runtime: refactor/fix asmcgocall/asmcgocall_errno

Instead of making asmcgocall call asmcgocall_errno,
make both load args into registers and call a shared
assembly function.

On amd64, this costs 1 word in the asmcgocall_errno path
but saves 3 words in the asmcgocall path, and the latter
is what happens on critical nosplit paths on Windows.

On arm, this fixes build failures: asmcgocall was writing
the arguments for asmcgocall_errno into the wrong
place on the stack. Passing them in registers avoids the
decision entirely.

On 386, this isn't really needed, since the nosplit paths
have twice as many words to work with, but do it for consistency.

Update issue 8635
Fixes arm build (except GOARM=5).

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/134390043

1 files changed, 7 insertions, 2 deletions

diff --git a/misc/cgo/test/callback.go b/misc/cgo/test/callback.go
index 67d271404..98f653ef7 100644
--- a/misc/cgo/test/callback.go
+++ b/misc/cgo/test/callback.go
@@ -156,6 +156,7 @@ func testCallbackCallers(t *testing.T) {
 		"runtime.cgocallbackg1",
 		"runtime.cgocallbackg",
 		"runtime.cgocallback_gofunc",
+		"asmcgocall",
 		"runtime.asmcgocall_errno",
 		"runtime.cgocall_errno",
 		"test._Cfunc_callback",
@@ -182,8 +183,12 @@ func testCallbackCallers(t *testing.T) {
 		if strings.HasPrefix(fname, "_") {
 			fname = path.Base(f.Name()[1:])
 		}
-		if fname != name[i] {
-			t.Errorf("expected function name %s, got %s", name[i], fname)
+		namei := ""
+		if i < len(name) {
+			namei = name[i]
+		}
+		if fname != namei {
+			t.Errorf("stk[%d] = %q, want %q", i, fname, namei)
 		}
 	}
 }