cmd/compile: add //go:uintptrkeepalive

This CL exports the existing ir.UintptrKeepAlive via the new directive
//go:uintptrkeepalive. This makes the compiler insert KeepAlives for
pointers converted to uintptr in calls, keeping them alive for the
duration of the call.

//go:uintptrkeepalive requires //go:nosplit, as stack growth can't
handle these arguments (it cannot know which are pointers). We currently
check this on the immediate function, but the actual restriction applies
to all transitive calls.

The existing //go:uintptrescapes is an extension of
//go:uintptrkeepalive which forces pointers to escape to the heap, thus
eliminating the stack growth issue.

This pragma is limited to the standard library.

For #51087

Change-Id: If9a19d484d3561b4219e5539b70c11a3cc09391e
Reviewed-on: https://go-review.googlesource.com/c/go/+/388095
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index cc5610acda..9a42b5afd1 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -340,6 +340,9 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 		if !base.Flag.CompilingRuntime && flag&runtimePragmas != 0 {
 			p.error(syntax.Error{Pos: pos, Msg: fmt.Sprintf("//%s only allowed in runtime", verb)})
 		}
+		if flag == ir.UintptrKeepAlive && !base.Flag.Std {
+			p.error(syntax.Error{Pos: pos, Msg: fmt.Sprintf("//%s is only allowed in the standard library", verb)})
+		}
 		if flag == 0 && !allowedStdPragmas[verb] && base.Flag.Std {
 			p.error(syntax.Error{Pos: pos, Msg: fmt.Sprintf("//%s is not allowed in the standard library", verb)})
 		}