cmd/compile: allow conversion from slice to array ptr

Panic if the slice is too short.

Updates #395

Change-Id: I90f4bff2da5d8f3148ba06d2482084f32b25c29a
Reviewed-on: https://go-review.googlesource.com/c/go/+/301650
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/escape/escape.go

@@ -669,7 +669,10 @@ func (e *escape) exprSkipInit(k hole, n ir.Node) {
 			k = e.spill(k, n)
 		}
 		e.expr(k.note(n, "interface-converted"), n.X)
-
+	case ir.OSLICE2ARRPTR:
+		// the slice pointer flows directly to the result
+		n := n.(*ir.ConvExpr)
+		e.expr(k, n.X)
 	case ir.ORECV:
 		n := n.(*ir.UnaryExpr)
 		e.discard(n.X)

src/cmd/compile/internal/ir/expr.go

@@ -277,7 +277,7 @@ func (n *ConvExpr) SetOp(op Op) {
 	switch op {
 	default:
 		panic(n.no("SetOp " + op.String()))
-	case OCONV, OCONVIFACE, OCONVNOP, OBYTES2STR, OBYTES2STRTMP, ORUNES2STR, OSTR2BYTES, OSTR2BYTESTMP, OSTR2RUNES, ORUNESTR:
+	case OCONV, OCONVIFACE, OCONVNOP, OBYTES2STR, OBYTES2STRTMP, ORUNES2STR, OSTR2BYTES, OSTR2BYTESTMP, OSTR2RUNES, ORUNESTR, OSLICE2ARRPTR:
 		n.op = op
 	}
 }

src/cmd/compile/internal/ir/fmt.go

@@ -206,6 +206,7 @@ var OpPrec = []int{
 	OPRINT:         8,
 	ORUNESTR:       8,
 	OSIZEOF:        8,
+	OSLICE2ARRPTR:  8,
 	OSTR2BYTES:     8,
 	OSTR2RUNES:     8,
 	OSTRUCTLIT:     8,
@@ -804,7 +805,8 @@ func exprFmt(n Node, s fmt.State, prec int) {
 		ORUNES2STR,
 		OSTR2BYTES,
 		OSTR2RUNES,
-		ORUNESTR:
+		ORUNESTR,
+		OSLICE2ARRPTR:
 		n := n.(*ConvExpr)
 		if n.Type() == nil || n.Type().Sym() == nil {
 			fmt.Fprintf(s, "(%v)", n.Type())

src/cmd/compile/internal/ir/node.go

@@ -137,6 +137,7 @@ const (
 	OSTR2BYTES    // Type(Left) (Type is []byte, Left is a string)
 	OSTR2BYTESTMP // Type(Left) (Type is []byte, Left is a string, ephemeral)
 	OSTR2RUNES    // Type(Left) (Type is []rune, Left is a string)
+	OSLICE2ARRPTR // Type(Left) (Type is *[N]T, Left is a []T)
 	// Left = Right or (if Colas=true) Left := Right
 	// If Colas, then Ninit includes a DCL node for Left.
 	OAS

