cmd/compile/internal: reuse memory for valueToProgAfter

Not a big improvement, but does help edge cases like the SSA package.
Change-Id: I40e531110b97efd5f45955be477fd0f4faa8d545
Reviewed-on: https://go-review.googlesource.com/92396
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -4653,7 +4653,13 @@ func genssa(f *ssa.Func, pp *Progs) {
 	s.ScratchFpMem = e.scratchFpMem
 
 	if Ctxt.Flag_locationlists {
-		valueToProgAfter = make([]*obj.Prog, f.NumValues())
+		if cap(f.Cache.ValueToProgAfter) < f.NumValues() {
+			f.Cache.ValueToProgAfter = make([]*obj.Prog, f.NumValues())
+		}
+		valueToProgAfter = f.Cache.ValueToProgAfter[:f.NumValues()]
+		for i := range valueToProgAfter {
+			valueToProgAfter[i] = nil
+		}
 	}
 
 	// Emit basic blocks

src/cmd/compile/internal/ssa/cache.go

@@ -4,7 +4,10 @@
 
 package ssa
 
-import "sort"
+import (
+	"cmd/internal/obj"
+	"sort"
+)
 
 // A Cache holds reusable compiler state.
 // It is intended to be re-used for multiple Func compilations.
@@ -21,6 +24,7 @@ type Cache struct {
 	domblockstore []ID         // scratch space for computing dominators
 	scrSparse     []*sparseSet // scratch sparse sets to be re-used.
 
+	ValueToProgAfter []*obj.Prog
 	blockDebug       []BlockDebug
 	valueNames       [][]SlotID
 	slotLocs         []VarLoc