[dev.ssa] cmd/compile/internal/ssa: New register allocator

Implement a global (whole function) register allocator.
This replaces the local (per basic block) register allocator.

Clobbering of registers by instructions is handled properly.
A separate change will add the correct clobbers to all the instructions.

Change-Id: I38ce4dc7dccb8303c1c0e0295fe70247b0a3f2ea
Reviewed-on: https://go-review.googlesource.com/13622Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Todd Neal <todd@tneal.org>

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

@@ -2277,7 +2277,10 @@ func genValue(v *ssa.Value) {
 		p.To.Reg = x86.REG_SP
 		p.To.Offset = localOffset(v)
 	case ssa.OpPhi:
-		// just check to make sure regalloc did it right
+		// just check to make sure regalloc and stackalloc did it right
+		if v.Type.IsMemory() {
+			return
+		}
 		f := v.Block.Func
 		loc := f.RegAlloc[v.ID]
 		for _, a := range v.Args {
@@ -2376,13 +2379,16 @@ func genValue(v *ssa.Value) {
 	case ssa.OpAMD64InvertFlags:
 		v.Fatalf("InvertFlags should never make it to codegen %v", v)
 	case ssa.OpAMD64REPSTOSQ:
+		p := Prog(x86.AXORL) // TODO: lift out zeroing into its own instruction?
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = x86.REG_AX
+		p.To.Type = obj.TYPE_REG
+		p.To.Reg = x86.REG_AX
 		Prog(x86.AREP)
 		Prog(x86.ASTOSQ)
-		v.Unimplementedf("REPSTOSQ clobbers not implemented: %s", v.LongString())
 	case ssa.OpAMD64REPMOVSB:
 		Prog(x86.AREP)
 		Prog(x86.AMOVSB)
-		v.Unimplementedf("REPMOVSB clobbers not implemented: %s", v.LongString())
 	default:
 		v.Unimplementedf("genValue not implemented: %s", v.LongString())
 	}

src/cmd/compile/internal/gc/testdata/regalloc_ssa.go

+// run
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests phi implementation
+
+package main
+
+func phiOverwrite_ssa() int {
+	var n int
+	for i := 0; i < 10; i++ {
+		if i == 6 {
+			break
+		}
+		n = i
+	}
+	return n
+}
+
+func phiOverwrite() {
+	want := 5
+	got := phiOverwrite_ssa()
+	if got != want {
+		println("phiOverwrite_ssa()=", want, ", got", got)
+		failed = true
+	}
+}
+
+func phiOverwriteBig_ssa() int {
+	var a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z int
+	a = 1
+	for idx := 0; idx < 26; idx++ {
+		a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z = b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, a
+	}
+	return a*1 + b*2 + c*3 + d*4 + e*5 + f*6 + g*7 + h*8 + i*9 + j*10 + k*11 + l*12 + m*13 + n*14 + o*15 + p*16 + q*17 + r*18 + s*19 + t*20 + u*21 + v*22 + w*23 + x*24 + y*25 + z*26
+}
+
+func phiOverwriteBig() {
+	want := 1
+	got := phiOverwriteBig_ssa()
+	if got != want {
+		println("phiOverwriteBig_ssa()=", want, ", got", got)
+		failed = true
+	}
+}
+
+var failed = false
+
+func main() {
+	phiOverwrite()
+	phiOverwriteBig()
+	if failed {
+		panic("failed")
+	}
+}

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

@@ -59,6 +59,14 @@ func findlive(f *Func) (reachable []bool, live []bool) {
 
 // deadcode removes dead code from f.
 func deadcode(f *Func) {
+	// deadcode after regalloc is forbidden for now.  Regalloc
+	// doesn't quite generate legal SSA which will lead to some
+	// required moves being eliminated.  See the comment at the
+	// top of regalloc.go for details.
+	if f.RegAlloc != nil {
+		f.Fatalf("deadcode after regalloc")
+	}
+
 	reachable, live := findlive(f)
 
 	// Remove dead values from blocks' value list.  Return dead

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

@@ -72,13 +72,14 @@ func init() {
 
 	// Common individual register masks
 	var (
-		cx     = buildReg("CX")
-		x15    = buildReg("X15")
-		gp     = buildReg("AX CX DX BX BP SI DI R8 R9 R10 R11 R12 R13 R14 R15")
-		fp     = buildReg("X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15")
-		gpsp   = gp | buildReg("SP")
-		gpspsb = gpsp | buildReg("SB")
-		flags  = buildReg("FLAGS")
+		cx         = buildReg("CX")
+		x15        = buildReg("X15")
+		gp         = buildReg("AX CX DX BX BP SI DI R8 R9 R10 R11 R12 R13 R14 R15")
+		fp         = buildReg("X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15")
+		gpsp       = gp | buildReg("SP")
+		gpspsb     = gpsp | buildReg("SB")
+		flags      = buildReg("FLAGS")
+		callerSave = gp | fp | flags
 	)
 
