cmd/compile: Drop references to Prog structs after each function

Don't accumulate a massive list of Prog structs during
compilation and write them all out at the end of compilation.
Instead, convert them to code+relocs (or data+relocs) after each
function is compiled.

Track down a few other places that were keeping Progs alive
and nil them out so the Progs get GCd promptly.

Saves ~20% in peak memory usage for the compiler.  Surprisingly not much
help speed-wise (only because we end up doing more GCs.  With a
compensating GOGC=120, it does help a bit), but this provides a base for
more changes (e.g. reusing a cache of Progs).

Change-Id: I838e01017c228995a687a8110d0cd67bf8596407
Reviewed-on: https://go-review.googlesource.com/19867
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -1449,8 +1449,13 @@ func funccompile(n *Node) {
 	Funcdepth = n.Func.Depth + 1
 	compile(n)
 	Curfn = nil
+	Pc = nil
+	continpc = nil
+	breakpc = nil
 	Funcdepth = 0
 	dclcontext = PEXTERN
+	flushdata()
+	obj.Flushplist(Ctxt) // convert from Prog list to machine code
 }
 
 func funcsym(s *Sym) *Sym {

src/cmd/compile/internal/gc/gsubr.go

@@ -173,6 +173,18 @@ func dumpdata() {
 	Clearp(Pc)
 }
 
+func flushdata() {
+	if dfirst == nil {
+		return
+	}
+	newplist()
+	*Pc = *dfirst
+	Pc = dpc
+	Clearp(Pc)
+	dfirst = nil
+	dpc = nil
+}
+
 // Fixup instructions after allocauto (formerly compactframe) has moved all autos around.
 func fixautoused(p *obj.Prog) {
 	for lp := &p; ; {

src/cmd/internal/obj/objfile.go

@@ -302,16 +302,22 @@ func Flushplist(ctxt *Link) {
 		ctxt.Arch.Assemble(ctxt, s)
 		fieldtrack(ctxt, s)
 		linkpcln(ctxt, s)
+		s.Text = nil
+		s.Etext = nil
 	}
 
 	// Add to running list in ctxt.
-	if ctxt.Etext == nil {
+	if text != nil {
+		if ctxt.Text == nil {
 			ctxt.Text = text
 		} else {
 			ctxt.Etext.Next = text
 		}
 		ctxt.Etext = etext
+	}
 	ctxt.Plist = nil
+	ctxt.Plast = nil
+	ctxt.Curp = nil
 }
 
 func Writeobjfile(ctxt *Link, b *Biobuf) {