cmd, runtime: remove PC_F & PC_B globals on Wasm

Following the previous CL, this removes more global variables on
Wasm.

PC_B is used mostly for intra-function jumps, and for a function
telling its callee where to start or resume. This usage can be
served by a parameter. The top level loop (wasm_pc_f_loop) uses
PC_B for resuming a function. This value is either set by gogo,
or loaded from the Go stack at function return. Instead of
loading PC_B at each function return, we could make gogo stores
PC_B at the same stack location, and let the top level loop do
the load. This way, we don't need to use global PC_B to
communicate with the top level loop, and we can replace global
PC_B with a parameter.

PC_F is similar. It is even more so in that the only reader is
the top level loop. Let the top level loop read it from the stack,
and we can get rid of PC_F entirely.

PC_F and PC_B are used less entensively as SP, so this CL has
smaller performance gain.

Running on Chrome 74.0.3729.108 on Linux/AMD64,

name                   old time/op    new time/op    delta
BinaryTree17              16.6s ± 0%     16.2s ± 1%   -2.59%  (p=0.016 n=4+5)
Fannkuch11                11.1s ± 1%     10.8s ± 0%   -2.65%  (p=0.008 n=5+5)
FmtFprintfEmpty           231ns ± 1%     217ns ± 0%   -6.06%  (p=0.008 n=5+5)
FmtFprintfString          407ns ± 3%     375ns ± 2%   -7.81%  (p=0.008 n=5+5)
FmtFprintfInt             466ns ± 2%     430ns ± 0%   -7.79%  (p=0.016 n=5+4)
FmtFprintfIntInt          719ns ± 2%     673ns ± 2%   -6.37%  (p=0.008 n=5+5)
FmtFprintfPrefixedInt     706ns ± 1%     676ns ± 3%   -4.31%  (p=0.008 n=5+5)
FmtFprintfFloat          1.01µs ± 1%    0.97µs ± 1%   -4.30%  (p=0.008 n=5+5)
FmtManyArgs              2.67µs ± 1%    2.51µs ± 1%   -5.95%  (p=0.008 n=5+5)
GobDecode                30.7ms ± 9%    31.3ms ±34%     ~     (p=0.222 n=5+5)
GobEncode                24.2ms ±23%    20.2ms ± 0%  -16.36%  (p=0.016 n=5+4)
Gzip                      852ms ± 0%     823ms ± 0%   -3.38%  (p=0.016 n=4+5)
Gunzip                    160ms ± 1%     151ms ± 1%   -5.37%  (p=0.008 n=5+5)
JSONEncode               35.7ms ± 1%    34.3ms ± 1%   -3.81%  (p=0.008 n=5+5)
JSONDecode                247ms ± 8%     254ms ± 7%     ~     (p=0.548 n=5+5)
Mandelbrot200            5.39ms ± 0%    5.41ms ± 0%   +0.42%  (p=0.008 n=5+5)
GoParse                  18.5ms ± 1%    18.3ms ± 2%     ~     (p=0.343 n=4+4)
RegexpMatchEasy0_32       424ns ± 2%     397ns ± 0%   -6.23%  (p=0.008 n=5+5)
RegexpMatchEasy0_1K      2.88µs ± 0%    2.86µs ± 1%     ~     (p=0.079 n=5+5)
RegexpMatchEasy1_32       395ns ± 2%     370ns ± 1%   -6.23%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K      3.26µs ± 0%    3.19µs ± 1%   -2.06%  (p=0.008 n=5+5)
RegexpMatchMedium_32      564ns ± 1%     532ns ± 0%   -5.71%  (p=0.008 n=5+5)
RegexpMatchMedium_1K      146µs ± 2%     140µs ± 1%   -4.62%  (p=0.008 n=5+5)
RegexpMatchHard_32       8.47µs ± 1%    7.91µs ± 1%   -6.65%  (p=0.008 n=5+5)
RegexpMatchHard_1K        253µs ± 1%     236µs ± 2%   -6.66%  (p=0.008 n=5+5)
Revcomp                   1.78s ± 4%     1.76s ± 5%     ~     (p=1.000 n=5+5)
Template                  292ms ±29%     269ms ± 5%     ~     (p=0.690 n=5+5)
TimeParse                1.61µs ± 4%    1.54µs ± 1%   -4.42%  (p=0.008 n=5+5)
TimeFormat               1.66µs ± 3%    1.58µs ± 1%   -5.22%  (p=0.008 n=5+5)
[Geo mean]                232µs          221µs        -4.54%