src/cmd/compile/internal/ir/op_string.go

@@ -29,143 +29,144 @@ func _() {
 	_ = x[OSTR2BYTES-18]
 	_ = x[OSTR2BYTESTMP-19]
 	_ = x[OSTR2RUNES-20]
-	_ = x[OAS-21]
+	_ = x[OSLICE2ARRPTR-21]
-	_ = x[OAS2-22]
+	_ = x[OAS-22]
-	_ = x[OAS2DOTTYPE-23]
+	_ = x[OAS2-23]
-	_ = x[OAS2FUNC-24]
+	_ = x[OAS2DOTTYPE-24]
-	_ = x[OAS2MAPR-25]
+	_ = x[OAS2FUNC-25]
-	_ = x[OAS2RECV-26]
+	_ = x[OAS2MAPR-26]
-	_ = x[OASOP-27]
+	_ = x[OAS2RECV-27]
-	_ = x[OCALL-28]
+	_ = x[OASOP-28]
-	_ = x[OCALLFUNC-29]
+	_ = x[OCALL-29]
-	_ = x[OCALLMETH-30]
+	_ = x[OCALLFUNC-30]
-	_ = x[OCALLINTER-31]
+	_ = x[OCALLMETH-31]
-	_ = x[OCALLPART-32]
+	_ = x[OCALLINTER-32]
-	_ = x[OCAP-33]
+	_ = x[OCALLPART-33]
-	_ = x[OCLOSE-34]
+	_ = x[OCAP-34]
-	_ = x[OCLOSURE-35]
+	_ = x[OCLOSE-35]
-	_ = x[OCOMPLIT-36]
+	_ = x[OCLOSURE-36]
-	_ = x[OMAPLIT-37]
+	_ = x[OCOMPLIT-37]
-	_ = x[OSTRUCTLIT-38]
+	_ = x[OMAPLIT-38]
-	_ = x[OARRAYLIT-39]
+	_ = x[OSTRUCTLIT-39]
-	_ = x[OSLICELIT-40]
+	_ = x[OARRAYLIT-40]
-	_ = x[OPTRLIT-41]
+	_ = x[OSLICELIT-41]
-	_ = x[OCONV-42]
+	_ = x[OPTRLIT-42]
-	_ = x[OCONVIFACE-43]
+	_ = x[OCONV-43]
-	_ = x[OCONVNOP-44]
+	_ = x[OCONVIFACE-44]
-	_ = x[OCOPY-45]
+	_ = x[OCONVNOP-45]
-	_ = x[ODCL-46]
+	_ = x[OCOPY-46]
-	_ = x[ODCLFUNC-47]
+	_ = x[ODCL-47]
-	_ = x[ODCLCONST-48]
+	_ = x[ODCLFUNC-48]
-	_ = x[ODCLTYPE-49]
+	_ = x[ODCLCONST-49]
-	_ = x[ODELETE-50]
+	_ = x[ODCLTYPE-50]
-	_ = x[ODOT-51]
+	_ = x[ODELETE-51]
-	_ = x[ODOTPTR-52]
+	_ = x[ODOT-52]
-	_ = x[ODOTMETH-53]
+	_ = x[ODOTPTR-53]
-	_ = x[ODOTINTER-54]
+	_ = x[ODOTMETH-54]
-	_ = x[OXDOT-55]
+	_ = x[ODOTINTER-55]
-	_ = x[ODOTTYPE-56]
+	_ = x[OXDOT-56]
-	_ = x[ODOTTYPE2-57]
+	_ = x[ODOTTYPE-57]
-	_ = x[OEQ-58]
+	_ = x[ODOTTYPE2-58]
-	_ = x[ONE-59]
+	_ = x[OEQ-59]
-	_ = x[OLT-60]
+	_ = x[ONE-60]
-	_ = x[OLE-61]
+	_ = x[OLT-61]
-	_ = x[OGE-62]
+	_ = x[OLE-62]
-	_ = x[OGT-63]
+	_ = x[OGE-63]
-	_ = x[ODEREF-64]
+	_ = x[OGT-64]
-	_ = x[OINDEX-65]
+	_ = x[ODEREF-65]
-	_ = x[OINDEXMAP-66]
+	_ = x[OINDEX-66]
-	_ = x[OKEY-67]
+	_ = x[OINDEXMAP-67]
-	_ = x[OSTRUCTKEY-68]
+	_ = x[OKEY-68]
-	_ = x[OLEN-69]
+	_ = x[OSTRUCTKEY-69]
-	_ = x[OMAKE-70]
+	_ = x[OLEN-70]
-	_ = x[OMAKECHAN-71]
+	_ = x[OMAKE-71]
-	_ = x[OMAKEMAP-72]
+	_ = x[OMAKECHAN-72]
-	_ = x[OMAKESLICE-73]
+	_ = x[OMAKEMAP-73]
-	_ = x[OMAKESLICECOPY-74]
+	_ = x[OMAKESLICE-74]
-	_ = x[OMUL-75]
+	_ = x[OMAKESLICECOPY-75]
-	_ = x[ODIV-76]
+	_ = x[OMUL-76]
-	_ = x[OMOD-77]
+	_ = x[ODIV-77]
-	_ = x[OLSH-78]
+	_ = x[OMOD-78]
-	_ = x[ORSH-79]
+	_ = x[OLSH-79]
-	_ = x[OAND-80]
+	_ = x[ORSH-80]
-	_ = x[OANDNOT-81]
+	_ = x[OAND-81]
-	_ = x[ONEW-82]
+	_ = x[OANDNOT-82]
-	_ = x[ONOT-83]
+	_ = x[ONEW-83]
-	_ = x[OBITNOT-84]
+	_ = x[ONOT-84]
-	_ = x[OPLUS-85]
+	_ = x[OBITNOT-85]
-	_ = x[ONEG-86]
+	_ = x[OPLUS-86]
-	_ = x[OOROR-87]
