cmd/link: add trampolines for too far calls in ppc64x

This change adds support for trampolines on ppc64x when using
internal linking, in the case where the offset to the branch
target is larger than what fits in the field provided by the
branch instruction.

Fixes #16665

Change-Id: Icfee72910f38c94588d2adce517b64dee6176145
Reviewed-on: https://go-review.googlesource.com/30850
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ppc64/asm.go

@@ -176,8 +176,8 @@ func genaddmoduledata(ctxt *ld.Link) {
 	// blr
 	o(0x4e800020)
 
-	ctxt.Textp = append(ctxt.Textp, initfunc)
 	initarray_entry := ctxt.Syms.Lookup("go.link.addmoduledatainit", 0)
+	ctxt.Textp = append(ctxt.Textp, initfunc)
 	initarray_entry.Attr |= ld.AttrReachable
 	initarray_entry.Attr |= ld.AttrLocal
 	initarray_entry.Type = obj.SINITARR
@@ -513,6 +513,69 @@ func archrelocaddr(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol, val *int64) int {
 	return 0
 }
 
+// resolve direct jump relocation r in s, and add trampoline if necessary
+func trampoline(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol) {
+
+	t := ld.Symaddr(r.Sym) + r.Add - (s.Value + int64(r.Off))
+	switch r.Type {
+	case obj.R_CALLPOWER:
+
+		// If branch offset is too far then create a trampoline.
+
+		if int64(int32(t<<6)>>6) != t || (*ld.FlagDebugTramp > 1 && s.File != r.Sym.File) {
+			var tramp *ld.Symbol
+			for i := 0; ; i++ {
+
+				// Using r.Add as part of the name is significant in functions like duffzero where the call
+				// target is at some offset within the function.  Calls to duff+8 and duff+256 must appear as
+				// distinct trampolines.
+
+				name := r.Sym.Name
+				if r.Add == 0 {
+					name = name + fmt.Sprintf("-tramp%d", i)
+				} else {
+					name = name + fmt.Sprintf("%+x-tramp%d", r.Add, i)
+				}
+
+				// Look up the trampoline in case it already exists
+
+				tramp = ctxt.Syms.Lookup(name, int(r.Sym.Version))
+				if tramp.Value == 0 {
+					break
+				}
+
+				t = ld.Symaddr(tramp) + r.Add - (s.Value + int64(r.Off))
+
+				// If the offset of the trampoline that has been found is within range, use it.
+				if int64(int32(t<<6)>>6) == t {
+					break
+				}
+			}
+			if tramp.Type == 0 {
+				ctxt.AddTramp(tramp)
+				tramp.Size = 16 // 4 instructions
+				tramp.P = make([]byte, tramp.Size)
+				t = ld.Symaddr(r.Sym) + r.Add
+				f := t & 0xffff0000
+				o1 := uint32(0x3fe00000 | (f >> 16)) // lis r31,trampaddr hi (r31 is temp reg)
+				f = t & 0xffff
+				o2 := uint32(0x63ff0000 | f) // ori r31,trampaddr lo
+				o3 := uint32(0x7fe903a6)     // mtctr
+				o4 := uint32(0x4e800420)     // bctr
+				ld.SysArch.ByteOrder.PutUint32(tramp.P, o1)
+				ld.SysArch.ByteOrder.PutUint32(tramp.P[4:], o2)
+				ld.SysArch.ByteOrder.PutUint32(tramp.P[8:], o3)
+				ld.SysArch.ByteOrder.PutUint32(tramp.P[12:], o4)
+			}
+			r.Sym = tramp
+			r.Add = 0 // This was folded into the trampoline target address
+			r.Done = 0
+		}
+	default:
+		ld.Errorf(s, "trampoline called with non-jump reloc: %v", r.Type)
+	}
+}
+
 func archreloc(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol, val *int64) int {
 	if ld.Linkmode == ld.LinkExternal {
 		switch r.Type {
@@ -573,15 +636,15 @@ func archreloc(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol, val *int64) int {
 		// Bits 6 through 29 = (S + A - P) >> 2
 
 		t := ld.Symaddr(r.Sym) + r.Add - (s.Value + int64(r.Off))
+
 		if t&3 != 0 {
 			ld.Errorf(s, "relocation for %s+%d is not aligned: %d", r.Sym.Name, r.Off, t)
 		}
-		if int64(int32(t<<6)>>6) != t {
-			// TODO(austin) This can happen if text > 32M.
-			// Add a call trampoline to .text in that case.
-			ld.Errorf(s, "relocation for %s+%d is too big: %d", r.Sym.Name, r.Off, t)
-		}
+		// If branch offset is too far then create a trampoline.
 
+		if int64(int32(t<<6)>>6) != t {
+			ld.Errorf(s, "direct call too far: %s %x", r.Sym.Name, t)
+		}
 		*val |= int64(uint32(t) &^ 0xfc000003)
 		return 0
 

src/cmd/link/internal/ppc64/obj.go

@@ -58,6 +58,7 @@ func Init() {
 	ld.Thearch.Elfreloc1 = elfreloc1
 	ld.Thearch.Elfsetupplt = elfsetupplt
 	ld.Thearch.Gentext = gentext
+	ld.Thearch.Trampoline = trampoline
 	ld.Thearch.Machoreloc1 = machoreloc1
 	if ld.SysArch == sys.ArchPPC64LE {
 		ld.Thearch.Lput = ld.Lputl

src/cmd/link/linkbig_test.go

@@ -77,15 +77,31 @@ func TestLargeText(t *testing.T) {
 		t.Fatalf("can't write output: %v\n", err)
 	}
 
+	// Build and run with internal linking.
+
 	os.Chdir(tmpdir)
-	cmd := exec.Command("go", "build", "-o", "bigtext", "-ldflags", "'-linkmode=external'")
+	cmd := exec.Command("go", "build", "-o", "bigtext")
 	out, err := cmd.CombinedOutput()
 	if err != nil {
-		t.Fatalf("Build of big text program failed: %v, output: %s", err, out)
+		t.Fatalf("Build failed for big text program with internal linking: %v, output: %s", err, out)
 	}
 	cmd = exec.Command(tmpdir + "/bigtext")
 	out, err = cmd.CombinedOutput()
 	if err != nil {
-		t.Fatalf("Program failed with err %v, output: %s", err, out)
+		t.Fatalf("Program built with internal linking failed to run with err %v, output: %s", err, out)
+	}
+
+	// Build and run with external linking
+
+	os.Chdir(tmpdir)
+	cmd = exec.Command("go", "build", "-o", "bigtext", "-ldflags", "'-linkmode=external'")
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Build failed for big text program with external linking: %v, output: %s", err, out)
+	}
+	cmd = exec.Command(tmpdir + "/bigtext")
+	out, err = cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Program built with external linking failed to run with err %v, output: %s", err, out)
 	}
 }