cmd/link: reduce the size of Reloc struct

For linking large binaries, the slice of Relocs consumes a large
amount of memory. We can reduce this memory consumption by
shrinking the size of the Reloc struct. This CL moves the fields
used only in external linking or only on PPC64 and S390X to a
lazily initialized side struct.

Linking k8s.io/kubernetes/cmd/kube-apiserver on Linux/AMD64,
before:
inuse_space 1240.25MB total
438.11MB 35.32% 35.32%   438.11MB 35.32%  cmd/link/internal/objfile.(*objReader).readSlices

after:
inuse_space 1123.39MB total
306.85MB 27.31% 55.03%   306.85MB 27.31%  cmd/link/internal/objfile.(*objReader).readSlices

Under GOGC=5 (to simulate system under memory pressure), the max
RSS reduces from ~2.05G to ~1.83G. Even with external linking the
max RSS doesn't increase.

Change-Id: I9bb8a8effaed24a8b3c4b1cfb93e6992b8740f91
Reviewed-on: https://go-review.googlesource.com/c/go/+/173437
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/objabi/reloctype.go

@@ -30,7 +30,7 @@
 
 package objabi
 
-type RelocType int32
+type RelocType int16
 
 //go:generate stringer -type=RelocType
 const (

src/cmd/link/internal/ld/data.go

@@ -189,13 +189,19 @@ func relocsym(ctxt *Link, s *sym.Symbol) {
 			Errorf(s, "unreachable sym in relocation: %s", r.Sym.Name)
 		}
 
+		if ctxt.LinkMode == LinkExternal {
+			r.InitExt()
+		}
+
 		// TODO(mundaym): remove this special case - see issue 14218.
 		if ctxt.Arch.Family == sys.S390X {
 			switch r.Type {
 			case objabi.R_PCRELDBL:
+				r.InitExt()
 				r.Type = objabi.R_PCREL
 				r.Variant = sym.RV_390_DBL
 			case objabi.R_CALL:
+				r.InitExt()
 				r.Variant = sym.RV_390_DBL
 			}
 		}
@@ -499,9 +505,12 @@ func relocsym(ctxt *Link, s *sym.Symbol) {
 			o = r.Add
 		}
 
+		if ctxt.Arch.Family == sys.PPC64 || ctxt.Arch.Family == sys.S390X {
+			r.InitExt()
 			if r.Variant != sym.RV_NONE {
 				o = thearch.Archrelocvariant(ctxt, r, s, o)
 			}
+		}
 
 		if false {
 			nam := "<nil>"

src/cmd/link/internal/ppc64/asm.go

@@ -272,6 +272,7 @@ func adddynrel(ctxt *ld.Link, s *sym.Symbol, r *sym.Reloc) bool {
 }
 func addelfdynrel(ctxt *ld.Link, s *sym.Symbol, r *sym.Reloc) bool {
 	targ := r.Sym
+	r.InitExt()
 
 	switch r.Type {
 	default:

src/cmd/link/internal/s390x/asm.go

@@ -104,6 +104,7 @@ func gentext(ctxt *ld.Link) {
 
 func adddynrel(ctxt *ld.Link, s *sym.Symbol, r *sym.Reloc) bool {
 	targ := r.Sym
+	r.InitExt()
 
 	switch r.Type {
 	default:

src/cmd/link/internal/sym/reloc.go

@@ -25,12 +25,24 @@ type Reloc struct {
 	Off       int32            // offset to rewrite
 	Siz       uint8            // number of bytes to rewrite, 1, 2, or 4
 	Done      bool             // set to true when relocation is complete
-	Variant RelocVariant     // variation on Type
 	Type      objabi.RelocType // the relocation type
 	Add       int64            // addend
-	Xadd    int64            // addend passed to external linker
 	Sym       *Symbol          // symbol the relocation addresses
+	*relocExt                  // extra fields (see below), may be nil, call InitExt before use
+}
+
+// relocExt contains extra fields in Reloc that are used only in
+// certain cases.
+type relocExt struct {
+	Xadd    int64        // addend passed to external linker
 	Xsym    *Symbol      // symbol passed to external linker
+	Variant RelocVariant // variation on Type, currently used only on PPC64 and S390X
+}
+
+func (r *Reloc) InitExt() {
+	if r.relocExt == nil {
+		r.relocExt = new(relocExt)
+	}
 }
 
 // RelocVariant is a linker-internal variation on a relocation.

src/cmd/link/internal/sym/symbol.go

@@ -211,6 +211,9 @@ func (s *Symbol) AddPCRelPlus(arch *sys.Arch, t *Symbol, add int64) int64 {
 	r.Add = add
 	r.Type = objabi.R_PCREL
 	r.Siz = 4
+	if arch.Family == sys.S390X || arch.Family == sys.PPC64 {
+		r.InitExt()
+	}
 	if arch.Family == sys.S390X {
 		r.Variant = RV_390_DBL
 	}