1 files changed, 23 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/ssa/location.go b/src/cmd/compile/internal/ssa/location.go
index 41b48947aa..70afa47e9d 100644
--- a/src/cmd/compile/internal/ssa/location.go
+++ b/src/cmd/compile/internal/ssa/location.go
@@ -26,12 +26,32 @@ func (r *Register) Name() string {
 	return r.name
 }
 
-// A LocalSlot is a location in the stack frame.
-// It is (possibly a subpiece of) a PPARAM, PPARAMOUT, or PAUTO ONAME node.
+// A LocalSlot is a location in the stack frame, which identifies and stores
+// part or all of a PPARAM, PPARAMOUT, or PAUTO ONAME node.
+// It can represent a whole variable, part of a larger stack slot, or part of a
+// variable that has been decomposed into multiple stack slots.
+// As an example, a string could have the following configurations:
+//
+//           stack layout              LocalSlots
+//
+// Optimizations are disabled. s is on the stack and represented in its entirety.
+// [ ------- s string ---- ] { N: s, Type: string, Off: 0 }
+//
+// s was not decomposed, but the SSA operates on its parts individually, so
+// there is a LocalSlot for each of its fields that points into the single stack slot.
+// [ ------- s string ---- ] { N: s, Type: *uint8, Off: 0 }, {N: s, Type: int, Off: 8}
+//
+// s was decomposed. Each of its fields is in its own stack slot and has its own LocalSLot.
+// [ ptr *uint8 ] [ len int] { N: ptr, Type: *uint8, Off: 0, SplitOf: parent, SplitOffset: 0},
+//                           { N: len, Type: int, Off: 0, SplitOf: parent, SplitOffset: 8}
+//                           parent = &{N: s, Type: string}
 type LocalSlot struct {
-	N    GCNode      // an ONAME *gc.Node representing a variable on the stack
+	N    GCNode      // an ONAME *gc.Node representing a stack location.
 	Type *types.Type // type of slot
 	Off  int64       // offset of slot in N
+
+	SplitOf     *LocalSlot // slot is a decomposition of SplitOf
+	SplitOffset int64      // .. at this offset.
 }
 
 func (s LocalSlot) Name() string {