name                   old speed      new speed      delta
GobDecode              25.0MB/s ± 8%  25.1MB/s ±27%     ~     (p=0.222 n=5+5)
GobEncode              32.8MB/s ±21%  38.0MB/s ± 0%  +15.84%  (p=0.016 n=5+4)
Gzip                   22.8MB/s ± 0%  23.6MB/s ± 0%   +3.49%  (p=0.016 n=4+5)
Gunzip                  121MB/s ± 1%   128MB/s ± 1%   +5.68%  (p=0.008 n=5+5)
JSONEncode             54.4MB/s ± 1%  56.5MB/s ± 1%   +3.97%  (p=0.008 n=5+5)
JSONDecode             7.88MB/s ± 8%  7.65MB/s ± 8%     ~     (p=0.548 n=5+5)
GoParse                3.07MB/s ± 8%  3.00MB/s ±22%     ~     (p=0.579 n=5+5)
RegexpMatchEasy0_32    75.6MB/s ± 2%  80.5MB/s ± 0%   +6.58%  (p=0.008 n=5+5)
RegexpMatchEasy0_1K     356MB/s ± 0%   358MB/s ± 1%     ~     (p=0.095 n=5+5)
RegexpMatchEasy1_32    81.1MB/s ± 2%  86.5MB/s ± 1%   +6.69%  (p=0.008 n=5+5)
RegexpMatchEasy1_1K     314MB/s ± 0%   320MB/s ± 0%   +2.10%  (p=0.008 n=5+5)
RegexpMatchMedium_32   1.77MB/s ± 1%  1.88MB/s ± 0%   +6.09%  (p=0.016 n=5+4)
RegexpMatchMedium_1K   6.99MB/s ± 2%  7.33MB/s ± 1%   +4.83%  (p=0.008 n=5+5)
RegexpMatchHard_32     3.78MB/s ± 1%  4.04MB/s ± 1%   +7.04%  (p=0.008 n=5+5)
RegexpMatchHard_1K     4.04MB/s ± 1%  4.33MB/s ± 2%   +7.17%  (p=0.008 n=5+5)
Revcomp                 143MB/s ± 4%   145MB/s ± 5%     ~     (p=1.000 n=5+5)
Template               6.77MB/s ±24%  7.22MB/s ± 5%     ~     (p=0.690 n=5+5)
[Geo mean]             24.4MB/s       25.4MB/s        +4.18%

Change-Id: Ib80716e62992aec28b2c4a96af280c278f83aa49
Reviewed-on: https://go-review.googlesource.com/c/go/+/173980
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>

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

@@ -250,9 +250,7 @@ const (
 
 const (
 	// globals
-	REG_PC_F = obj.RBaseWasm + iota
+	REG_SP = obj.RBaseWasm + iota // SP is currently 32-bit, until 64-bit memory operations are available
-	REG_PC_B
-	REG_SP // SP is currently 32-bit, until 64-bit memory operations are available
 	REG_CTXT
 	REG_g
 	// RET* are used by runtime.return0 and runtime.reflectcall. These functions pass return values in registers.
@@ -296,9 +294,11 @@ const (
 	REG_F14
 	REG_F15
 
