cmd/link: trampoline support for external linking on ARM

all.bash passes with -debugtramp=2 (except the unavoidable
disassembly test as we change instructions). And successfully
build k8s.io/kubernetes/cmd/hyperkube in both internal linking
and external linking mode.

Fixes #17028.

Change-Id: Ic8fac6a394488155c5eba9215662db1c1086e24b
Reviewed-on: https://go-review.googlesource.com/31143Reviewed-by: David Chase <drchase@google.com>

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

@@ -416,6 +416,17 @@ func signext24(x int64) int32 {
 	return (int32(x) << 8) >> 8
 }
 
+// encode an immediate in ARM's imm12 format. copied from ../../../internal/obj/arm/asm5.go
+func immrot(v uint32) uint32 {
+	for i := 0; i < 16; i++ {
+		if v&^0xff == 0 {
+			return uint32(i<<8) | v | 1<<25
+		}
+		v = v<<2 | v>>30
+	}
+	return 0
+}
+
 // Convert the direct jump relocation r to refer to a trampoline if the target is too far
 func trampoline(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol) {
 	switch r.Type {
@@ -424,12 +435,18 @@ func trampoline(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol) {
 		// low 24-bit encodes the target address
 		t := (ld.Symaddr(r.Sym) + int64(signext24(r.Add&0xffffff)*4) - (s.Value + int64(r.Off))) / 4
 		if t > 0x7fffff || t < -0x800000 || (*ld.FlagDebugTramp > 1 && s.File != r.Sym.File) {
-			// direct call too far, need to insert trampoline
+			// direct call too far, need to insert trampoline.
+			// look up existing trampolines first. if we found one within the range
+			// of direct call, we can reuse it. otherwise create a new one.
 			offset := (signext24(r.Add&0xffffff) + 2) * 4
 			var tramp *ld.Symbol
 			for i := 0; ; i++ {
 				name := r.Sym.Name + fmt.Sprintf("%+d-tramp%d", offset, i)
 				tramp = ctxt.Syms.Lookup(name, int(r.Sym.Version))
+				if tramp.Type == obj.SDYNIMPORT {
+					// don't reuse trampoline defined in other module
+					continue
+				}
 				if tramp.Value == 0 {
 					// either the trampoline does not exist -- we need to create one,
 					// or found one the address which is not assigned -- this will be
@@ -447,15 +464,16 @@ func trampoline(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol) {
 			if tramp.Type == 0 {
 				// trampoline does not exist, create one
 				ctxt.AddTramp(tramp)
-				tramp.Size = 12 // 3 instructions
-				tramp.P = make([]byte, tramp.Size)
-				t = ld.Symaddr(r.Sym) + int64(offset)
-				o1 := uint32(0xe5900000 | 11<<12 | 15<<16) // MOVW (R15), R11 // R15 is actual pc + 8
-				o2 := uint32(0xe12fff10 | 11)              // JMP  (R11)
-				o3 := uint32(t)                            // WORD $target
-				ld.SysArch.ByteOrder.PutUint32(tramp.P, o1)
-				ld.SysArch.ByteOrder.PutUint32(tramp.P[4:], o2)
-				ld.SysArch.ByteOrder.PutUint32(tramp.P[8:], o3)
+				if ctxt.DynlinkingGo() {
+					if immrot(uint32(offset)) == 0 {
+						ld.Errorf(s, "odd offset in dynlink direct call: %v+%d", r.Sym, offset)
+					}
+					gentrampdyn(tramp, r.Sym, int64(offset))
+				} else if ld.Buildmode == ld.BuildmodeCArchive || ld.Buildmode == ld.BuildmodeCShared || ld.Buildmode == ld.BuildmodePIE {
+					gentramppic(tramp, r.Sym, int64(offset))
+				} else {
+					gentramp(tramp, r.Sym, int64(offset))
+				}
 			}
 			// modify reloc to point to tramp, which will be resolved later
 			r.Sym = tramp
@@ -467,6 +485,89 @@ func trampoline(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol) {
 	}
 }
 
