cmd/compile: recognize (*[Big]T)(ptr)[:n:m] pattern for -d=checkptr

A common idiom for turning an unsafe.Pointer into a slice is to write:

    s := (*[Big]T)(ptr)[:n:m]

This technically violates Go's unsafe pointer rules (rule #1 says T2
can't be bigger than T1), but it's fairly common and not too difficult
to recognize, so might as well allow it for now so we can make
progress on #34972.

This should be revisited if #19367 is accepted.

Updates #22218.
Updates #34972.

Change-Id: Id824e2461904e770910b6e728b4234041d2cc8bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/201839
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/runtime/checkptr.go b/src/runtime/checkptr.go
index a6d33c5af1..d1fc651509 100644
--- a/src/runtime/checkptr.go
+++ b/src/runtime/checkptr.go
@@ -9,18 +9,19 @@ import "unsafe"
 type ptrAlign struct {
 	ptr  unsafe.Pointer
 	elem *_type
+	n    uintptr
 }
 
-func checkptrAlignment(p unsafe.Pointer, elem *_type) {
-	// Check that (*T)(p) is appropriately aligned.
+func checkptrAlignment(p unsafe.Pointer, elem *_type, n uintptr) {
+	// Check that (*[n]elem)(p) is appropriately aligned.
 	// TODO(mdempsky): What about fieldAlign?
 	if uintptr(p)&(uintptr(elem.align)-1) != 0 {
-		panic(ptrAlign{p, elem})
+		panic(ptrAlign{p, elem, n})
 	}
 
-	// Check that (*T)(p) doesn't straddle multiple heap objects.
-	if elem.size != 1 && checkptrBase(p) != checkptrBase(add(p, elem.size-1)) {
-		panic(ptrAlign{p, elem})
+	// Check that (*[n]elem)(p) doesn't straddle multiple heap objects.
+	if size := n * elem.size; size > 1 && checkptrBase(p) != checkptrBase(add(p, size-1)) {
+		panic(ptrAlign{p, elem, n})
 	}
 }
 
@@ -34,6 +35,9 @@ func checkptrArithmetic(p unsafe.Pointer, originals []unsafe.Pointer) {
 		panic(ptrArith{p, originals})
 	}
 
+	// Check that if the computed pointer p points into a heap
+	// object, then one of the original pointers must have pointed
+	// into the same object.
 	base := checkptrBase(p)
 	if base == 0 {
 		return