cmd/compile: add saturating conversions on wasm

This change adds the GOWASM option "satconv" to enable the generation
of experimental saturating (non-trapping) float-to-int conversions.
It improves the performance of the conversion by 42%.

Previously the conversions had already been augmented with helper
functions to have saturating behavior. Now Wasm.rules is always using
the new operation names and wasm/ssa.go is falling back to the helpers
if the feature is not enabled.

The feature is in phase 4 of the WebAssembly proposal process:
https://github.com/WebAssembly/meetings/blob/master/process/phases.md

More information on the feature can be found at:
https://github.com/WebAssembly/nontrapping-float-to-int-conversions/blob/master/proposals/nontrapping-float-to-int-conversion/Overview.md

Change-Id: Ic6c3688017054ede804b02b6b0ffd4a02ef33ad7
Reviewed-on: https://go-review.googlesource.com/c/go/+/170119Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/install-source.html

@@ -645,6 +645,7 @@ for which the compiler will target. The default is <code>power8</code>.
 	The default is to use no experimental features.
 	</p>
 	<ul>
+		<li><code>GOWASM=satconv</code>: generate <a href="https://github.com/WebAssembly/nontrapping-float-to-int-conversions/blob/master/proposals/nontrapping-float-to-int-conversion/Overview.md">saturating (non-trapping) float-to-int conversions</a></li>
 		<li><code>GOWASM=signext</code>: generate <a href="https://github.com/WebAssembly/sign-extension-ops/blob/master/proposals/sign-extension-ops/Overview.md">sign-extension operators</a></li>
 	</ul>
 </li>

src/cmd/compile/internal/ssa/gen/Wasm.rules

@@ -84,14 +84,14 @@
 (Cvt64Uto32F x) -> (LoweredRound32F (F64ConvertI64U x))
 (Cvt64Uto64F x) -> (F64ConvertI64U x)
 
-(Cvt32Fto32 x) -> (I64TruncF64S x)
-(Cvt32Fto64 x) -> (I64TruncF64S x)
-(Cvt64Fto32 x) -> (I64TruncF64S x)
-(Cvt64Fto64 x) -> (I64TruncF64S x)
-(Cvt32Fto32U x) -> (I64TruncF64U x)
-(Cvt32Fto64U x) -> (I64TruncF64U x)
-(Cvt64Fto32U x) -> (I64TruncF64U x)
-(Cvt64Fto64U x) -> (I64TruncF64U x)
+(Cvt32Fto32 x) -> (I64TruncSatF64S x)
+(Cvt32Fto64 x) -> (I64TruncSatF64S x)
+(Cvt64Fto32 x) -> (I64TruncSatF64S x)
+(Cvt64Fto64 x) -> (I64TruncSatF64S x)
+(Cvt32Fto32U x) -> (I64TruncSatF64U x)
+(Cvt32Fto64U x) -> (I64TruncSatF64U x)
+(Cvt64Fto32U x) -> (I64TruncSatF64U x)
+(Cvt64Fto64U x) -> (I64TruncSatF64U x)
 
 (Cvt32Fto64F x) -> x
 (Cvt64Fto32F x) -> (LoweredRound32F x)

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

@@ -187,8 +187,8 @@ func init() {
 		{name: "F64Mul", asm: "F64Mul", argLength: 2, reg: fp21, typ: "Float64"}, // arg0 * arg1
 		{name: "F64Div", asm: "F64Div", argLength: 2, reg: fp21, typ: "Float64"}, // arg0 / arg1
 
