cmd/link, cmd/cgo: support -flto in CFLAGS

The linker now accepts unrecognized object files in external linking mode.
These objects will simply be passed to the external linker.
This permits using -flto which can generate pure byte code objects,
whose symbol table the linker does not know how to read.

The cgo tool now passes -fno-lto when generating objects whose symbols
it needs to read. The cgo tool now emits matching types in different
objects, so that the lto linker does not report a mismatch.

This is based on https://golang.org/cl/293290 by Derek Parker.

For #43505
Fixes #43830
Fixes #46295

Change-Id: I6787de213417466784ddef5af8899e453b4ae1ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/322614
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

1 files changed, 22 insertions, 7 deletions

diff --git a/src/cmd/dist/test.go b/src/cmd/dist/test.go
index 50bf80ba59..bc49c6d804 100644
--- a/src/cmd/dist/test.go
+++ b/src/cmd/dist/test.go
@@ -722,14 +722,29 @@ func (t *tester) registerTests() {
 				},
 			})
 			if t.hasCxx() {
-				t.tests = append(t.tests, distTest{
-					name:    "swig_callback",
-					heading: "../misc/swig/callback",
-					fn: func(dt *distTest) error {
-						t.addCmd(dt, "misc/swig/callback", t.goTest())
-						return nil
+				t.tests = append(t.tests,
+					distTest{
+						name:    "swig_callback",
+						heading: "../misc/swig/callback",
+						fn: func(dt *distTest) error {
+							t.addCmd(dt, "misc/swig/callback", t.goTest())
+							return nil
+						},
+					},
+					distTest{
+						name:    "swig_callback_lto",
+						heading: "../misc/swig/callback",
+						fn: func(dt *distTest) error {
+							cmd := t.addCmd(dt, "misc/swig/callback", t.goTest())
+							cmd.Env = append(os.Environ(),
+								"CGO_CFLAGS=-flto",
+								"CGO_CXXFLAGS=-flto",
+								"CGO_LDFLAGS=-flto",
+							)
+							return nil
+						},
 					},
-				})
+				)
 			}
 		}
 	}