+// generate a trampoline to target+offset
+func gentramp(tramp, target *ld.Symbol, offset int64) {
+	tramp.Size = 12 // 3 instructions
+	tramp.P = make([]byte, tramp.Size)
+	t := ld.Symaddr(target) + int64(offset)
+	o1 := uint32(0xe5900000 | 11<<12 | 15<<16) // MOVW (R15), R11 // R15 is actual pc + 8
+	o2 := uint32(0xe12fff10 | 11)              // JMP  (R11)
+	o3 := uint32(t)                            // WORD $target
+	ld.SysArch.ByteOrder.PutUint32(tramp.P, o1)
+	ld.SysArch.ByteOrder.PutUint32(tramp.P[4:], o2)
+	ld.SysArch.ByteOrder.PutUint32(tramp.P[8:], o3)
+
+	if ld.Linkmode == ld.LinkExternal {
+		r := ld.Addrel(tramp)
+		r.Off = 8
+		r.Type = obj.R_ADDR
+		r.Siz = 4
+		r.Sym = target
+		r.Add = offset
+	}
+}
+
+// generate a trampoline to target+offset in position independent code
+func gentramppic(tramp, target *ld.Symbol, offset int64) {
+	tramp.Size = 16 // 4 instructions
+	tramp.P = make([]byte, tramp.Size)
+	o1 := uint32(0xe5900000 | 11<<12 | 15<<16 | 4)  // MOVW 4(R15), R11 // R15 is actual pc + 8
+	o2 := uint32(0xe0800000 | 11<<12 | 15<<16 | 11) // ADD R15, R11, R11
+	o3 := uint32(0xe12fff10 | 11)                   // JMP  (R11)
+	o4 := uint32(0)                                 // WORD $(target-pc) // filled in with relocation
+	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 := ld.Addrel(tramp)
+	r.Off = 12
+	r.Type = obj.R_PCREL
+	r.Siz = 4
+	r.Sym = target
+	r.Add = offset + 4
+}
+
+// generate a trampoline to target+offset in dynlink mode (using GOT)
+func gentrampdyn(tramp, target *ld.Symbol, offset int64) {
+	tramp.Size = 20                                 // 5 instructions
+	o1 := uint32(0xe5900000 | 11<<12 | 15<<16 | 8)  // MOVW 8(R15), R11 // R15 is actual pc + 8
+	o2 := uint32(0xe0800000 | 11<<12 | 15<<16 | 11) // ADD R15, R11, R11
+	o3 := uint32(0xe5900000 | 11<<12 | 11<<16)      // MOVW (R11), R11
+	o4 := uint32(0xe12fff10 | 11)                   // JMP  (R11)
+	o5 := uint32(0)                                 // WORD $target@GOT // filled in with relocation
+	o6 := uint32(0)
+	if offset != 0 {
+		// insert an instruction to add offset
+		tramp.Size = 24 // 6 instructions
+		o6 = o5
+		o5 = o4
+		o4 = uint32(0xe2800000 | 11<<12 | 11<<16 | immrot(uint32(offset))) // ADD $offset, R11, R11
+		o1 = uint32(0xe5900000 | 11<<12 | 15<<16 | 12)                     // MOVW 12(R15), R11
+	}
+	tramp.P = make([]byte, tramp.Size)
+	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)
+	ld.SysArch.ByteOrder.PutUint32(tramp.P[16:], o5)
+	if offset != 0 {
+		ld.SysArch.ByteOrder.PutUint32(tramp.P[20:], o6)
+	}
+
+	r := ld.Addrel(tramp)
+	r.Off = 16
+	r.Type = obj.R_GOTPCREL
+	r.Siz = 4
+	r.Sym = target
+	r.Add = 8
+	if offset != 0 {
+		// increase reloc offset by 4 as we inserted an ADD instruction
+		r.Off = 20
+		r.Add = 12
+	}
+}
+
 func archreloc(ctxt *ld.Link, r *ld.Reloc, s *ld.Symbol, val *int64) int {
 	if ld.Linkmode == ld.LinkExternal {
 		switch r.Type {

