cmd/compile: remove scratchFpMem global

Instead, add a scratchFpMem field to ssafn,
so that it may be passed on to genssa.

Updates #15756

Change-Id: Icdeae290d3098d14d31659fa07a9863964bb76ed
Reviewed-on: https://go-review.googlesource.com/38728Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -171,8 +171,6 @@ func (s byStackVar) Len() int           { return len(s) }
 func (s byStackVar) Less(i, j int) bool { return cmpstackvarlt(s[i], s[j]) }
 func (s byStackVar) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 
-var scratchFpMem *Node
-
 func (s *ssafn) AllocFrame(f *ssa.Func) {
 	s.stksize = 0
 	s.stkptrsize = 0
@@ -208,9 +206,8 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 		}
 	}
 
-	if f.Config.NeedsFpScratch {
-		scratchFpMem = tempAt(src.NoXPos, s.curfn, Types[TUINT64])
-		scratchFpMem.SetUsed(scratchUsed)
+	if f.Config.NeedsFpScratch && scratchUsed {
+		s.scratchFpMem = tempAt(src.NoXPos, s.curfn, Types[TUINT64])
 	}
 
 	sort.Sort(byStackVar(fn.Dcl))

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

@@ -4306,8 +4306,7 @@ func genssa(f *ssa.Func, pp *Progs) {
 		s.SSEto387 = map[int16]int16{}
 	}
 
-	s.ScratchFpMem = scratchFpMem
-	scratchFpMem = nil
+	s.ScratchFpMem = e.scratchFpMem
 
 	// Emit basic blocks
 	for i, b := range f.Blocks {
@@ -4686,11 +4685,12 @@ func fieldIdx(n *Node) int {
 // ssafn holds frontend information about a function that the backend is processing.
 // It also exports a bunch of compiler services for the ssa backend.
 type ssafn struct {
-	curfn      *Node
-	strings    map[string]interface{} // map from constant string to data symbols
-	stksize    int64                  // stack size for current frame
-	stkptrsize int64                  // prefix of stack containing pointers
-	log        bool
+	curfn        *Node
+	strings      map[string]interface{} // map from constant string to data symbols
+	scratchFpMem *Node                  // temp for floating point register / memory moves on some architectures
+	stksize      int64                  // stack size for current frame
+	stkptrsize   int64                  // prefix of stack containing pointers
+	log          bool
 }
 
 // StringData returns a symbol (a *Sym wrapped in an interface) which