+	REG_PC_B // also first parameter, i32
+
 	MAXREG
 
-	MINREG  = REG_PC_F
+	MINREG  = REG_SP
 	REGSP   = REG_SP
 	REGCTXT = REG_CTXT
 	REGG    = REG_g

src/cmd/internal/obj/wasm/wasmobj.go

@@ -16,8 +16,6 @@ import (
 )
 
 var Register = map[string]int16{
-	"PC_F":  REG_PC_F,
-	"PC_B":  REG_PC_B,
 	"SP":    REG_SP,
 	"CTXT":  REG_CTXT,
 	"g":     REG_g,
@@ -60,6 +58,8 @@ var Register = map[string]int16{
 	"F13": REG_F13,
 	"F14": REG_F14,
 	"F15": REG_F15,
+
+	"PC_B": REG_PC_B,
 }
 
 var registerNames []string
@@ -368,20 +368,31 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 				break
 			}
 
-			// reset PC_B to function entry
-			p = appendp(p, AI32Const, constAddr(0))
-			p = appendp(p, ASet, regAddr(REG_PC_B))
-
 			// low-level WebAssembly call to function
 			switch jmp.To.Type {
 			case obj.TYPE_MEM:
+				if !notUsePC_B[jmp.To.Sym.Name] {
+					// Set PC_B parameter to function entry.
+					p = appendp(p, AI32Const, constAddr(0))
+				}
 				p = appendp(p, ACall, jmp.To)
+
 			case obj.TYPE_NONE:
 				// (target PC is on stack)
 				p = appendp(p, AI32WrapI64)
 				p = appendp(p, AI32Const, constAddr(16)) // only needs PC_F bits (16-31), PC_B bits (0-15) are zero
 				p = appendp(p, AI32ShrU)
+
+				// Set PC_B parameter to function entry.
+				// We need to push this before pushing the target PC_F,
+				// so temporarily pop PC_F, using our REG_PC_B as a
+				// scratch register, and push it back after pushing 0.
+				p = appendp(p, ASet, regAddr(REG_PC_B))
+				p = appendp(p, AI32Const, constAddr(0))
+				p = appendp(p, AGet, regAddr(REG_PC_B))
+
 				p = appendp(p, ACallIndirect)
+
 			default:
 				panic("bad target for JMP")
 			}
@@ -419,20 +430,31 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			})
 			p = appendp(p, AI64Store, constAddr(0))
 
-			// reset PC_B to function entry
-			p = appendp(p, AI32Const, constAddr(0))
-			p = appendp(p, ASet, regAddr(REG_PC_B))
-
 			// low-level WebAssembly call to function
 			switch call.To.Type {
 			case obj.TYPE_MEM:
+				if !notUsePC_B[call.To.Sym.Name] {
+					// Set PC_B parameter to function entry.
+					p = appendp(p, AI32Const, constAddr(0))
+				}
 				p = appendp(p, ACall, call.To)
+
 			case obj.TYPE_NONE:
 				// (target PC is on stack)
 				p = appendp(p, AI32WrapI64)
 				p = appendp(p, AI32Const, constAddr(16)) // only needs PC_F bits (16-31), PC_B bits (0-15) are zero
 				p = appendp(p, AI32ShrU)
+
+				// Set PC_B parameter to function entry.
+				// We need to push this before pushing the target PC_F,
+				// so temporarily pop PC_F, using our PC_B as a
+				// scratch register, and push it back after pushing 0.
+				p = appendp(p, ASet, regAddr(REG_PC_B))
+				p = appendp(p, AI32Const, constAddr(0))
+				p = appendp(p, AGet, regAddr(REG_PC_B))
+
 				p = appendp(p, ACallIndirect)
+
 			default:
 				panic("bad target for CALL")
 			}
@@ -465,7 +487,13 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 
 			// jump to before the call if jmpdefer has reset the return address to the call's PC
 			if call.To.Sym == deferreturn {
-				p = appendp(p, AGet, regAddr(REG_PC_B))
+				// get PC_B from -8(SP)
+				p = appendp(p, AGet, regAddr(REG_SP))
+				p = appendp(p, AI32Const, constAddr(8))
+				p = appendp(p, AI32Sub)
+				p = appendp(p, AI32Load16U, constAddr(0))
+				p = appendp(p, ATee, regAddr(REG_PC_B))
+
 				p = appendp(p, AI32Const, constAddr(call.Pc))
 				p = appendp(p, AI32Eq)
 				p = appendp(p, ABrIf, constAddr(0))
@@ -487,9 +515,8 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 
 			if ret.To.Type == obj.TYPE_MEM {
-				// reset PC_B to function entry
+				// Set PC_B parameter to function entry.
 				p = appendp(p, AI32Const, constAddr(0))
-				p = appendp(p, ASet, regAddr(REG_PC_B))
 
