compiler, runtime: allocate defer records on the stack

    
    When a defer is executed at most once in a function body,
    we can allocate the defer record for it on the stack instead
    of on the heap.
    
    This should make defers like this (which are very common) faster.
    
    This is a port of CL 171758 from the gc repo.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/190410

From-SVN: r274613

1 files changed, 5 insertions, 0 deletions

diff --git a/libgo/go/runtime/mgcmark.go b/libgo/go/runtime/mgcmark.go
index 1b8a7a3ddd7..2463a48d428 100644
--- a/libgo/go/runtime/mgcmark.go
+++ b/libgo/go/runtime/mgcmark.go
@@ -657,6 +657,11 @@ func scanstack(gp *g, gcw *gcWork) {
 		scanstackblock(uintptr(unsafe.Pointer(&gp.context)), unsafe.Sizeof(gp.context), gcw)
 	}
 
+	// Note: in the gc runtime scanstack also scans defer records.
+	// This is necessary as it uses stack objects (a.k.a. stack tracing).
+	// We don't (yet) do stack objects, and regular stack/heap scan
+	// will take care of defer records just fine.
+
 	gp.gcscanvalid = true
 }