cmd/internal/obj/mips et al.: introduce SB register on mips64x

SB register (R28) is introduced for access external addresses with shorter
instruction sequences. It is loaded at entry points. External data within
2G of SB can be accessed this way.

cmd/internal/obj: relocaltion R_ADDRMIPS is split into two relocations
R_ADDRMIPS and R_ADDRMIPSU, handling the low 16 bits and the "upper" 16
bits of external addresses, respectively, since the instructios may not
be adjacent. It might be better if relocation Variant could be used.

cmd/link/internal/mips64: support new relocations.

cmd/compile/internal/mips64: reserve SB register.

runtime: initialize SB register at entry points.

Change-Id: I5f34868f88c5a9698c042a8a1f12f76806c187b9
Reviewed-on: https://go-review.googlesource.com/19802Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/compile/internal/mips64/gsubr.go

@@ -41,6 +41,7 @@ import (
 var resvd = []int{
 	mips.REGZERO,
 	mips.REGSP,   // reserved for SP
+	mips.REGSB,   // reserved for SB
 	mips.REGLINK, // reserved for link
 	mips.REGG,
 	mips.REGTMP,

src/cmd/compile/internal/mips64/reg.go

@@ -111,7 +111,7 @@ func regnames(n *int) []string {
 
 func excludedregs() uint64 {
 	// Exclude registers with fixed functions
-	regbits := 1<<0 | RtoB(mips.REGSP) | RtoB(mips.REGG) | RtoB(mips.REGTMP) | RtoB(mips.REGLINK) | RtoB(mips.REG_R26) | RtoB(mips.REG_R27)
+	regbits := 1<<0 | RtoB(mips.REGSP) | RtoB(mips.REGG) | RtoB(mips.REGSB) | RtoB(mips.REGTMP) | RtoB(mips.REGLINK) | RtoB(mips.REG_R26) | RtoB(mips.REG_R27)
 
 	// Also exclude floating point registers with fixed constants
 	regbits |= RtoB(mips.FREGZERO) | RtoB(mips.FREGHALF) | RtoB(mips.FREGONE) | RtoB(mips.FREGTWO)

src/cmd/internal/obj/link.go

@@ -454,8 +454,8 @@ const (
 	// R_ADDRARM64 relocates an adrp, add pair to compute the address of the
 	// referenced symbol.
 	R_ADDRARM64
-	// R_ADDRMIPS (only used on mips64) resolves to a 32-bit external address,
-	// by loading the address into a register with two instructions (lui, ori).
+	// R_ADDRMIPS (only used on mips64) resolves to the low 16 bits of an external
+	// address, by encoding it into the instruction.
 	R_ADDRMIPS
 	// R_ADDROFF resolves to a 32-bit offset from the beginning of the section
 	// holding the data being relocated to the referenced symbol.
@@ -581,6 +581,10 @@ const (
 	// R_PCRELDBL relocates s390x 2-byte aligned PC-relative addresses.
 	// TODO(mundaym): remove once variants can be serialized - see issue 14218.
 	R_PCRELDBL
+
+	// R_ADDRMIPSU (only used on mips64) resolves to the sign-adjusted "upper" 16
+	// bits (bit 16-31) of an external address, by encoding it into the instruction.
+	R_ADDRMIPSU
 )
 
 type Auto struct {

src/cmd/link/internal/mips64/asm.go

@@ -34,7 +34,6 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/sys"
 	"cmd/link/internal/ld"
-	"encoding/binary"
 	"fmt"
 	"log"
 )
@@ -71,24 +70,14 @@ func archreloc(r *ld.Reloc, s *ld.LSym, val *int64) int {
 		*val = ld.Symaddr(r.Sym) + r.Add - ld.Symaddr(ld.Linklookup(ld.Ctxt, ".got", 0))
 		return 0
 
-	case obj.R_ADDRMIPS:
+	case obj.R_ADDRMIPS,
+		obj.R_ADDRMIPSU:
 		t := ld.Symaddr(r.Sym) + r.Add
-		if t >= 1<<32 || t < -1<<32 {
-			ld.Diag("program too large, address relocation = %v", t)
-		}
-
-		// the first instruction is always at the lower address, this is endian neutral;
-		// but note that o1 and o2 should still use the target endian.
 		o1 := ld.SysArch.ByteOrder.Uint32(s.P[r.Off:])
-		o2 := ld.SysArch.ByteOrder.Uint32(s.P[r.Off+4:])
-		o1 = o1&0xffff0000 | uint32(t>>16)&0xffff
-		o2 = o2&0xffff0000 | uint32(t)&0xffff
-
-		// when laid out, the instruction order must always be o1, o2.
-		if ld.Ctxt.Arch.ByteOrder == binary.BigEndian {
-			*val = int64(o1)<<32 | int64(o2)
+		if r.Type == obj.R_ADDRMIPS {
+			*val = int64(o1&0xffff0000 | uint32(t)&0xffff)
 		} else {
-			*val = int64(o2)<<32 | int64(o1)
+			*val = int64(o1&0xffff0000 | uint32((t+1<<15)>>16)&0xffff)
 		}
 		return 0
 

src/runtime/rt0_linux_mips64x.s

@@ -27,5 +27,10 @@ TEXT _main<>(SB),NOSPLIT,$-8
 	JMP main(SB)
 
 TEXT main(SB),NOSPLIT,$-8
+	// initalize REGSB = PC&0xffffffff00000000
+	BGEZAL	R0, 1(PC)
+	SRLV	$32, R31, RSB
+	SLLV	$32, RSB
+
 	MOVV	$runtime·rt0_go(SB), R4
 	JMP	(R4)

src/runtime/sys_linux_mips64x.s

@@ -233,6 +233,11 @@ TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
 	RET
 
 TEXT runtime·sigtramp(SB),NOSPLIT,$64
+	// initialize REGSB = PC&0xffffffff00000000
+	BGEZAL	R0, 1(PC)
+	SRLV	$32, R31, RSB
+	SLLV	$32, RSB
+
 	// initialize essential registers (just in case)
 	JAL	runtime·reginit(SB)
 
@@ -250,8 +255,7 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$64
 	RET
 
 TEXT runtime·cgoSigtramp(SB),NOSPLIT,$0
-	MOVV	$runtime·sigtramp(SB), R1
-	JMP	(R1)
+	JMP	runtime·sigtramp(SB)
 
 TEXT runtime·mmap(SB),NOSPLIT,$-8
 	MOVV	addr+0(FP), R4