[dev.ssa] cmd/compile: disable xor clearing when flags must be preserved

The x86 backend automatically rewrites MOV $0, AX to
XOR AX, AX.  That rewrite isn't ok when the flags register
is live across the MOV.  Keep track of which moves care
about preserving flags, then disable this rewrite for them.

On x86, Prog.Mark was being used to hold the length of the
instruction.  We already store that in Prog.Isize, so no
need to store it in Prog.Mark also.  This frees up Prog.Mark
to hold a bitmask on x86 just like all the other architectures.

Update #12405

Change-Id: Ibad8a8f41fc6222bec1e4904221887d3cc3ca029
Reviewed-on: https://go-review.googlesource.com/18861Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -3405,6 +3405,7 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 	for i, b := range f.Blocks {
 		s.bstart[b.ID] = Pc
 		// Emit values in block
+		s.markMoves(b)
 		for _, v := range b.Values {
 			x := Pc
 			s.genValue(v)
@@ -3864,6 +3865,11 @@ func (s *genState) genValue(v *ssa.Value) {
 		p.From.Offset = i
 		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 := regnum(v)
 		p := Prog(v.Op.Asm())
@@ -4237,6 +4243,29 @@ func (s *genState) genValue(v *ssa.Value) {
 	}
 }
 
+// markMoves marks any MOVXconst ops that need to avoid clobbering flags.
+func (s *genState) markMoves(b *ssa.Block) {
+	flive := b.FlagsLiveAtEnd
+	if b.Control != nil && b.Control.Type.IsFlags() {
+		flive = true
+	}
+	for i := len(b.Values) - 1; i >= 0; i-- {
+		v := b.Values[i]
+		if flive && (v.Op == ssa.OpAMD64MOVWconst || v.Op == ssa.OpAMD64MOVLconst || v.Op == ssa.OpAMD64MOVQconst) {
+			// The "mark" is any non-nil Aux value.
+			v.Aux = v
+		}
+		if v.Type.IsFlags() {
+			flive = false
+		}
+		for _, a := range v.Args {
+			if a.Type.IsFlags() {
+				flive = true
+			}
+		}
+	}
+}
+
 // movZero generates a register indirect move with a 0 immediate and keeps track of bytes left and next offset
 func movZero(as int, width int64, nbytes int64, offset int64, regnum int16) (nleft int64, noff int64) {
 	p := Prog(as)

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

@@ -50,6 +50,9 @@ type Block struct {
 	// Ignored if len(Succs) < 2.
 	// Fatal if not BranchUnknown and len(Succs) > 2.
 	Likely BranchPrediction
+
+	// After flagalloc, records whether flags are live at the end of the block.
+	FlagsLiveAtEnd bool
 }
 
 //     kind           control    successors

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

@@ -120,4 +120,9 @@ func flagalloc(f *Func) {
 			// standard regs, and it runs next.)
 		}
 	}
+
+	// Save live flag state for later.
+	for _, b := range f.Blocks {
+		b.FlagsLiveAtEnd = end[b.ID] != nil
+	}
 }

src/cmd/compile/internal/ssa/gen/AMD64Ops.go

@@ -93,7 +93,6 @@ func init() {
 	// Common regInfo
 	var (
 		gp01      = regInfo{inputs: []regMask{}, outputs: gponly}
-		gp01flags = regInfo{inputs: []regMask{}, outputs: gponly, clobbers: flags}
 		gp11      = regInfo{inputs: []regMask{gpsp}, outputs: gponly, clobbers: flags}
 		gp11nf    = regInfo{inputs: []regMask{gpsp}, outputs: gponly} // nf: no flags clobbered
 		gp11sb    = regInfo{inputs: []regMask{gpspsb}, outputs: gponly}
@@ -340,12 +339,10 @@ func init() {
 		{name: "MOVLQSX", reg: gp11nf, asm: "MOVLQSX"}, // sign extend arg0 from int32 to int64
 		{name: "MOVLQZX", reg: gp11nf, asm: "MOVLQZX"}, // zero extend arg0 from int32 to int64
 