+	_ = x[ONEG-87]
-	_ = x[OPANIC-88]
+	_ = x[OOROR-88]
-	_ = x[OPRINT-89]
+	_ = x[OPANIC-89]
-	_ = x[OPRINTN-90]
+	_ = x[OPRINT-90]
-	_ = x[OPAREN-91]
+	_ = x[OPRINTN-91]
-	_ = x[OSEND-92]
+	_ = x[OPAREN-92]
-	_ = x[OSLICE-93]
+	_ = x[OSEND-93]
-	_ = x[OSLICEARR-94]
+	_ = x[OSLICE-94]
-	_ = x[OSLICESTR-95]
+	_ = x[OSLICEARR-95]
-	_ = x[OSLICE3-96]
+	_ = x[OSLICESTR-96]
-	_ = x[OSLICE3ARR-97]
+	_ = x[OSLICE3-97]
-	_ = x[OSLICEHEADER-98]
+	_ = x[OSLICE3ARR-98]
-	_ = x[ORECOVER-99]
+	_ = x[OSLICEHEADER-99]
-	_ = x[ORECV-100]
+	_ = x[ORECOVER-100]
-	_ = x[ORUNESTR-101]
+	_ = x[ORECV-101]
-	_ = x[OSELRECV2-102]
+	_ = x[ORUNESTR-102]
-	_ = x[OIOTA-103]
+	_ = x[OSELRECV2-103]
-	_ = x[OREAL-104]
+	_ = x[OIOTA-104]
-	_ = x[OIMAG-105]
+	_ = x[OREAL-105]
-	_ = x[OCOMPLEX-106]
+	_ = x[OIMAG-106]
-	_ = x[OALIGNOF-107]
+	_ = x[OCOMPLEX-107]
-	_ = x[OOFFSETOF-108]
+	_ = x[OALIGNOF-108]
-	_ = x[OSIZEOF-109]
+	_ = x[OOFFSETOF-109]
-	_ = x[OMETHEXPR-110]
+	_ = x[OSIZEOF-110]
-	_ = x[OSTMTEXPR-111]
+	_ = x[OMETHEXPR-111]
-	_ = x[OBLOCK-112]
+	_ = x[OSTMTEXPR-112]
-	_ = x[OBREAK-113]
+	_ = x[OBLOCK-113]
-	_ = x[OCASE-114]
+	_ = x[OBREAK-114]
-	_ = x[OCONTINUE-115]
+	_ = x[OCASE-115]
-	_ = x[ODEFER-116]
+	_ = x[OCONTINUE-116]
-	_ = x[OFALL-117]
+	_ = x[ODEFER-117]
-	_ = x[OFOR-118]
+	_ = x[OFALL-118]
-	_ = x[OFORUNTIL-119]
+	_ = x[OFOR-119]
-	_ = x[OGOTO-120]
+	_ = x[OFORUNTIL-120]
-	_ = x[OIF-121]
+	_ = x[OGOTO-121]
-	_ = x[OLABEL-122]
+	_ = x[OIF-122]
-	_ = x[OGO-123]
+	_ = x[OLABEL-123]
-	_ = x[ORANGE-124]
+	_ = x[OGO-124]
-	_ = x[ORETURN-125]
+	_ = x[ORANGE-125]
-	_ = x[OSELECT-126]
+	_ = x[ORETURN-126]
-	_ = x[OSWITCH-127]
+	_ = x[OSELECT-127]
-	_ = x[OTYPESW-128]
+	_ = x[OSWITCH-128]
-	_ = x[OFUNCINST-129]
+	_ = x[OTYPESW-129]
-	_ = x[OTCHAN-130]
+	_ = x[OFUNCINST-130]
-	_ = x[OTMAP-131]
+	_ = x[OTCHAN-131]
-	_ = x[OTSTRUCT-132]
+	_ = x[OTMAP-132]
-	_ = x[OTINTER-133]
+	_ = x[OTSTRUCT-133]
-	_ = x[OTFUNC-134]
+	_ = x[OTINTER-134]
-	_ = x[OTARRAY-135]
+	_ = x[OTFUNC-135]
-	_ = x[OTSLICE-136]
+	_ = x[OTARRAY-136]
-	_ = x[OINLCALL-137]
+	_ = x[OTSLICE-137]
-	_ = x[OEFACE-138]
+	_ = x[OINLCALL-138]
-	_ = x[OITAB-139]
+	_ = x[OEFACE-139]
-	_ = x[OIDATA-140]
+	_ = x[OITAB-140]
-	_ = x[OSPTR-141]
+	_ = x[OIDATA-141]
-	_ = x[OCFUNC-142]
+	_ = x[OSPTR-142]
-	_ = x[OCHECKNIL-143]
+	_ = x[OCFUNC-143]
-	_ = x[OVARDEF-144]
+	_ = x[OCHECKNIL-144]
-	_ = x[OVARKILL-145]
+	_ = x[OVARDEF-145]
-	_ = x[OVARLIVE-146]
+	_ = x[OVARKILL-146]
-	_ = x[ORESULT-147]
+	_ = x[OVARLIVE-147]
-	_ = x[OINLMARK-148]
+	_ = x[ORESULT-148]
-	_ = x[OLINKSYMOFFSET-149]
+	_ = x[OINLMARK-149]
-	_ = x[OTAILCALL-150]
+	_ = x[OLINKSYMOFFSET-150]
-	_ = x[OGETG-151]
+	_ = x[OTAILCALL-151]
-	_ = x[OEND-152]
+	_ = x[OGETG-152]
+	_ = x[OEND-153]
 }
 
