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>

src/cmd/cgo/gcc.go

@@ -1638,6 +1638,8 @@ func (p *Package) gccCmd() []string {
 		c = append(c, "-maix64")
 		c = append(c, "-mcmodel=large")
 	}
+	// disable LTO so we get an object whose symbols we can read
+	c = append(c, "-fno-lto")
 	c = append(c, "-") //read input from standard input
 	return c
 }

src/cmd/cgo/out.go

@@ -168,8 +168,18 @@ func (p *Package) writeDefs() {
 			if *gccgo {
 				fmt.Fprintf(fc, "extern byte *%s;\n", n.C)
 			} else {
-				fmt.Fprintf(fm, "extern char %s[];\n", n.C)
+				// Force a reference to all symbols so that
-				fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				// the external linker will add DT_NEEDED
+				// entries as needed on ELF systems.
+				// Treat function variables differently
+				// to avoid type confict errors from LTO
+				// (Link Time Optimization).
+				if n.Kind == "fpvar" {
+					fmt.Fprintf(fm, "extern void %s();\n", n.C)
+				} else {
+					fmt.Fprintf(fm, "extern char %s[];\n", n.C)
+					fmt.Fprintf(fm, "void *_cgohack_%s = %s;\n\n", n.C, n.C)
+				}
 				fmt.Fprintf(fgo2, "//go:linkname __cgo_%s %s\n", n.C, n.C)
 				fmt.Fprintf(fgo2, "//go:cgo_import_static %s\n", n.C)
 				fmt.Fprintf(fgo2, "var __cgo_%s byte\n", n.C)
@@ -1042,7 +1052,7 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
 		// This unpacks the argument struct above and calls the Go function.
 		fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a *%s) {\n", cPrefix, exp.ExpName, gotype)
 
-		fmt.Fprintf(fm, "int _cgoexp%s_%s;\n", cPrefix, exp.ExpName)
+		fmt.Fprintf(fm, "void _cgoexp%s_%s(void* p){}\n", cPrefix, exp.ExpName)
 
 		if gccResult != "void" {
 			// Write results back to frame.

src/cmd/dist/test.go

@@ -722,14 +722,29 @@ func (t *tester) registerTests() {
 				},
 			})
 			if t.hasCxx() {
-				t.tests = append(t.tests, distTest{
+				t.tests = append(t.tests,
-					name:    "swig_callback",
+					distTest{
-					heading: "../misc/swig/callback",
+						name:    "swig_callback",
-					fn: func(dt *distTest) error {
+						heading: "../misc/swig/callback",
-						t.addCmd(dt, "misc/swig/callback", t.goTest())
+						fn: func(dt *distTest) error {
-						return nil
+							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
+						},
+					},
+				)
 			}
 		}
 	}

src/cmd/go/testdata/script/cgo_lto2_issue43830.txt

@@ -0,0 +1,33 @@
+# tests golang.org/issue/43830
+
+[!cgo] skip 'skipping test without cgo'
+[openbsd] env CC='clang'
+[openbsd] [!exec:clang] skip 'skipping test without clang present'
+[!openbsd] env CC='gcc'
+[!openbsd] [!exec:gcc] skip 'skipping test without gcc present'
+
+env CGO_CFLAGS='-Wno-ignored-optimization-argument -flto -ffat-lto-objects'
+
+go build main.go
+
+-- main.go --
+
+package main
+
+import "fmt"
+
+// #include "hello.h"
+import "C"
+
+func main() {
+	hello := C.hello
+	fmt.Printf("%v\n", hello)
+}
+
+-- hello.h --
+
+#include <stdio.h>
+
+void hello(void) {
+  printf("hello\n");
+}

src/cmd/go/testdata/script/cgo_lto_issue43830.txt

@@ -0,0 +1,39 @@
+# tests golang.org/issue/43830
+
+[!cgo] skip 'skipping test without cgo'
+[openbsd] env CC='clang'
+[openbsd] [!exec:clang] skip 'skipping test without clang present'
+[!openbsd] env CC='gcc'
+[!openbsd] [!exec:gcc] skip 'skipping test without gcc present'
+
+env CGO_CFLAGS='-Wno-ignored-optimization-argument -flto -ffat-lto-objects'
+
+go build main.go add.go
+
+-- main.go --
+
+package main
+
+/*
+int c_add(int a, int b) {
+	return myadd(a, b);
+}
+*/
+import "C"
+
+func main() {
+	println(C.c_add(1, 2))
+}
+
+-- add.go --
+
+package main
+
+import "C"
+
+/* test */
+
+//export myadd
+func myadd(a C.int, b C.int) C.int {
+	return a + 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)
 		}

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 := ""

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
+	// externalobj is set to true if we see an object compiled by
-	// from a package that is known to support internal linking mode.
+	// 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
 	}