-		// clobbers flags as liblink will rewrite these to XOR reg, reg if the constant is zero
-		// TODO: revisit when issue 12405 is fixed
-		{name: "MOVBconst", reg: gp01flags, asm: "MOVB", typ: "UInt8"},  // 8 low bits of auxint
-		{name: "MOVWconst", reg: gp01flags, asm: "MOVW", typ: "UInt16"}, // 16 low bits of auxint
-		{name: "MOVLconst", reg: gp01flags, asm: "MOVL", typ: "UInt32"}, // 32 low bits of auxint
-		{name: "MOVQconst", reg: gp01flags, asm: "MOVQ", typ: "UInt64"}, // auxint
+		{name: "MOVBconst", reg: gp01, asm: "MOVB", typ: "UInt8"},  // 8 low bits of auxint
+		{name: "MOVWconst", reg: gp01, asm: "MOVW", typ: "UInt16"}, // 16 low bits of auxint
+		{name: "MOVLconst", reg: gp01, asm: "MOVL", typ: "UInt32"}, // 32 low bits of auxint
+		{name: "MOVQconst", reg: gp01, asm: "MOVQ", typ: "UInt64"}, // auxint
 
 		{name: "CVTTSD2SL", reg: fpgp, asm: "CVTTSD2SL"}, // convert float64 to int32
 		{name: "CVTTSD2SQ", reg: fpgp, asm: "CVTTSD2SQ"}, // convert float64 to int64

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

@@ -2694,7 +2694,6 @@ var opcodeTable = [...]opInfo{
 		name: "MOVBconst",
 		asm:  x86.AMOVB,
 		reg: regInfo{
-			clobbers: 8589934592, // .FLAGS
 			outputs: []regMask{
 				65519, // .AX .CX .DX .BX .BP .SI .DI .R8 .R9 .R10 .R11 .R12 .R13 .R14 .R15
 			},
@@ -2704,7 +2703,6 @@ var opcodeTable = [...]opInfo{
 		name: "MOVWconst",
 		asm:  x86.AMOVW,
 		reg: regInfo{
-			clobbers: 8589934592, // .FLAGS
 			outputs: []regMask{
 				65519, // .AX .CX .DX .BX .BP .SI .DI .R8 .R9 .R10 .R11 .R12 .R13 .R14 .R15
 			},
@@ -2714,7 +2712,6 @@ var opcodeTable = [...]opInfo{
 		name: "MOVLconst",
 		asm:  x86.AMOVL,
 		reg: regInfo{
-			clobbers: 8589934592, // .FLAGS
 			outputs: []regMask{
 				65519, // .AX .CX .DX .BX .BP .SI .DI .R8 .R9 .R10 .R11 .R12 .R13 .R14 .R15
 			},
@@ -2724,7 +2721,6 @@ var opcodeTable = [...]opInfo{
 		name: "MOVQconst",
 		asm:  x86.AMOVQ,
 		reg: regInfo{
-			clobbers: 8589934592, // .FLAGS
 			outputs: []regMask{
 				65519, // .AX .CX .DX .BX .BP .SI .DI .R8 .R9 .R10 .R11 .R12 .R13 .R14 .R15
 			},

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

@@ -1415,15 +1415,6 @@ func (v *Value) rematerializeable() bool {
 	// We can't rematerialize instructions which
 	// clobber the flags register.
 	if regspec.clobbers&flagRegMask != 0 {
-		if v.Op == OpAMD64MOVQconst && v.AuxInt != 0 ||
-			v.Op == OpAMD64MOVLconst && int32(v.AuxInt) != 0 ||
-			v.Op == OpAMD64MOVWconst && int16(v.AuxInt) != 0 ||
-			v.Op == OpAMD64MOVBconst && int8(v.AuxInt) != 0 {
-			// These are marked as clobbering flags, but only
-			// the 0 versions actually do.  TODO: fix MOV->XOR rewrites
-			// to understand when they are allowed to clobber flags?
-			return true
-		}
 		return false
 	}
 

src/cmd/internal/obj/link.go

@@ -215,13 +215,13 @@ type Prog struct {
 	As     int16
 	Reg    int16
 	RegTo2 int16  // 2nd register output operand
-	Mark   uint16
+	Mark   uint16 // bitmask of arch-specific items
 	Optab  uint16
 	Scond  uint8
 	Back   uint8
 	Ft     uint8
 	Tt     uint8
-	Isize  uint8
+	Isize  uint8 // size of the instruction in bytes (x86 only)
 	Mode   int8
 