src/cmd/link/internal/ld/data.go

@@ -330,9 +330,6 @@ func trampoline(ctxt *Link, s *Symbol) {
 	if Thearch.Trampoline == nil {
 		return // no need or no support of trampolines on this arch
 	}
-	if Linkmode == LinkExternal {
-		return // currently only support internal linking
-	}
 
 	for ri := range s.R {
 		r := &s.R[ri]

src/cmd/link/linkbig_test.go

@@ -12,6 +12,7 @@ import (
 	"bytes"
 	"cmd/internal/obj"
 	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"os/exec"
@@ -19,29 +20,33 @@ import (
 )
 
 func TestLargeText(t *testing.T) {
+	if testing.Short() || (obj.GOARCH != "ppc64le" && obj.GOARCH != "ppc64" && obj.GOARCH != "arm") {
+		t.Skip("Skipping large text section test in short mode or on %s", obj.GOARCH)
+	}
+	testenv.MustHaveGoBuild(t)
 
 	var w bytes.Buffer
-
-	if testing.Short() || (obj.GOARCH != "ppc64le" && obj.GOARCH != "ppc64") {
-		t.Skip("Skipping large text section test in short mode or if not ppc64x")
-	}
 	const FN = 4
 	tmpdir, err := ioutil.TempDir("", "bigtext")
 
 	defer os.RemoveAll(tmpdir)
 
 	// Generate the scenario where the total amount of text exceeds the
-	// limit for the bl instruction, on RISC architectures like ppc64le,
+	// limit for the jmp/call instruction, on RISC architectures like ppc64le,
 	// which is 2^26.  When that happens the call requires special trampolines or
 	// long branches inserted by the linker where supported.
-
 	// Multiple .s files are generated instead of one.
-
+	instOnArch := map[string]string{
+		"ppc64":   "\tMOVD\tR0,R3\n",
+		"ppc64le": "\tMOVD\tR0,R3\n",
+		"arm":     "\tMOVW\tR0,R1\n",
+	}
+	inst := instOnArch[obj.GOARCH]
 	for j := 0; j < FN; j++ {
 		testname := fmt.Sprintf("bigfn%d", j)
 		fmt.Fprintf(&w, "TEXT ·%s(SB),$0\n", testname)
 		for i := 0; i < 2200000; i++ {
-			fmt.Fprintf(&w, "\tMOVD\tR0,R3\n")
+			fmt.Fprintf(&w, inst)
 		}
 		fmt.Fprintf(&w, "\tRET\n")
 		err := ioutil.WriteFile(tmpdir+"/"+testname+".s", w.Bytes(), 0666)
@@ -64,7 +69,6 @@ func TestLargeText(t *testing.T) {
 	// There are lots of dummy code generated in the .s files just to generate a lot
 	// of text. Link them in but guard their call so their code is not executed but
 	// the main part of the program can be run.
-
 	fmt.Fprintf(&w, "\tif os.Getenv(\"LINKTESTARG\") != \"\" {\n")
 	for i := 0; i < FN; i++ {
 		fmt.Fprintf(&w, "\t\tbigfn%d()\n", i)
@@ -78,9 +82,8 @@ func TestLargeText(t *testing.T) {
 	}
 
 	// Build and run with internal linking.
-
 	os.Chdir(tmpdir)
-	cmd := exec.Command("go", "build", "-o", "bigtext")
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", "bigtext")
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("Build failed for big text program with internal linking: %v, output: %s", err, out)
@@ -92,9 +95,8 @@ func TestLargeText(t *testing.T) {
 	}
 
 	// Build and run with external linking
-
 	os.Chdir(tmpdir)
-	cmd = exec.Command("go", "build", "-o", "bigtext", "-ldflags", "'-linkmode=external'")
+	cmd = exec.Command(testenv.GoToolPath(t), "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)