sym, infoEpilog loader.Sym) []loader.Sym {
 	syms := []loader.Sym{}
-	if len(u.Textp) == 0 && u.DWInfo.Child == nil {
+	if len(u.Textp) == 0 && u.DWInfo.Child == nil && len(u.VarDIEs) == 0 {
 		return syms
 	}
 
@@ -1583,6 +1558,7 @@ func (d *dwctxt) writeUnitInfo(u *sym.CompilationUnit, abbrevsym loader.Sym, inf
 	if u.Consts != 0 {
 		cu = append(cu, loader.Sym(u.Consts))
 	}
+	cu = appendSyms(cu, u.VarDIEs)
 	var cusize int64
 	for _, child := range cu {
 		cusize += int64(len(d.ldr.Data(child)))
@@ -1907,10 +1883,11 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 		checkStrictDups = 1
 	}
 
-	// Create DIEs for global variables and the types they use.
-	// FIXME: ideally this should be done in the compiler, since
-	// for globals there isn't any abiguity about which package
-	// a global belongs to.
+	// Make a pass through all data symbols, looking for those
+	// corresponding to reachable, Go-generated, user-visible
+	// global variables. For each global of this sort, locate
+	// the corresponding compiler-generated DIE symbol and tack
+	// it onto the list associated with the unit.
 	for idx := loader.Sym(1); idx < loader.Sym(d.ldr.NDef()); idx++ {
 		if !d.ldr.AttrReachable(idx) ||
 			d.ldr.AttrNotInSymbolTable(idx) ||
@@ -1925,7 +1902,8 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 			continue
 		}
 		// Skip things with no type
-		if d.ldr.SymGoType(idx) == 0 {
+		gt := d.ldr.SymGoType(idx)
+		if gt == 0 {
 			continue
 		}
 		// Skip file local symbols (this includes static tmps, stack
@@ -1939,10 +1917,20 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 			continue
 		}
 
-		// Create DIE for global.
-		sv := d.ldr.SymValue(idx)
-		gt := d.ldr.SymGoType(idx)
-		d.dwarfDefineGlobal(ctxt, idx, sn, sv, gt)
+		// Find compiler-generated DWARF info sym for global in question,
+		// and tack it onto the appropriate unit.  Note that there are
+		// circumstances under which we can't find the compiler-generated
+		// symbol-- this typically happens as a result of compiler options
+		// (e.g. compile package X with "-dwarf=0").
+
+		// FIXME: use an aux sym or a relocation here instead of a
+		// name lookup.
+		varDIE := d.ldr.Lookup(dwarf.InfoPrefix+sn, 0)
+		if varDIE != 0 {
+			unit := d.ldr.SymUnit(idx)
+			d.defgotype(gt)
+			unit.VarDIEs = append(unit.VarDIEs, sym.LoaderSym(varDIE))
+		}
 	}
 
 	d.synthesizestringtypes(ctxt, dwtypes.Child)
diff --git a/src/cmd/link/internal/sym/compilation_unit.go b/src/cmd/link/internal/sym/compilation_unit.go
index 5d7206db66..926cbc3d20 100644
--- a/src/cmd/link/internal/sym/compilation_unit.go
+++ b/src/cmd/link/internal/sym/compilation_unit.go
@@ -29,6 +29,7 @@ type CompilationUnit struct {
 
 	Consts    LoaderSym   // Package constants DIEs
 	FuncDIEs  []LoaderSym // Function DIE subtrees
+	VarDIEs   []LoaderSym // Global variable DIEs
 	AbsFnDIEs []LoaderSym // Abstract function DIE subtrees
 	RangeSyms []LoaderSym // Symbols for debug_range
 	Textp     []LoaderSym // Text symbols in this CU