cmd/compile: free value earlier in nilcheck

When we remove a nil check, add it back to the free Value pool immediately.

Fixes #18732

Change-Id: I8d644faabbfb52157d3f2d071150ff0342ac28dc
Reviewed-on: https://go-review.googlesource.com/58810
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -175,7 +175,7 @@ func (f *Func) LogStat(key string, args ...interface{}) {
 	f.Warnl(f.Entry.Pos, "\t%s\t%s%s\t%s", n, key, value, f.Name)
 }
 
-// freeValue frees a value. It must no longer be referenced.
+// freeValue frees a value. It must no longer be referenced or have any args.
 func (f *Func) freeValue(v *Value) {
 	if v.Block == nil {
 		f.Fatalf("trying to free an already freed value")
@@ -183,6 +183,9 @@ func (f *Func) freeValue(v *Value) {
 	if v.Uses != 0 {
 		f.Fatalf("value %s still has %d uses", v, v.Uses)
 	}
+	if len(v.Args) != 0 {
+		f.Fatalf("value %s still has %d args", v, len(v.Args))
+	}
 	// Clear everything but ID (which we reuse).
 	id := v.ID
 

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

@@ -126,7 +126,7 @@ func nilcheckelim(f *Func) {
 							f.Warnl(v.Pos, "removed nil check")
 						}
 						v.reset(OpUnknown)
-						// TODO: f.freeValue(v)
+						f.freeValue(v)
 						i--
 						continue
 					}

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

@@ -1569,7 +1569,7 @@ func (s *regAllocState) regalloc(f *Func) {
 				if s.f.pass.debug > regDebug {
 					fmt.Printf("delete copied value %s\n", c.LongString())
 				}
-				c.Args[0].Uses--
+				c.RemoveArg(0)
 				f.freeValue(c)
 				delete(s.copies, c)
 				progress = true