cmd/compile: explain special treatment when rewrite slice literal args

Followup discussion in CL 360055.

Change-Id: I36212c2a497b152d01ed86d244d5f57bd34a64a6
Reviewed-on: https://go-review.googlesource.com/c/go/+/360614
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: cuonglm

src/cmd/compile/internal/escape/call.go

@@ -333,7 +333,24 @@ func (e *escape) rewriteArgument(argp *ir.Node, init *ir.Nodes, call ir.Node, fn
 		}
 	}
 
-	// Peel away any slice lits.
+	// Peel away any slice literals for better escape analyze
+	// them. For example:
+	//
+	//     go F([]int{a, b})
+	//
+	// If F doesn't escape its arguments, then the slice can
+	// be allocated on the new goroutine's stack.
+	//
+	// For variadic functions, the compiler has already rewritten:
+	//
+	//     f(a, b, c)
+	//
+	// to:
+	//
+	//     f([]T{a, b, c}...)
+	//
+	// So we need to look into slice elements to handle uintptr(ptr)
+	// arguments to syscall-like functions correctly.
 	if arg := *argp; arg.Op() == ir.OSLICELIT {
 		list := arg.(*ir.CompLitExpr).List
 		for i := range list {