-const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFMETHEXPRSTMTEXPRBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWFUNCINSTTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETTAILCALLGETGEND"
+const _Op_name = "XXXNAMENONAMETYPEPACKLITERALNILADDSUBORXORADDSTRADDRANDANDAPPENDBYTES2STRBYTES2STRTMPRUNES2STRSTR2BYTESSTR2BYTESTMPSTR2RUNESSLICE2ARRPTRASAS2AS2DOTTYPEAS2FUNCAS2MAPRAS2RECVASOPCALLCALLFUNCCALLMETHCALLINTERCALLPARTCAPCLOSECLOSURECOMPLITMAPLITSTRUCTLITARRAYLITSLICELITPTRLITCONVCONVIFACECONVNOPCOPYDCLDCLFUNCDCLCONSTDCLTYPEDELETEDOTDOTPTRDOTMETHDOTINTERXDOTDOTTYPEDOTTYPE2EQNELTLEGEGTDEREFINDEXINDEXMAPKEYSTRUCTKEYLENMAKEMAKECHANMAKEMAPMAKESLICEMAKESLICECOPYMULDIVMODLSHRSHANDANDNOTNEWNOTBITNOTPLUSNEGORORPANICPRINTPRINTNPARENSENDSLICESLICEARRSLICESTRSLICE3SLICE3ARRSLICEHEADERRECOVERRECVRUNESTRSELRECV2IOTAREALIMAGCOMPLEXALIGNOFOFFSETOFSIZEOFMETHEXPRSTMTEXPRBLOCKBREAKCASECONTINUEDEFERFALLFORFORUNTILGOTOIFLABELGORANGERETURNSELECTSWITCHTYPESWFUNCINSTTCHANTMAPTSTRUCTTINTERTFUNCTARRAYTSLICEINLCALLEFACEITABIDATASPTRCFUNCCHECKNILVARDEFVARKILLVARLIVERESULTINLMARKLINKSYMOFFSETTAILCALLGETGEND"
 
