cmd/compile: initialize loop depths

Regalloc uses loop depths - make sure they are initialized!

Test to make sure we aren't pushing spills into loops.

This fixes a generated-code performance bug introduced with
the better spill placement change:
https://go-review.googlesource.com/c/34822/

Update #19595

Change-Id: Ib9f0da6fb588503518847d7aab51e569fd3fa61e
Reviewed-on: https://go-review.googlesource.com/38434Reviewed-by: David Chase <drchase@google.com>

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

@@ -2176,6 +2176,7 @@ func (s *regAllocState) computeLive() {
 	// out to all of them.
 	po := f.postorder()
 	s.loopnest = f.loopnest()
+	s.loopnest.calculateDepths()
 	for {
 		changed := false
 

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

@@ -31,3 +31,38 @@ func TestLiveControlOps(t *testing.T) {
 	regalloc(f.f)
 	checkFunc(f.f)
 }
+
+// Test to make sure we don't push spills into loops.
+// See issue #19595.
+func TestSpillWithLoop(t *testing.T) {
+	c := testConfig(t)
+	f := c.Fun("entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, TypeMem, 0, nil),
+			Valu("ptr", OpArg, TypeInt64Ptr, 0, c.Frontend().Auto(TypeInt64)),
+			Valu("cond", OpArg, TypeBool, 0, c.Frontend().Auto(TypeBool)),
+			Valu("ld", OpAMD64MOVQload, TypeInt64, 0, nil, "ptr", "mem"), // this value needs a spill
+			Goto("loop"),
+		),
+		Bloc("loop",
+			Valu("memphi", OpPhi, TypeMem, 0, nil, "mem", "call"),
+			Valu("call", OpAMD64CALLstatic, TypeMem, 0, nil, "memphi"),
+			Valu("test", OpAMD64CMPBconst, TypeFlags, 0, nil, "cond"),
+			Eq("test", "next", "exit"),
+		),
+		Bloc("next",
+			Goto("loop"),
+		),
+		Bloc("exit",
+			Valu("store", OpAMD64MOVQstore, TypeMem, 0, nil, "ptr", "ld", "call"),
+			Exit("store"),
+		),
+	)
+	regalloc(f.f)
+	checkFunc(f.f)
+	for _, v := range f.blocks["loop"].Values {
+		if v.Op == OpStoreReg {
+			t.Errorf("spill inside loop %s", v.LongString())
+		}
+	}
+}