 	// Common slices of register masks
@@ -90,16 +91,16 @@ func init() {
 
 	// Common regInfo
 	var (
-		gp01      = regInfo{inputs: []regMask{}, outputs: gponly}
-		gp11      = regInfo{inputs: []regMask{gpsp}, outputs: gponly}
-		gp11sb    = regInfo{inputs: []regMask{gpspsb}, outputs: gponly}
-		gp21      = regInfo{inputs: []regMask{gpsp, gpsp}, outputs: gponly}
-		gp21sb    = regInfo{inputs: []regMask{gpspsb, gpsp}, outputs: gponly}
-		gp21shift = regInfo{inputs: []regMask{gpsp, cx}, outputs: []regMask{gp &^ cx}}
+		gp01      = regInfo{inputs: []regMask{}, outputs: gponly, clobbers: flags}
+		gp11      = regInfo{inputs: []regMask{gpsp}, outputs: gponly, clobbers: flags}
+		gp11sb    = regInfo{inputs: []regMask{gpspsb}, outputs: gponly, clobbers: flags}
+		gp21      = regInfo{inputs: []regMask{gpsp, gpsp}, outputs: gponly, clobbers: flags}
+		gp21sb    = regInfo{inputs: []regMask{gpspsb, gpsp}, outputs: gponly, clobbers: flags}
+		gp21shift = regInfo{inputs: []regMask{gpsp, cx}, outputs: []regMask{gp &^ cx}, clobbers: flags}
 
 		gp2flags = regInfo{inputs: []regMask{gpsp, gpsp}, outputs: flagsonly}
 		gp1flags = regInfo{inputs: []regMask{gpsp}, outputs: flagsonly}
-		flagsgp  = regInfo{inputs: flagsonly, outputs: gponly}
+		flagsgp  = regInfo{inputs: flagsonly, outputs: gponly, clobbers: flags}
 
 		gpload    = regInfo{inputs: []regMask{gpspsb, 0}, outputs: gponly}
 		gploadidx = regInfo{inputs: []regMask{gpspsb, gpsp, 0}, outputs: gponly}
@@ -122,6 +123,7 @@ func init() {
 		fpstore    = regInfo{inputs: []regMask{gpspsb, fp, 0}}
 		fpstoreidx = regInfo{inputs: []regMask{gpspsb, gpsp, fp, 0}}
 	)
+	// TODO: most ops clobber flags
 
 	// Suffixes encode the bit width of various instructions.
 	// Q = 64 bit, L = 32 bit, W = 16 bit, B = 8 bit
@@ -318,8 +320,8 @@ func init() {
 		{name: "REPSTOSQ", reg: regInfo{[]regMask{buildReg("DI"), buildReg("CX")}, buildReg("DI AX CX"), nil}}, // store arg1 8-byte words containing zero into arg0 using STOSQ. arg2=mem.
 
 		//TODO: set register clobber to everything?
-		{name: "CALLstatic"},                                                            // call static function aux.(*gc.Sym).  arg0=mem, returns mem
-		{name: "CALLclosure", reg: regInfo{[]regMask{gpsp, buildReg("DX"), 0}, 0, nil}}, // call function via closure.  arg0=codeptr, arg1=closure, arg2=mem returns mem
+		{name: "CALLstatic", reg: regInfo{clobbers: callerSave}},                                 // call static function aux.(*gc.Sym).  arg0=mem, returns mem
+		{name: "CALLclosure", reg: regInfo{[]regMask{gpsp, buildReg("DX"), 0}, callerSave, nil}}, // call function via closure.  arg0=codeptr, arg1=closure, arg2=mem returns mem
 
 		{name: "REPMOVSB", reg: regInfo{[]regMask{buildReg("DI"), buildReg("SI"), buildReg("CX")}, buildReg("DI SI CX"), nil}}, // move arg2 bytes from arg1 to arg0.  arg3=mem, returns memory
 

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

@@ -15,6 +15,7 @@ import (
 	"io/ioutil"
 	"log"
 	"regexp"
+	"sort"
 )
 
