cmd/compile: document Move's type

Fixes #37381

Change-Id: I8abf07d6342c10fc8d52e11c6a70fb0ec09220d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/220683
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -1923,6 +1923,7 @@
 (IsNonNil (LocalAddr _ _)) -> (ConstBool [1])
 
 // Inline small or disjoint runtime.memmove calls with constant length.
+// See the comment in op Move in genericOps.go for discussion of the type.
 (StaticCall {sym} s1:(Store _ (Const(64|32) [sz]) s2:(Store  _ src s3:(Store {t} _ dst mem))))
 	&& sz >= 0
 	&& isSameSym(sym,"runtime.memmove")

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -351,6 +351,13 @@ var genericOps = []opData{
 	// The source and destination of Move may overlap in some cases. See e.g.
 	// memmove inlining in generic.rules. When inlineablememmovesize (in ../rewrite.go)
 	// returns true, we must do all loads before all stores, when lowering Move.
+	// The type of Move is used for the write barrier pass to insert write barriers
+	// and for alignment on some architectures.
+	// For pointerless types, it is possible for the type to be inaccurate.
+	// For type alignment and pointer information, use the type in Aux;
+	// for type size, use the size in AuxInt.
+	// The "inline runtime.memmove" rewrite rule generates Moves with inaccurate types,
+	// such as type byte instead of the more accurate type [8]byte.
 	{name: "Move", argLength: 3, typ: "Mem", aux: "TypSize"}, // arg0=destptr, arg1=srcptr, arg2=mem, auxint=size, aux=type.  Returns memory.
 	{name: "Zero", argLength: 2, typ: "Mem", aux: "TypSize"}, // arg0=destptr, arg1=mem, auxint=size, aux=type. Returns memory.