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>

3 files changed, 31 insertions, 5 deletions

diff --git a/src/cmd/link/internal/ld/ar.go b/src/cmd/link/internal/ld/ar.go
index 22f53a4df2..23915f9032 100644
--- a/src/cmd/link/internal/ld/ar.go
+++ b/src/cmd/link/internal/ld/ar.go
@@ -124,6 +124,10 @@ func hostArchive(ctxt *Link, name string) {
 
 			libgcc := sym.Library{Pkg: "libgcc"}
 			h := ldobj(ctxt, f, &libgcc, l, pname, name)
+			if h.ld == nil {
+				Errorf(nil, "%s unrecognized object file at offset %d", name, off)
+				continue
+			}
 			f.MustSeek(h.off, 0)
 			h.ld(ctxt, f, h.pkg, h.length, h.pn)
 		}
diff --git a/src/cmd/link/internal/ld/config.go b/src/cmd/link/internal/ld/config.go
index ae0d7520eb..20f1d0b8c1 100644
--- a/src/cmd/link/internal/ld/config.go
+++ b/src/cmd/link/internal/ld/config.go
@@ -241,6 +241,10 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 		return true, "dynamically linking with a shared library"
 	}
 
+	if unknownObjFormat {
+		return true, "some input objects have an unrecognized file format"
+	}
+
 	return false, ""
 }
 
@@ -248,7 +252,7 @@ func mustLinkExternal(ctxt *Link) (res bool, reason string) {
 //
 // It is called after flags are processed and inputs are processed,
 // so the ctxt.LinkMode variable has an initial value from the -linkmode
-// flag and the iscgo externalobj variables are set.
+// flag and the iscgo, externalobj, and unknownObjFormat variables are set.
 func determineLinkMode(ctxt *Link) {
 	extNeeded, extReason := mustLinkExternal(ctxt)
 	via := ""
diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index e8f001ba8e..644faeb2fb 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -343,10 +343,16 @@ var (
 const pkgdef = "__.PKGDEF"
 
 var (
-	// Set if we see an object compiled by the host compiler that is not
-	// from a package that is known to support internal linking mode.
+	// externalobj is set to true if we see an object compiled by
+	// the host compiler that is not from a package that is known
+	// to support internal linking mode.
 	externalobj = false
-	theline     string
+
+	// unknownObjFormat is set to true if we see an object whose
+	// format we don't recognize.
+	unknownObjFormat = false
+
+	theline string
 )
 
 func Lflag(ctxt *Link, arg string) {
@@ -1065,6 +1071,10 @@ func hostobjs(ctxt *Link) {
 		}
 
 		f.MustSeek(h.off, 0)
+		if h.ld == nil {
+			Errorf(nil, "%s: unrecognized object file format", h.pn)
+			continue
+		}
 		h.ld(ctxt, f, h.pkg, h.length, h.pn)
 		f.Close()
 	}
@@ -1855,6 +1865,14 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 		return ldhostobj(ldxcoff, ctxt.HeadType, f, pkg, length, pn, file)
 	}
 
+	if c1 != 'g' || c2 != 'o' || c3 != ' ' || c4 != 'o' {
+		// An unrecognized object is just passed to the external linker.
+		// If we try to read symbols from this object, we will
+		// report an error at that time.
+		unknownObjFormat = true
+		return ldhostobj(nil, ctxt.HeadType, f, pkg, length, pn, file)
+	}
+
 	/* check the header */
 	line, err := f.ReadString('\n')
 	if err != nil {
@@ -1874,7 +1892,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 			return nil
 		}
 
-		Errorf(nil, "%s: not an object file: @%d %02x%02x%02x%02x", pn, start, c1, c2, c3, c4)
+		Errorf(nil, "%s: not an object file: @%d %q", pn, start, line)
 		return nil
 	}