 				// low-level WebAssembly call to function
 				p = appendp(p, ACall, ret.To)
@@ -497,16 +524,6 @@ func preprocess(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 				break
 			}
 
-			// read return PC_F from Go stack
-			p = appendp(p, AGet, regAddr(REG_SP))
-			p = appendp(p, AI32Load16U, constAddr(2))
-			p = appendp(p, ASet, regAddr(REG_PC_F))
-
-			// read return PC_B from Go stack
-			p = appendp(p, AGet, regAddr(REG_SP))
-			p = appendp(p, AI32Load16U, constAddr(0))
-			p = appendp(p, ASet, regAddr(REG_PC_B))
-
 			// SP += 8
 			p = appendp(p, AGet, regAddr(REG_SP))
 			p = appendp(p, AI32Const, constAddr(8))
@@ -771,16 +788,38 @@ func countRegisters(s *obj.LSym) (numI, numF int16) {
 	return
 }
 
+// Most of the Go functions has a single parameter (PC_B) in
+// Wasm ABI. This is a list of exceptions.
+var notUsePC_B = map[string]bool{
+	"_rt0_wasm_js":           true,
+	"wasm_export_run":        true,
+	"wasm_export_resume":     true,
+	"wasm_export_getsp":      true,
+	"wasm_pc_f_loop":         true,
+	"runtime.wasmMove":       true,
+	"runtime.wasmZero":       true,
+	"runtime.wasmDiv":        true,
+	"runtime.wasmTruncS":     true,
+	"runtime.wasmTruncU":     true,
+	"runtime.gcWriteBarrier": true,
+	"cmpbody":                true,
+	"memeqbody":              true,
+	"memcmp":                 true,
+	"memchr":                 true,
+}
+
 func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 	w := new(bytes.Buffer)
 
 	hasLocalSP := false
+	hasPC_B := false
 	var r0, f0 int16
 
 	// Function starts with declaration of locals: numbers and types.
 	// Some functions use a special calling convention.
 	switch s.Name {
-	case "wasm_export_run", "runtime.wasmMove", "runtime.wasmZero", "runtime.wasmDiv", "runtime.wasmTruncS", "runtime.wasmTruncU", "memeqbody":
+	case "_rt0_wasm_js", "wasm_export_run", "wasm_export_resume", "wasm_export_getsp", "wasm_pc_f_loop",
+		"runtime.wasmMove", "runtime.wasmZero", "runtime.wasmDiv", "runtime.wasmTruncS", "runtime.wasmTruncU", "memeqbody":
 		writeUleb128(w, 0) // number of sets of locals
 	case "memchr", "memcmp":
 		writeUleb128(w, 1) // number of sets of locals
@@ -797,9 +836,10 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 	default:
 		// Normal calling convention: No WebAssembly parameters. First local variable is local SP cache.
 		hasLocalSP = true
+		hasPC_B = true
 		numI, numF := countRegisters(s)
-		r0 = 1
+		r0 = 2
-		f0 = 1 + numI
+		f0 = 2 + numI
 
