cmd/link: optimize TLS IE to LE in build mode PIE

When cmd/compile generates position-independent code on linux
(the -shared flag), it refers to runtime.tlsg as a TLS IE variable.

When cmd/link is linking a PIE executable internally, all TLS IE
relocations are generated by cmd/compile, and the variable they
refer to, runtime.tlsg, is local to the binary. This means we can
optimize this particular IE case to LE, and thus implement IE
support when internally linking.

To do this optimization in the linker, we need to rewrite the
PC-relative MOVD to a constant load. This may seem like an
unconscionable act born of enthusiasm, but it turns out this is
standard operating procedure for linkers. GNU gold does exactly
the same optimization. I spent some time reading it and documented
at least one missing feature from this version.

Part of adding PIE internal linking on linux/amd64.

Change-Id: I1eb068d0ec774724944c6b308aa5084582ecde0b
Reviewed-on: https://go-review.googlesource.com/28540Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/internal/amd64/asm.go

@@ -789,3 +789,36 @@ func asmb(ctxt *ld.Link) {
 
 	ld.Cflush()
 }
+
+func tlsIEtoLE(s *ld.Symbol, off, size int) {
+	// Transform the PC-relative instruction into a constant load.
+	// That is,
+	//
+	//	MOVQ X(IP), REG  ->  MOVQ $Y, REG
+	//
+	// To determine the instruction and register, we study the op codes.
+	// Consult an AMD64 instruction encoding guide to decipher this.
+	op := s.P[off-3 : off]
+	reg := op[2] >> 3
+
+	if op[1] == 0x8b || reg == 4 {
+		// MOVQ
+		if op[0] == 0x4c {
+			op[0] = 0x49
+		} else if size == 4 && op[0] == 0x44 {
+			op[0] = 0x41
+		}
+		if op[1] == 0x8b {
+			op[1] = 0xc7
+		} else {
+			op[1] = 0x81 // special case for SP
+		}
+		op[2] = 0xc0 | reg
+	} else {
+		// An alternate op is ADDQ. This is handled by GNU gold,
+		// but right now is not generated by the Go compiler:
+		//	ADDQ X(IP), REG  ->  ADDQ $Y, REG
+		// Consider adding support for it here.
+		log.Fatalf("expected TLS IE op to be MOVQ, got %v", op)
+	}
+}

src/cmd/link/internal/amd64/obj.go

@@ -73,6 +73,7 @@ func linkarchinit() {
 	ld.Thearch.Append16 = ld.Append16l
 	ld.Thearch.Append32 = ld.Append32l
 	ld.Thearch.Append64 = ld.Append64l
+	ld.Thearch.TLSIEtoLE = tlsIEtoLE
 
 	ld.Thearch.Linuxdynld = "/lib64/ld-linux-x86-64.so.2"
 	ld.Thearch.Freebsddynld = "/libexec/ld-elf.so.1"

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

@@ -448,7 +448,17 @@ func relocsym(ctxt *Link, s *Symbol) {
 				}
 				break
 			}
-			log.Fatalf("cannot handle R_TLS_IE when linking internally")
+			if Buildmode == BuildmodePIE && Iself {
+				// We are linking the final executable, so we
+				// can optimize any TLS IE relocation to LE.
+				if Thearch.TLSIEtoLE == nil {
+					log.Fatalf("internal linking of TLS IE not supported on %s", SysArch.Family)
+				}
+				Thearch.TLSIEtoLE(s, int(off), int(r.Siz))
+				o = int64(ctxt.Tlsoffset)
+			} else {
+				log.Fatalf("cannot handle R_TLS_IE (sym %s) when linking internally", s.Name)
+			}
 
 		case obj.R_ADDR:
 			if Linkmode == LinkExternal && r.Sym.Type != obj.SCONST {

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

@@ -111,6 +111,14 @@ type Arch struct {
 	Append16         func(b []byte, v uint16) []byte
 	Append32         func(b []byte, v uint32) []byte
 	Append64         func(b []byte, v uint64) []byte
+
+	// TLSIEtoLE converts a TLS Initial Executable relocation to
+	// a TLS Local Executable relocation.
+	//
+	// This is possible when a TLS IE relocation refers to a local
+	// symbol in an executable, which is typical when internally
+	// linking PIE binaries.
+	TLSIEtoLE func(s *Symbol, off, size int)
 }
 
 var (