compiler: optimize 0,1,2-case select statement

    
    For a select statement with zero-, one-, or two-case with a
    default case, we can generate simpler code instead of calling the
    generic selectgo. A zero-case select is just blocking the
    execution. A one-case select is mostly just executing the case. A
    two-case select with a default case is a non-blocking send or
    receive. We add these special cases for lowering a select
    statement.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/184998

From-SVN: r273034

2 files changed, 4 insertions, 0 deletions

diff --git a/libgo/go/runtime/chan.go b/libgo/go/runtime/chan.go
index 6dfe2f3..6c8d6f7 100644
--- a/libgo/go/runtime/chan.go
+++ b/libgo/go/runtime/chan.go
@@ -32,6 +32,9 @@ import (
 //go:linkname chanrecv1 runtime.chanrecv1
 //go:linkname chanrecv2 runtime.chanrecv2
 //go:linkname closechan runtime.closechan
+//go:linkname selectnbsend runtime.selectnbsend
+//go:linkname selectnbrecv runtime.selectnbrecv
+//go:linkname selectnbrecv2 runtime.selectnbrecv2
 
 const (
 	maxAlign  = 8
diff --git a/libgo/go/runtime/select.go b/libgo/go/runtime/select.go
index d658a34..16de9b8 100644
--- a/libgo/go/runtime/select.go
+++ b/libgo/go/runtime/select.go
@@ -14,6 +14,7 @@ import (
 // themselves, so that the compiler will export them.
 //
 //go:linkname selectgo runtime.selectgo
+//go:linkname block runtime.block
 
 const debugSelect = false