 		numTypes := 1
 		if numI > 0 {
@@ -826,6 +866,7 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 		// Copy SP from its global variable into a local variable. Accessing a local variable is more efficient.
 		updateLocalSP(w)
 	}
+
 	for p := s.Func.Text; p != nil; p = p.Link {
 		switch p.As {
 		case AGet:
@@ -836,10 +877,16 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			switch {
 			case reg == REG_SP && hasLocalSP:
 				w.WriteByte(0x20)  // local.get
-				writeUleb128(w, 0) // local SP
+				writeUleb128(w, 1) // local SP
-			case reg >= REG_PC_F && reg <= REG_PAUSE:
+			case reg >= REG_SP && reg <= REG_PAUSE:
 				w.WriteByte(0x23) // global.get
-				writeUleb128(w, uint64(reg-REG_PC_F))
+				writeUleb128(w, uint64(reg-REG_SP))
+			case reg == REG_PC_B:
+				if !hasPC_B {
+					panic(fmt.Sprintf("PC_B is not used in %s", s.Name))
+				}
+				w.WriteByte(0x20)  // local.get (i32)
+				writeUleb128(w, 0) // local PC_B
 			case reg >= REG_R0 && reg <= REG_R15:
 				w.WriteByte(0x20) // local.get (i64)
 				writeUleb128(w, uint64(r0+(reg-REG_R0)))
@@ -857,21 +904,26 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 			reg := p.To.Reg
 			switch {
-			case reg >= REG_PC_F && reg <= REG_PAUSE:
+			case reg >= REG_SP && reg <= REG_PAUSE:
 				if reg == REG_SP && hasLocalSP {
 					w.WriteByte(0x22)  // local.tee
-					writeUleb128(w, 0) // local SP
+					writeUleb128(w, 1) // local SP
 				}
 				w.WriteByte(0x24) // global.set
-				writeUleb128(w, uint64(reg-REG_PC_F))
+				writeUleb128(w, uint64(reg-REG_SP))
-			case reg >= REG_R0 && reg <= REG_F15:
+			case reg >= REG_R0 && reg <= REG_PC_B:
 				if p.Link.As == AGet && p.Link.From.Reg == reg {
 					w.WriteByte(0x22) // local.tee
 					p = p.Link
 				} else {
 					w.WriteByte(0x21) // local.set
 				}
-				if reg <= REG_R15 {
+				if reg == REG_PC_B {
+					if !hasPC_B {
+						panic(fmt.Sprintf("PC_B is not used in %s", s.Name))
+					}
+					writeUleb128(w, 0) // local PC_B
+				} else if reg <= REG_R15 {
 					writeUleb128(w, uint64(r0+(reg-REG_R0)))
 				} else {
 					writeUleb128(w, uint64(f0+(reg-REG_F0)))
@@ -887,6 +939,12 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			}
 			reg := p.To.Reg
 			switch {
+			case reg == REG_PC_B:
+				if !hasPC_B {
+					panic(fmt.Sprintf("PC_B is not used in %s", s.Name))
+				}
+				w.WriteByte(0x22)  // local.tee (i32)
+				writeUleb128(w, 0) // local PC_B
 			case reg >= REG_R0 && reg <= REG_R15:
 				w.WriteByte(0x22) // local.tee (i64)
 				writeUleb128(w, uint64(r0+(reg-REG_R0)))
@@ -1037,9 +1095,9 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 
 func updateLocalSP(w *bytes.Buffer) {
 	w.WriteByte(0x23)  // global.get
-	writeUleb128(w, uint64(REG_SP-REG_PC_F)) // SP
+	writeUleb128(w, 0) // global SP
 	w.WriteByte(0x21)  // local.set
-	writeUleb128(w, 0)                       // local SP
+	writeUleb128(w, 1) // local SP
 }
 
 func align(as obj.As) uint64 {

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

@@ -102,10 +102,11 @@ func asmb2(ctxt *ld.Link) {
 	}
 
 	types := []*wasmFuncType{
-		// For normal Go functions the return value is
+		// For normal Go functions, the single parameter is PC_B,
+		// the return value is
 		// 0 if the function returned normally or
 		// 1 if the stack needs to be unwound.
-		{Results: []byte{I32}},
+		{Params: []byte{I32}, Results: []byte{I32}},
 	}
 