-		{name: "I64TruncF64S", asm: "I64TruncF64S", argLength: 1, reg: regInfo{inputs: []regMask{fp}, outputs: []regMask{gp}}, typ: "Int64"},       // truncates the float arg0 to a signed integer
-		{name: "I64TruncF64U", asm: "I64TruncF64U", argLength: 1, reg: regInfo{inputs: []regMask{fp}, outputs: []regMask{gp}}, typ: "Int64"},       // truncates the float arg0 to an unsigned integer
+		{name: "I64TruncSatF64S", asm: "I64TruncSatF64S", argLength: 1, reg: regInfo{inputs: []regMask{fp}, outputs: []regMask{gp}}, typ: "Int64"}, // truncates the float arg0 to a signed integer (saturating)
+		{name: "I64TruncSatF64U", asm: "I64TruncSatF64U", argLength: 1, reg: regInfo{inputs: []regMask{fp}, outputs: []regMask{gp}}, typ: "Int64"}, // truncates the float arg0 to an unsigned integer (saturating)
 		{name: "F64ConvertI64S", asm: "F64ConvertI64S", argLength: 1, reg: regInfo{inputs: []regMask{gp}, outputs: []regMask{fp}}, typ: "Float64"}, // converts the signed integer arg0 to a float
 		{name: "F64ConvertI64U", asm: "F64ConvertI64U", argLength: 1, reg: regInfo{inputs: []regMask{gp}, outputs: []regMask{fp}}, typ: "Float64"}, // converts the unsigned integer arg0 to a float
 

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

@@ -2132,8 +2132,8 @@ const (
 	OpWasmF64Sub
 	OpWasmF64Mul
 	OpWasmF64Div
-	OpWasmI64TruncF64S
-	OpWasmI64TruncF64U
+	OpWasmI64TruncSatF64S
+	OpWasmI64TruncSatF64U
 	OpWasmF64ConvertI64S
 	OpWasmF64ConvertI64U
 	OpWasmI64Extend8S
@@ -28676,9 +28676,9 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:   "I64TruncF64S",
+		name:   "I64TruncSatF64S",
 		argLen: 1,
-		asm:    wasm.AI64TruncF64S,
+		asm:    wasm.AI64TruncSatF64S,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 4294901760}, // F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15
@@ -28689,9 +28689,9 @@ var opcodeTable = [...]opInfo{
 		},
 	},
 	{
-		name:   "I64TruncF64U",
+		name:   "I64TruncSatF64U",
 		argLen: 1,
-		asm:    wasm.AI64TruncF64U,
+		asm:    wasm.AI64TruncSatF64U,
 		reg: regInfo{
 			inputs: []inputInfo{
 				{0, 4294901760}, // F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12 F13 F14 F15

