cmd/internal/obj/x86: move MOV->XOR rewriting into compiler

Fixes #20986.

Change-Id: Ic3cf5c0ab260f259ecff7b92cfdf5f4ae432aef3
Reviewed-on: https://go-review.googlesource.com/73072
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/amd64/ssa.go

@@ -494,6 +494,18 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Reg = v.Args[0].Reg()
 	case ssa.OpAMD64MOVLconst, ssa.OpAMD64MOVQconst:
 		x := v.Reg()
+
+		// If flags aren't live (indicated by v.Aux == nil),
+		// then we can rewrite MOV $0, AX into XOR AX, AX.
+		if v.AuxInt == 0 && v.Aux == nil {
+			p := s.Prog(x86.AXORL)
+			p.From.Type = obj.TYPE_REG
+			p.From.Reg = x
+			p.To.Type = obj.TYPE_REG
+			p.To.Reg = x
+			break
+		}
+
 		asm := v.Op.Asm()
 		// Use MOVL to move a small constant into a register
 		// when the constant is positive and fits into 32 bits.
@@ -506,11 +518,6 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.From.Offset = v.AuxInt
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = x
-		// If flags are live at this instruction, suppress the
-		// MOV $0,AX -> XOR AX,AX optimization.
-		if v.Aux != nil {
-			p.Mark |= x86.PRESERVEFLAGS
-		}
 	case ssa.OpAMD64MOVSSconst, ssa.OpAMD64MOVSDconst:
 		x := v.Reg()
 		p := s.Prog(v.Op.Asm())

src/cmd/compile/internal/x86/ssa.go

@@ -426,16 +426,23 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Reg = v.Args[0].Reg()
 	case ssa.Op386MOVLconst:
 		x := v.Reg()
+
+		// If flags aren't live (indicated by v.Aux == nil),
+		// then we can rewrite MOV $0, AX into XOR AX, AX.
+		if v.AuxInt == 0 && v.Aux == nil {
+			p := s.Prog(x86.AXORL)
+			p.From.Type = obj.TYPE_REG
+			p.From.Reg = x
+			p.To.Type = obj.TYPE_REG
+			p.To.Reg = x
+			break
+		}
+
 		p := s.Prog(v.Op.Asm())
 		p.From.Type = obj.TYPE_CONST
 		p.From.Offset = v.AuxInt
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = x
-		// If flags are live at this instruction, suppress the
-		// MOV $0,AX -> XOR AX,AX optimization.
-		if v.Aux != nil {
-			p.Mark |= x86.PRESERVEFLAGS
-		}
 	case ssa.Op386MOVSSconst, ssa.Op386MOVSDconst:
 		x := v.Reg()
 		p := s.Prog(v.Op.Asm())

src/cmd/internal/obj/x86/a.out.go

@@ -36,8 +36,7 @@ import "cmd/internal/obj"
 
 const (
 	/* mark flags */
-	DONE          = 1 << iota
-	PRESERVEFLAGS // not allowed to clobber flags
+	DONE = 1 << iota
 )
 
