cmd/link: remove absolute address for c-archive on darwin/arm

Now it is possible to build a c-archive as PIC on darwin/arm (this is
now the default). Then the system linker can link the binary using
the archive as PIE.

Fixes #12896.

Change-Id: Iad84131572422190f5fa036e7d71910dc155f155
Reviewed-on: https://go-review.googlesource.com/22461Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/build.go

@@ -334,6 +334,11 @@ func buildModeInit() {
 			}
 			return p
 		}
+		switch platform {
+		case "darwin/arm":
+			codegenArg = "-shared"
+		default:
+		}
 		exeSuffix = ".a"
 		ldBuildmode = "c-archive"
 	case "c-shared":

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

@@ -330,6 +330,36 @@ func machoreloc1(r *ld.Reloc, sectoff int64) int {
 
 	rs := r.Xsym
 
+	if r.Type == obj.R_PCREL {
+		if rs.Type == obj.SHOSTOBJ {
+			ld.Diag("pc-relative relocation of external symbol is not supported")
+			return -1
+		}
+		if r.Siz != 4 {
+			return -1
+		}
+
+		// emit a pair of "scattered" relocations that
+		// resolve to the difference of section addresses of
+		// the symbol and the instruction
+		// this value is added to the field being relocated
+		o1 := uint32(sectoff)
+		o1 |= 1 << 31 // scattered bit
+		o1 |= ld.MACHO_ARM_RELOC_SECTDIFF << 24
+		o1 |= 2 << 28 // size = 4
+
+		o2 := uint32(0)
+		o2 |= 1 << 31 // scattered bit
+		o2 |= ld.MACHO_ARM_RELOC_PAIR << 24
+		o2 |= 2 << 28 // size = 4
+
+		ld.Thearch.Lput(o1)
+		ld.Thearch.Lput(uint32(ld.Symaddr(rs)))
+		ld.Thearch.Lput(o2)
+		ld.Thearch.Lput(uint32(ld.Ctxt.Cursym.Value + int64(r.Off)))
+		return 0
+	}
+
 	if rs.Type == obj.SHOSTOBJ || r.Type == obj.R_CALLARM {
 		if rs.Dynid < 0 {
 			ld.Diag("reloc %d to non-macho symbol %s type=%d", r.Type, rs.Name, rs.Type)

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

@@ -560,6 +560,9 @@ func relocsym(s *LSym) {
 							o += int64(uint64(Symaddr(rs)) - rs.Sect.Vaddr)
 						}
 						o -= int64(r.Off) // relative to section offset, not symbol
+					} else if SysArch.Family == sys.ARM {
+						// see ../arm/asm.go:/machoreloc1
+						o += Symaddr(rs) - int64(Ctxt.Cursym.Value) - int64(r.Off)
 					} else {
 						o += int64(r.Siz)
 					}

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

@@ -79,6 +79,8 @@ const (
 	MACHO_X86_64_RELOC_SIGNED_2   = 7
 	MACHO_X86_64_RELOC_SIGNED_4   = 8
 	MACHO_ARM_RELOC_VANILLA       = 0
+	MACHO_ARM_RELOC_PAIR          = 1
+	MACHO_ARM_RELOC_SECTDIFF      = 2
 	MACHO_ARM_RELOC_BR24          = 5
 	MACHO_ARM64_RELOC_UNSIGNED    = 0
 	MACHO_ARM64_RELOC_BRANCH26    = 2
@@ -350,8 +352,9 @@ func machoshbits(mseg *MachoSeg, sect *Section, segname string) {
 
 	var msect *MachoSect
 	if sect.Rwx&1 == 0 && segname != "__DWARF" && (SysArch.Family == sys.ARM64 ||
-		(SysArch.Family == sys.AMD64 && (Buildmode == BuildmodeCShared || Buildmode == BuildmodeCArchive))) {
-		// Darwin external linker on arm64 and on amd64 in c-shared/c-archive buildmode
+		(SysArch.Family == sys.AMD64 && (Buildmode == BuildmodeCShared || Buildmode == BuildmodeCArchive)) ||
+		(SysArch.Family == sys.ARM && (Buildmode == BuildmodeCShared || Buildmode == BuildmodeCArchive))) {
+		// Darwin external linker on arm64 and on amd64 and arm in c-shared/c-archive buildmode
 		// complains about absolute relocs in __TEXT, so if the section is not
 		// executable, put it in __DATA segment.
 		msect = newMachoSect(mseg, buf, "__DATA")

src/runtime/cgo/signal_darwin_armx.go

@@ -13,10 +13,14 @@ import "unsafe"
 //go:linkname x_cgo_panicmem x_cgo_panicmem
 var x_cgo_panicmem uintptr
 
+// use a pointer to avoid relocation of external symbol in __TEXT
+// make linker happy
+var _cgo_panicmem = &x_cgo_panicmem
+
 // TODO(crawshaw): move this into x_cgo_init, it will not run until
 // runtime has finished loading, which may be after its use.
 func init() {
-	x_cgo_panicmem = funcPC(panicmem)
+	*_cgo_panicmem = funcPC(panicmem)
 }
 
 func funcPC(f interface{}) uintptr {