 type arch struct {
@@ -125,11 +126,22 @@ func genOp() {
 				fmt.Fprintf(w, "asm: x86.A%s,\n", v.asm)
 			}
 			fmt.Fprintln(w, "reg:regInfo{")
-			// reg inputs
-			if len(v.reg.inputs) > 0 {
-				fmt.Fprintln(w, "inputs: []regMask{")
-				for _, r := range v.reg.inputs {
-					fmt.Fprintf(w, "%d,%s\n", r, a.regMaskComment(r))
+
+			// Compute input allocation order.  We allocate from the
+			// most to the least constrained input.  This order guarantees
+			// that we will always be able to find a register.
+			var s []intPair
+			for i, r := range v.reg.inputs {
+				if r != 0 {
+					s = append(s, intPair{countRegs(r), i})
+				}
+			}
+			if len(s) > 0 {
+				sort.Sort(byKey(s))
+				fmt.Fprintln(w, "inputs: []inputInfo{")
+				for _, p := range s {
+					r := v.reg.inputs[p.val]
+					fmt.Fprintf(w, "{%d,%d},%s\n", p.val, r, a.regMaskComment(r))
 				}
 				fmt.Fprintln(w, "},")
 			}
@@ -205,3 +217,23 @@ func genLower() {
 		genRules(a)
 	}
 }
+
+// countRegs returns the number of set bits in the register mask.
+func countRegs(r regMask) int {
+	n := 0
+	for r != 0 {
+		n += int(r & 1)
+		r >>= 1
+	}
+	return n
+}
+
+// for sorting a pair of integers by key
+type intPair struct {
+	key, val int
+}
+type byKey []intPair
+
+func (a byKey) Len() int           { return len(a) }
+func (a byKey) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byKey) Less(i, j int) bool { return a[i].key < a[j].key }

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

@@ -362,7 +362,7 @@ func (v *Value) LongHTML() string {
 		s += fmt.Sprintf(" %s", a.HTML())
 	}
 	r := v.Block.Func.RegAlloc
-	if r != nil && r[v.ID] != nil {
+	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + r[v.ID].Name()
 	}
 

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

@@ -19,8 +19,13 @@ type opInfo struct {
 	generic bool // this is a generic (arch-independent) opcode
 }
 
+type inputInfo struct {
+	idx  int     // index in Args array
+	regs regMask // allowed input registers
+}
+
 type regInfo struct {
-	inputs   []regMask
+	inputs   []inputInfo // ordered in register allocation order
 	clobbers regMask
 	outputs  []regMask // NOTE: values can only have 1 output for now.
 }

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

@@ -4,6 +4,15 @@
 
 package ssa
 
+// setloc sets the home location of v to loc.
+func setloc(home []Location, v *Value, loc Location) []Location {
+	for v.ID >= ID(len(home)) {
+		home = append(home, nil)
+	}
+	home[v.ID] = loc
+	return home
+}
+
 // stackalloc allocates storage in the stack frame for
 // all Values that did not get a register.
 func stackalloc(f *Func) {
@@ -26,7 +35,7 @@ func stackalloc(f *Func) {
 	// so stackmap is smaller.
 
 	// Assign stack locations to phis first, because we
-	// must also assign the same locations to the phi copies
+	// must also assign the same locations to the phi stores
 	// introduced during regalloc.
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
@@ -36,12 +45,19 @@ func stackalloc(f *Func) {
 			if v.Type.IsMemory() { // TODO: only "regallocable" types
 				continue
 			}
+			if int(v.ID) < len(home) && home[v.ID] != nil {
+				continue // register-based phi
+			}
+			// stack-based phi
 			n = align(n, v.Type.Alignment())
 			f.Logf("stackalloc: %d-%d for %v\n", n, n+v.Type.Size(), v)
 			loc := &LocalSlot{n}
 			n += v.Type.Size()
 			home = setloc(home, v, loc)
 			for _, w := range v.Args {
+				if w.Op != OpStoreReg {
+					f.Fatalf("stack-based phi must have StoreReg args")
+				}
 				home = setloc(home, w, loc)
 			}
 		}

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

@@ -57,13 +57,6 @@ func tighten(f *Func) {
 				if v.Op == OpPhi {
 					continue
 				}
-				if v.Op == OpSB || v.Op == OpSP {
-					// regalloc expects OpSP and OpSB values to be in the entry block,
-					// so don't move them.
-					// TODO: Handle this more gracefully in regalloc and
-					// remove this restriction.
-					continue
-				}
 				if uses[v.ID] == 1 && !phi[v.ID] && home[v.ID] != b && len(v.Args) < 2 {
 					// v is used in exactly one block, and it is not b.
 					// Furthermore, it takes at most one input,

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

@@ -11,7 +11,7 @@ import "fmt"
 // if they preserve the value of the Value (e.g. changing a (mul 2 x) to an (add x x)).
 type Value struct {
 	// A unique identifier for the value.  For performance we allocate these IDs
-	// densely starting at 0.  There is no guarantee that there won't be occasional holes, though.
+	// densely starting at 1.  There is no guarantee that there won't be occasional holes, though.
 	ID ID
 
 	// The operation that computes this value.  See op.go.
@@ -69,7 +69,7 @@ func (v *Value) LongString() string {
 		s += fmt.Sprintf(" %v", a)
 	}
 	r := v.Block.Func.RegAlloc
-	if r != nil && r[v.ID] != nil {
+	if int(v.ID) < len(r) && r[v.ID] != nil {
 		s += " : " + r[v.ID].Name()
 	}
 	return s