cmd/compile: add -d=checkptr to validate unsafe.Pointer rules

This CL adds -d=checkptr as a compile-time option for adding
instrumentation to check that Go code is following unsafe.Pointer
safety rules dynamically. In particular, it currently checks two
things:

1. When converting unsafe.Pointer to *T, make sure the resulting
pointer is aligned appropriately for T.

2. When performing pointer arithmetic, if the result points to a Go
heap object, make sure we can find an unsafe.Pointer-typed operand
that pointed into the same object.

These checks are currently disabled for the runtime, and can also be
disabled through a new //go:nocheckptr annotation. The latter is
necessary for functions like strings.noescape, which intentionally
violate safety rules to workaround escape analysis limitations.

Fixes #22218.

Change-Id: If5a51273881d93048f74bcff10a3275c9c91da6a
Reviewed-on: https://go-review.googlesource.com/c/go/+/162237
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -40,6 +40,7 @@ var (
 
 var (
 	Debug_append       int
+	Debug_checkptr     int
 	Debug_closure      int
 	Debug_compilelater int
 	debug_dclstack     int
@@ -65,6 +66,7 @@ var debugtab = []struct {
 	val  interface{} // must be *int or *string
 }{
 	{"append", "print information about append compilation", &Debug_append},
+	{"checkptr", "instrument unsafe pointer conversions", &Debug_checkptr},
 	{"closure", "print information about closure compilation", &Debug_closure},
 	{"compilelater", "compile functions as late as possible", &Debug_compilelater},
 	{"disablenil", "disable nil checks", &disable_checknil},
@@ -433,6 +435,11 @@ func Main(archInit func(*Arch)) {
 		}
 	}
 
+	// Runtime can't use -d=checkptr, at least not yet.
+	if compiling_runtime {
+		Debug_checkptr = 0
+	}
+
 	// set via a -d flag
 	Ctxt.Debugpcln = Debug_pctab
 	if flagDWARF {