 /*

src/cmd/internal/obj/x86/asm6.go

@@ -202,7 +202,6 @@ const (
 	Zm_ilo
 	Zib_rr
 	Zil_rr
-	Zclr
 	Zbyte
 	Zvex_rm_v_r
 	Zvex_r_v_rm
@@ -412,7 +411,6 @@ var ybtl = []ytab{
 var ymovw = []ytab{
 	{Zr_m, 1, argList{Yrl, Yml}},
 	{Zm_r, 1, argList{Yml, Yrl}},
-	{Zclr, 1, argList{Yi0, Yrl}},
 	{Zil_rp, 1, argList{Yi32, Yrl}},
 	{Zilo_m, 2, argList{Yi32, Yml}},
 	{Zaut_r, 2, argList{Yiauto, Yrl}},
@@ -421,7 +419,6 @@ var ymovw = []ytab{
 var ymovl = []ytab{
 	{Zr_m, 1, argList{Yrl, Yml}},
 	{Zm_r, 1, argList{Yml, Yrl}},
-	{Zclr, 1, argList{Yi0, Yrl}},
 	{Zil_rp, 1, argList{Yi32, Yrl}},
 	{Zilo_m, 2, argList{Yi32, Yml}},
 	{Zm_r_xm, 1, argList{Yml, Ymr}}, // MMX MOVD
@@ -447,7 +444,6 @@ var ymovq = []ytab{
 	// valid only in 64-bit mode, usually with 64-bit prefix
 	{Zr_m, 1, argList{Yrl, Yml}},      // 0x89
 	{Zm_r, 1, argList{Yml, Yrl}},      // 0x8b
-	{Zclr, 1, argList{Yi0, Yrl}},      // 0x31
 	{Zilo_m, 2, argList{Ys32, Yrl}},   // 32 bit signed 0xc7,(0)
 	{Ziq_rp, 1, argList{Yi64, Yrl}},   // 0xb8 -- 32/64 bit immediate
 	{Zilo_m, 2, argList{Yi32, Yml}},   // 0xc7,(0)
@@ -1217,7 +1213,7 @@ var optab =
 	{AMOVHLPS, yxr, Pm, [23]uint8{0x12}},
 	{AMOVHPD, yxmov, Pe, [23]uint8{0x16, 0x17}},
 	{AMOVHPS, yxmov, Pm, [23]uint8{0x16, 0x17}},
-	{AMOVL, ymovl, Px, [23]uint8{0x89, 0x8b, 0x31, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
+	{AMOVL, ymovl, Px, [23]uint8{0x89, 0x8b, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
 	{AMOVLHPS, yxr, Pm, [23]uint8{0x16}},
 	{AMOVLPD, yxmov, Pe, [23]uint8{0x12, 0x13}},
 	{AMOVLPS, yxmov, Pm, [23]uint8{0x12, 0x13}},
@@ -1230,7 +1226,7 @@ var optab =
 	{AMOVNTPD, yxr_ml, Pe, [23]uint8{0x2b}},
 	{AMOVNTPS, yxr_ml, Pm, [23]uint8{0x2b}},
 	{AMOVNTQ, ymr_ml, Pm, [23]uint8{0xe7}},
-	{AMOVQ, ymovq, Pw8, [23]uint8{0x6f, 0x7f, Pf2, 0xd6, Pf3, 0x7e, Pe, 0xd6, 0x89, 0x8b, 0x31, 0xc7, 00, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
+	{AMOVQ, ymovq, Pw8, [23]uint8{0x6f, 0x7f, Pf2, 0xd6, Pf3, 0x7e, Pe, 0xd6, 0x89, 0x8b, 0xc7, 00, 0xb8, 0xc7, 00, 0x6e, 0x7e, Pe, 0x6e, Pe, 0x7e, 0}},
 	{AMOVQOZX, ymrxr, Pf3, [23]uint8{0xd6, 0x7e}},
 	{AMOVSB, ynone, Pb, [23]uint8{0xa4}},
 	{AMOVSD, yxmov, Pf2, [23]uint8{0x10, 0x11}},
@@ -1240,7 +1236,7 @@ var optab =
 	{AMOVSW, ynone, Pe, [23]uint8{0xa5}},
 	{AMOVUPD, yxmov, Pe, [23]uint8{0x10, 0x11}},
 	{AMOVUPS, yxmov, Pm, [23]uint8{0x10, 0x11}},
-	{AMOVW, ymovw, Pe, [23]uint8{0x89, 0x8b, 0x31, 0xb8, 0xc7, 00, 0}},
+	{AMOVW, ymovw, Pe, [23]uint8{0x89, 0x8b, 0xb8, 0xc7, 00, 0}},
 	{AMOVWLSX, yml_rl, Pm, [23]uint8{0xbf}},
 	{AMOVWLZX, yml_rl, Pm, [23]uint8{0xb7}},
 	{AMOVWQSX, yml_rl, Pw, [23]uint8{0x0f, 0xbf}},
@@ -2405,10 +2401,6 @@ func oclass(ctxt *obj.Link, p *obj.Prog, a *obj.Addr) int {
 			v = int64(int32(v))
 		}
 		if v == 0 {
-			if p.Mark&PRESERVEFLAGS != 0 {
-				// If PRESERVEFLAGS is set, avoid MOV $0, AX turning into XOR AX, AX.
-				return Yu7
-			}
 			return Yi0
 		}
 		if v == 1 {
@@ -3857,11 +3849,6 @@ func (asmbuf *AsmBuf) doasm(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 				asmbuf.rexflag |= regrex[p.From.Reg] & (Rxb | 0x40)
 				asmbuf.Put1(byte(op + reg[p.From.Reg]))
 
-			case Zclr:
-				asmbuf.rexflag &^= Pw
-				asmbuf.Put1(byte(op))
-				asmbuf.asmand(ctxt, cursym, p, &p.To, &p.To)
-
 			case Zcallcon, Zjmpcon:
 				if yt.zcase == Zcallcon {
 					asmbuf.Put1(byte(op))

src/math/big/arith_amd64.s

@@ -30,8 +30,6 @@ TEXT ·divWW(SB),NOSPLIT,$0
 // The carry bit is saved with SBBQ Rx, Rx: if the carry was set, Rx is -1, otherwise it is 0.
 // It is restored with ADDQ Rx, Rx: if Rx was -1 the carry is set, otherwise it is cleared.
 // This is faster than using rotate instructions.
-//
-// CAUTION: Note that MOVQ $0, Rx is translated to XORQ Rx, Rx which clears the carry bit!
 
 // func addVV(z, x, y []Word) (c Word)
 TEXT ·addVV(SB),NOSPLIT,$0