-var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 126, 129, 139, 146, 153, 160, 164, 168, 176, 184, 193, 201, 204, 209, 216, 223, 229, 238, 246, 254, 260, 264, 273, 280, 284, 287, 294, 302, 309, 315, 318, 324, 331, 339, 343, 350, 358, 360, 362, 364, 366, 368, 370, 375, 380, 388, 391, 400, 403, 407, 415, 422, 431, 444, 447, 450, 453, 456, 459, 462, 468, 471, 474, 480, 484, 487, 491, 496, 501, 507, 512, 516, 521, 529, 537, 543, 552, 563, 570, 574, 581, 589, 593, 597, 601, 608, 615, 623, 629, 637, 645, 650, 655, 659, 667, 672, 676, 679, 687, 691, 693, 698, 700, 705, 711, 717, 723, 729, 737, 742, 746, 753, 759, 764, 770, 776, 783, 788, 792, 797, 801, 806, 814, 820, 827, 834, 840, 847, 860, 868, 872, 875}
+var _Op_index = [...]uint16{0, 3, 7, 13, 17, 21, 28, 31, 34, 37, 39, 42, 48, 52, 58, 64, 73, 85, 94, 103, 115, 124, 136, 138, 141, 151, 158, 165, 172, 176, 180, 188, 196, 205, 213, 216, 221, 228, 235, 241, 250, 258, 266, 272, 276, 285, 292, 296, 299, 306, 314, 321, 327, 330, 336, 343, 351, 355, 362, 370, 372, 374, 376, 378, 380, 382, 387, 392, 400, 403, 412, 415, 419, 427, 434, 443, 456, 459, 462, 465, 468, 471, 474, 480, 483, 486, 492, 496, 499, 503, 508, 513, 519, 524, 528, 533, 541, 549, 555, 564, 575, 582, 586, 593, 601, 605, 609, 613, 620, 627, 635, 641, 649, 657, 662, 667, 671, 679, 684, 688, 691, 699, 703, 705, 710, 712, 717, 723, 729, 735, 741, 749, 754, 758, 765, 771, 776, 782, 788, 795, 800, 804, 809, 813, 818, 826, 832, 839, 846, 852, 859, 872, 880, 884, 887}
 
 func (i Op) String() string {
 	if i >= Op(len(_Op_index)-1) {

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

@@ -508,7 +508,7 @@ func (x *expandState) rewriteSelect(leaf *Value, selector *Value, offset int64,
 		ls := x.rewriteSelect(leaf, selector.Args[0], offset, regOffset)
 		locs = x.splitSlots(ls, ".ptr", 0, x.typs.BytePtr)
 
-	case OpSlicePtr:
+	case OpSlicePtr, OpSlicePtrUnchecked:
 		w := selector.Args[0]
 		ls := x.rewriteSelect(leaf, w, offset, regOffset)
 		locs = x.splitSlots(ls, ".ptr", 0, types.NewPtr(w.Type.Elem()))
@@ -1202,7 +1202,7 @@ func expandCalls(f *Func) {
 			case OpStructSelect, OpArraySelect,
 				OpIData, OpITab,
 				OpStringPtr, OpStringLen,
-				OpSlicePtr, OpSliceLen, OpSliceCap,
+				OpSlicePtr, OpSliceLen, OpSliceCap, OpSlicePtrUnchecked,
 				OpComplexReal, OpComplexImag,
 				OpInt64Hi, OpInt64Lo:
 				w := v.Args[0]

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

@@ -56,6 +56,7 @@
 (SlicePtr (SliceMake ptr _ _ )) => ptr
 (SliceLen (SliceMake _ len _)) => len
 (SliceCap (SliceMake _ _ cap)) => cap
+(SlicePtrUnchecked (SliceMake ptr _ _ )) => ptr
 
 (Load <t> ptr mem) && t.IsSlice() =>
   (SliceMake

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

@@ -479,6 +479,10 @@ var genericOps = []opData{
 	{name: "SlicePtr", argLength: 1, typ: "BytePtr"}, // ptr(arg0)
 	{name: "SliceLen", argLength: 1},                 // len(arg0)
 	{name: "SliceCap", argLength: 1},                 // cap(arg0)
+	// SlicePtrUnchecked, like SlicePtr, extracts the pointer from a slice.
+	// SlicePtr values are assumed non-nil, because they are guarded by bounds checks.
+	// SlicePtrUnchecked values can be nil.
+	{name: "SlicePtrUnchecked", argLength: 1},
 