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

@@ -1095,10 +1095,10 @@ func rewriteValueWasm_OpCtz8NonZero_0(v *Value) bool {
 func rewriteValueWasm_OpCvt32Fto32_0(v *Value) bool {
 	// match: (Cvt32Fto32 x)
 	// cond:
-	// result: (I64TruncF64S x)
+	// result: (I64TruncSatF64S x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64S)
+		v.reset(OpWasmI64TruncSatF64S)
 		v.AddArg(x)
 		return true
 	}
@@ -1106,10 +1106,10 @@ func rewriteValueWasm_OpCvt32Fto32_0(v *Value) bool {
 func rewriteValueWasm_OpCvt32Fto32U_0(v *Value) bool {
 	// match: (Cvt32Fto32U x)
 	// cond:
-	// result: (I64TruncF64U x)
+	// result: (I64TruncSatF64U x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64U)
+		v.reset(OpWasmI64TruncSatF64U)
 		v.AddArg(x)
 		return true
 	}
@@ -1117,10 +1117,10 @@ func rewriteValueWasm_OpCvt32Fto32U_0(v *Value) bool {
 func rewriteValueWasm_OpCvt32Fto64_0(v *Value) bool {
 	// match: (Cvt32Fto64 x)
 	// cond:
-	// result: (I64TruncF64S x)
+	// result: (I64TruncSatF64S x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64S)
+		v.reset(OpWasmI64TruncSatF64S)
 		v.AddArg(x)
 		return true
 	}
@@ -1140,10 +1140,10 @@ func rewriteValueWasm_OpCvt32Fto64F_0(v *Value) bool {
 func rewriteValueWasm_OpCvt32Fto64U_0(v *Value) bool {
 	// match: (Cvt32Fto64U x)
 	// cond:
-	// result: (I64TruncF64U x)
+	// result: (I64TruncSatF64U x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64U)
+		v.reset(OpWasmI64TruncSatF64U)
 		v.AddArg(x)
 		return true
 	}
@@ -1215,10 +1215,10 @@ func rewriteValueWasm_OpCvt32to64F_0(v *Value) bool {
 func rewriteValueWasm_OpCvt64Fto32_0(v *Value) bool {
 	// match: (Cvt64Fto32 x)
 	// cond:
-	// result: (I64TruncF64S x)
+	// result: (I64TruncSatF64S x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64S)
+		v.reset(OpWasmI64TruncSatF64S)
 		v.AddArg(x)
 		return true
 	}
@@ -1237,10 +1237,10 @@ func rewriteValueWasm_OpCvt64Fto32F_0(v *Value) bool {
 func rewriteValueWasm_OpCvt64Fto32U_0(v *Value) bool {
 	// match: (Cvt64Fto32U x)
 	// cond:
-	// result: (I64TruncF64U x)
+	// result: (I64TruncSatF64U x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64U)
+		v.reset(OpWasmI64TruncSatF64U)
 		v.AddArg(x)
 		return true
 	}
@@ -1248,10 +1248,10 @@ func rewriteValueWasm_OpCvt64Fto32U_0(v *Value) bool {
 func rewriteValueWasm_OpCvt64Fto64_0(v *Value) bool {
 	// match: (Cvt64Fto64 x)
 	// cond:
-	// result: (I64TruncF64S x)
+	// result: (I64TruncSatF64S x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64S)
+		v.reset(OpWasmI64TruncSatF64S)
 		v.AddArg(x)
 		return true
 	}
@@ -1259,10 +1259,10 @@ func rewriteValueWasm_OpCvt64Fto64_0(v *Value) bool {
 func rewriteValueWasm_OpCvt64Fto64U_0(v *Value) bool {
 	// match: (Cvt64Fto64U x)
 	// cond:
-	// result: (I64TruncF64U x)
+	// result: (I64TruncSatF64U x)
 	for {
 		x := v.Args[0]
-		v.reset(OpWasmI64TruncF64U)
+		v.reset(OpWasmI64TruncSatF64U)
 		v.AddArg(x)
 		return true
 	}

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

@@ -10,6 +10,7 @@ import (
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/obj/wasm"
+	"cmd/internal/objabi"
 )
 
 func Init(arch *gc.Arch) {
@@ -307,15 +308,23 @@ func ssaGenValueOnStack(s *gc.SSAGenState, v *ssa.Value) {
 		}
 		s.Prog(wasm.AI64DivS)
 
-	case ssa.OpWasmI64TruncF64S:
+	case ssa.OpWasmI64TruncSatF64S:
 		getValue64(s, v.Args[0])
-		p := s.Prog(wasm.ACall)
-		p.To = obj.Addr{Type: obj.TYPE_MEM, Name: obj.NAME_EXTERN, Sym: gc.WasmTruncS}
+		if objabi.GOWASM.SatConv {
+			s.Prog(v.Op.Asm())
+		} else {
+			p := s.Prog(wasm.ACall)
+			p.To = obj.Addr{Type: obj.TYPE_MEM, Name: obj.NAME_EXTERN, Sym: gc.WasmTruncS}
+		}
 