 	// collect host imports (functions that get imported from the WebAssembly host, usually JavaScript)
@@ -320,16 +321,14 @@ func writeGlobalSec(ctxt *ld.Link) {
 	sizeOffset := writeSecHeader(ctxt, sectionGlobal)
 
 	globalRegs := []byte{
-		I32, // 0: PC_F
+		I32, // 0: SP
-		I32, // 1: PC_B
+		I64, // 1: CTXT
-		I32, // 2: SP
+		I64, // 2: g
-		I64, // 3: CTXT
+		I64, // 3: RET0
-		I64, // 4: g
+		I64, // 4: RET1
-		I64, // 5: RET0
+		I64, // 5: RET2
-		I64, // 6: RET1
+		I64, // 6: RET3
-		I64, // 7: RET2
+		I32, // 7: PAUSE
-		I64, // 8: RET3
-		I32, // 9: PAUSE
 	}
 
 	writeUleb128(ctxt.Out, uint64(len(globalRegs))) // number of globals

src/runtime/asm_wasm.s

@@ -37,17 +37,12 @@ TEXT runtime·gogo(SB), NOSPLIT, $0-8
 	MOVD gobuf_g(R0), g
 	MOVD gobuf_sp(R0), SP
 
+	// Put target PC at -8(SP), wasm_pc_f_loop will pick it up
+	Get SP
+	I32Const $8
+	I32Sub
 	I64Load gobuf_pc(R0)
-	I32WrapI64
+	I64Store $0
-	I32Const $16
-	I32ShrU
-	Set PC_F
-
-	I64Load gobuf_pc(R0)
-	I64Const $0xFFFF
-	I64And
-	I32WrapI64
-	Set PC_B
 
 	MOVD gobuf_ret(R0), RET0
 	MOVD gobuf_ctxt(R0), CTXT

src/runtime/rt0_js_wasm.s

@@ -31,14 +31,9 @@ TEXT wasm_export_run(SB),NOSPLIT,$0
 	I64ExtendI32U
 	I64Store $8
 
-	I32Const $runtime·rt0_go(SB)
+	I32Const $0 // entry PC_B
-	I32Const $16
+	Call runtime·rt0_go(SB)
-	I32ShrU
+	Drop
-	Set PC_F
-
-	I32Const $0
-	Set PC_B
-
 	Call wasm_pc_f_loop(SB)
 
 	Return
@@ -46,14 +41,9 @@ TEXT wasm_export_run(SB),NOSPLIT,$0
 // wasm_export_resume gets called from JavaScript. It resumes the execution of Go code until it needs to wait for
 // an event.
 TEXT wasm_export_resume(SB),NOSPLIT,$0
-	I32Const $runtime·handleEvent(SB)
-	I32Const $16
-	I32ShrU
-	Set PC_F
-
 	I32Const $0
-	Set PC_B
+	Call runtime·handleEvent(SB)
-
+	Drop
 	Call wasm_pc_f_loop(SB)
 
 	Return
@@ -63,9 +53,23 @@ TEXT wasm_pc_f_loop(SB),NOSPLIT,$0
 // The WebAssembly stack may unwind, e.g. when switching goroutines.
 // The Go stack on the linear memory is then used to jump to the correct functions
 // with this loop, without having to restore the full WebAssembly stack.
+// It is expected to have a pending call before entering the loop, so check PAUSE first.
+	Get PAUSE
+	I32Eqz
+	If
 	loop:
 		Loop
-		Get PC_F
+			// Get PC_B & PC_F from -8(SP)
+			Get SP
+			I32Const $8
+			I32Sub
+			I32Load16U $0 // PC_B
+
+			Get SP
+			I32Const $8
+			I32Sub
+			I32Load16U $2 // PC_F
+
 			CallIndirect $0
 			Drop
 
@@ -73,6 +77,7 @@ loop:
 			I32Eqz
 			BrIf loop
 		End
+	End
 
 	I32Const $0
 	Set PAUSE
@@ -91,6 +96,7 @@ TEXT runtime·pause(SB), NOSPLIT, $0-8
 	RETUNWIND
 
 TEXT runtime·exit(SB), NOSPLIT, $0-4
+	I32Const $0
 	Call runtime·wasmExit(SB)
 	Drop
 	I32Const $1