 	Info ProgInfo

src/cmd/internal/obj/pass.go

@@ -203,7 +203,6 @@ func linkpatch(ctxt *Link, sym *LSym) {
 	}
 
 	for p := sym.Text; p != nil; p = p.Link {
-		p.Mark = 0 /* initialization for follow */
 		if p.Pcond != nil {
 			p.Pcond = brloop(ctxt, p.Pcond)
 			if p.Pcond != nil {

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

@@ -34,6 +34,12 @@ import "cmd/internal/obj"
 
 //go:generate go run ../stringer.go -i $GOFILE -o anames.go -p x86
 
+const (
+	/* mark flags */
+	DONE          = 1 << iota
+	PRESERVEFLAGS // not allowed to clobber flags
+)
+
 /*
  *	amd64
  */

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

@@ -1748,7 +1748,7 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 
 			// process forward jumps to p
 			for q = p.Rel; q != nil; q = q.Forwd {
-				v = int32(p.Pc - (q.Pc + int64(q.Mark)))
+				v = int32(p.Pc - (q.Pc + int64(q.Isize)))
 				if q.Back&2 != 0 { // short
 					if v > 127 {
 						loop++
@@ -1761,7 +1761,7 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 						s.P[q.Pc+1] = byte(v)
 					}
 				} else {
-					bp = s.P[q.Pc+int64(q.Mark)-4:]
+					bp = s.P[q.Pc+int64(q.Isize)-4:]
 					bp[0] = byte(v)
 					bp = bp[1:]
 					bp[0] = byte(v >> 8)
@@ -1784,7 +1784,6 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 
 			obj.Symgrow(ctxt, s, p.Pc+int64(m))
 			copy(s.P[p.Pc:][:m], ctxt.And[:m])
-			p.Mark = uint16(m)
 			c += int32(m)
 		}
 
@@ -2157,6 +2156,10 @@ 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 {

src/cmd/internal/obj/x86/obj6.go

@@ -1214,16 +1214,16 @@ loop:
 		q = p.Pcond
 		if q != nil && q.As != obj.ATEXT {
 			/* mark instruction as done and continue layout at target of jump */
-			p.Mark = 1
+			p.Mark |= DONE
 
 			p = q
-			if p.Mark == 0 {
+			if p.Mark&DONE == 0 {
 				goto loop
 			}
 		}
 	}
 
-	if p.Mark != 0 {
+	if p.Mark&DONE != 0 {
 		/*
 		 * p goes here, but already used it elsewhere.
 		 * copy up to 4 instructions or else branch to other copy.
@@ -1246,7 +1246,7 @@ loop:
 			if nofollow(a) || pushpop(a) {
 				break // NOTE(rsc): arm does goto copy
 			}
-			if q.Pcond == nil || q.Pcond.Mark != 0 {
+			if q.Pcond == nil || q.Pcond.Mark&DONE != 0 {
 				continue
 			}
 			if a == obj.ACALL || a == ALOOP {
@@ -1260,10 +1260,10 @@ loop:
 
 				q = obj.Copyp(ctxt, p)
 				p = p.Link
-				q.Mark = 1
+				q.Mark |= DONE
 				(*last).Link = q
 				*last = q
-				if int(q.As) != a || q.Pcond == nil || q.Pcond.Mark != 0 {
+				if int(q.As) != a || q.Pcond == nil || q.Pcond.Mark&DONE != 0 {
 					continue
 				}
 
@@ -1273,7 +1273,7 @@ loop:
 				q.Link = p
 				xfol(ctxt, q.Link, last)
 				p = q.Link
-				if p.Mark != 0 {
+				if p.Mark&DONE != 0 {
 					return
 				}
 				goto loop
@@ -1290,7 +1290,7 @@ loop:
 	}
 
 	/* emit p */
-	p.Mark = 1
+	p.Mark |= DONE
 
 	(*last).Link = p
 	*last = p
@@ -1328,7 +1328,7 @@ loop:
 			}
 		} else {
 			q = p.Link
-			if q.Mark != 0 {
+			if q.Mark&DONE != 0 {
 				if a != ALOOP {
 					p.As = relinv(int16(a))
 					p.Link = p.Pcond
@@ -1338,7 +1338,7 @@ loop:
 		}
 
 		xfol(ctxt, p.Link, last)
-		if p.Pcond.Mark != 0 {
+		if p.Pcond.Mark&DONE != 0 {
 			return
 		}
 		p = p.Pcond