Go patch committed: Add go:notinheap magic comment

[Ian Lance Taylor <iant at golang dot org>]

This patch to the Go frontend implements go:notinheap as the gc
compiler does. A type marked as go:notinheap may not live in the heap,
and does not require a write barrier. Struct and array types that
incorporate notinheap types are themselves notinheap. Allocating a
value of a notinheap type on the heap is an error.

This is not just an optimization. There is code where a write barrier
may not occur that was getting a write barrier with gccgo but not gc,
because the types in question were notinheap. The case I found was
setting the mcache field in exitsyscallfast.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian

Attachment: patch.txt
Description: Text document