-	case ssa.OpWasmI64TruncF64U:
+	case ssa.OpWasmI64TruncSatF64U:
 		getValue64(s, v.Args[0])
-		p := s.Prog(wasm.ACall)
-		p.To = obj.Addr{Type: obj.TYPE_MEM, Name: obj.NAME_EXTERN, Sym: gc.WasmTruncU}
+		if objabi.GOWASM.SatConv {
+			s.Prog(v.Op.Asm())
+		} else {
+			p := s.Prog(wasm.ACall)
+			p.To = obj.Addr{Type: obj.TYPE_MEM, Name: obj.NAME_EXTERN, Sym: gc.WasmTruncU}
+		}
 
 	case
 		ssa.OpWasmF64Neg, ssa.OpWasmF64ConvertI64S, ssa.OpWasmF64ConvertI64U,

src/cmd/go/alldocs.go

@@ -1581,7 +1581,7 @@
 // 		Valid values are hardfloat (default), softfloat.
 // 	GOWASM
 // 		For GOARCH=wasm, comma-separated list of experimental WebAssembly features to use.
-// 		Valid values are: signext.
+// 		Valid values are satconv, signext.
 //
 // Special-purpose environment variables:
 //

src/cmd/go/internal/help/helpdoc.go

@@ -565,7 +565,7 @@ Architecture-specific environment variables:
 		Valid values are hardfloat (default), softfloat.
 	GOWASM
 		For GOARCH=wasm, comma-separated list of experimental WebAssembly features to use.
-		Valid values are: signext.
+		Valid values are satconv, signext.
 
 Special-purpose environment variables:
 

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

@@ -216,6 +216,15 @@ const (
 	AI64Extend16S
 	AI64Extend32S
 
+	AI32TruncSatF32S // opcode 0xFC 0x00
+	AI32TruncSatF32U
+	AI32TruncSatF64S
+	AI32TruncSatF64U
+	AI64TruncSatF32S
+	AI64TruncSatF32U
+	AI64TruncSatF64S
+	AI64TruncSatF64U
+
 	ALast // Sentinel: End of low-level WebAssembly instructions.
 
 	ARESUMEPOINT

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

@@ -182,6 +182,14 @@ var Anames = []string{
 	"I64Extend8S",
 	"I64Extend16S",
 	"I64Extend32S",
+	"I32TruncSatF32S",
+	"I32TruncSatF32U",
+	"I32TruncSatF64S",
+	"I32TruncSatF64U",
+	"I64TruncSatF32S",
+	"I64TruncSatF32U",
+	"I64TruncSatF64S",
+	"I64TruncSatF64U",
 	"Last",
 	"RESUMEPOINT",
 	"CALLNORESUME",

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

@@ -886,7 +886,7 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 		}
 
 		switch {
-		case p.As < AUnreachable || p.As >= ALast:
+		case p.As < AUnreachable:
 			panic(fmt.Sprintf("unexpected assembler op: %s", p.As))
 		case p.As < AEnd:
 			w.WriteByte(byte(p.As - AUnreachable + 0x00))
@@ -894,8 +894,13 @@ func assemble(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			w.WriteByte(byte(p.As - AEnd + 0x0B))
 		case p.As < AI32Load:
 			w.WriteByte(byte(p.As - ADrop + 0x1A))
-		default:
+		case p.As < AI32TruncSatF32S:
 			w.WriteByte(byte(p.As - AI32Load + 0x28))
+		case p.As < ALast:
+			w.WriteByte(0xFC)
+			w.WriteByte(byte(p.As - AI32TruncSatF32S + 0x00))
+		default:
+			panic(fmt.Sprintf("unexpected assembler op: %s", p.As))
 		}
 
 		switch p.As {

src/cmd/internal/objabi/util.go

@@ -79,10 +79,14 @@ func goppc64() int {
 
 type gowasmFeatures struct {
 	SignExt bool
+	SatConv bool
 }
 
 func (f *gowasmFeatures) String() string {
 	var flags []string
+	if f.SatConv {
+		flags = append(flags, "satconv")
+	}
 	if f.SignExt {
 		flags = append(flags, "signext")
 	}
@@ -92,6 +96,8 @@ func (f *gowasmFeatures) String() string {
 func gowasm() (f gowasmFeatures) {
 	for _, opt := range strings.Split(envOr("GOWASM", ""), ",") {
 		switch opt {
+		case "satconv":
+			f.SatConv = true
 		case "signext":
 			f.SignExt = true
 		case "":