diff --git a/src/cmd/compile/internal/ssa/compile.go b/src/cmd/compile/internal/ssa/compile.go
index cbef6ea645..8080b7aaba 100644
--- a/src/cmd/compile/internal/ssa/compile.go
+++ b/src/cmd/compile/internal/ssa/compile.go
@@ -183,7 +183,7 @@ var passes = [...]pass{
 	{name: "prove", fn: prove},
 	{name: "decompose builtin", fn: decomposeBuiltIn, required: true},
 	{name: "dec", fn: dec, required: true},
-	{name: "late opt", fn: opt}, // TODO: split required rules and optimizing rules
+	{name: "late opt", fn: opt, required: true}, // TODO: split required rules and optimizing rules
 	{name: "generic deadcode", fn: deadcode},
 	{name: "fuse", fn: fuse},
 	{name: "dse", fn: dse},
diff --git a/src/cmd/compile/internal/ssa/gen/generic.rules b/src/cmd/compile/internal/ssa/gen/generic.rules
index cc24269418..2186d8921c 100644
--- a/src/cmd/compile/internal/ssa/gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/gen/generic.rules
@@ -534,6 +534,9 @@
     (Store [8] dst real mem))
 
 // string ops
+// Decomposing StringMake and lowering of StringPtr and StringLen
+// happens in a later pass, dec, so that these operations are available
+// to otherpasses for optimizations.
 (StringPtr (StringMake (Const64 <t> [c]) _)) -> (Const64 <t> [c])
 (StringLen (StringMake _ (Const64 <t> [c]))) -> (Const64 <t> [c])
 (ConstString {s}) && config.PtrSize == 4 && s.(string) == "" ->
@@ -563,6 +566,9 @@
     (Store [config.PtrSize] dst ptr mem))
 
 // slice ops
+// Decomposing SliceMake, and lowering of SlicePtr, SliceLen, and SliceCap
+// happens in a later pass, dec, so that these operations are available
+// to other passes for optimizations.
 (SlicePtr (SliceMake (Const64 <t> [c]) _ _)) -> (Const64 <t> [c])
 (SliceLen (SliceMake _ (Const64 <t> [c]) _)) -> (Const64 <t> [c])
 (SliceCap (SliceMake _ _ (Const64 <t> [c]))) -> (Const64 <t> [c])