 	// Complex (part/whole)
 	{name: "ComplexMake", argLength: 2}, // arg0=real, arg1=imag

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

@@ -469,6 +469,7 @@ const (
 	BoundsSlice3BU                      // ... with unsigned high
 	BoundsSlice3C                       // 3-arg slicing operation, 0 <= low <= high failed
 	BoundsSlice3CU                      // ... with unsigned low
+	BoundsConvert                       // conversion to array pointer failed
 	BoundsKindCount
 )
 
@@ -496,7 +497,8 @@ func boundsABI(b int64) int {
 	case BoundsSlice3Alen,
 		BoundsSlice3AlenU,
 		BoundsSlice3Acap,
-		BoundsSlice3AcapU:
+		BoundsSlice3AcapU,
+		BoundsConvert:
 		return 0
 	case BoundsSliceAlen,
 		BoundsSliceAlenU,

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

@@ -2834,6 +2834,7 @@ const (
 	OpSlicePtr
 	OpSliceLen
 	OpSliceCap
+	OpSlicePtrUnchecked
 	OpComplexMake
 	OpComplexReal
 	OpComplexImag
@@ -35898,6 +35899,11 @@ var opcodeTable = [...]opInfo{
 		argLen:  1,
 		generic: true,
 	},
+	{
+		name:    "SlicePtrUnchecked",
+		argLen:  1,
+		generic: true,
+	},
 	{
 		name:    "ComplexMake",
 		argLen:  2,

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

@@ -23,6 +23,8 @@ func rewriteValuedec(v *Value) bool {
 		return rewriteValuedec_OpSliceLen(v)
 	case OpSlicePtr:
 		return rewriteValuedec_OpSlicePtr(v)
+	case OpSlicePtrUnchecked:
+		return rewriteValuedec_OpSlicePtrUnchecked(v)
 	case OpStore:
 		return rewriteValuedec_OpStore(v)
 	case OpStringLen:
@@ -248,6 +250,20 @@ func rewriteValuedec_OpSlicePtr(v *Value) bool {
 	}
 	return false
 }
+func rewriteValuedec_OpSlicePtrUnchecked(v *Value) bool {
+	v_0 := v.Args[0]
+	// match: (SlicePtrUnchecked (SliceMake ptr _ _ ))
+	// result: ptr
+	for {
+		if v_0.Op != OpSliceMake {
+			break
+		}
+		ptr := v_0.Args[0]
+		v.copyOf(ptr)
+		return true
+	}
+	return false
+}
 func rewriteValuedec_OpStore(v *Value) bool {
 	v_2 := v.Args[2]
 	v_1 := v.Args[1]

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

@@ -164,6 +164,7 @@ func InitConfig() {
 		BoundsCheckFunc[ssa.BoundsSlice3BU] = typecheck.LookupRuntimeFunc("goPanicSlice3BU")
 		BoundsCheckFunc[ssa.BoundsSlice3C] = typecheck.LookupRuntimeFunc("goPanicSlice3C")
 		BoundsCheckFunc[ssa.BoundsSlice3CU] = typecheck.LookupRuntimeFunc("goPanicSlice3CU")
+		BoundsCheckFunc[ssa.BoundsConvert] = typecheck.LookupRuntimeFunc("goPanicSliceConvert")
 	} else {
 		BoundsCheckFunc[ssa.BoundsIndex] = typecheck.LookupRuntimeFunc("panicIndex")
 		BoundsCheckFunc[ssa.BoundsIndexU] = typecheck.LookupRuntimeFunc("panicIndexU")
@@ -181,6 +182,7 @@ func InitConfig() {
 		BoundsCheckFunc[ssa.BoundsSlice3BU] = typecheck.LookupRuntimeFunc("panicSlice3BU")
 		BoundsCheckFunc[ssa.BoundsSlice3C] = typecheck.LookupRuntimeFunc("panicSlice3C")
 		BoundsCheckFunc[ssa.BoundsSlice3CU] = typecheck.LookupRuntimeFunc("panicSlice3CU")
+		BoundsCheckFunc[ssa.BoundsConvert] = typecheck.LookupRuntimeFunc("panicSliceConvert")
 	}
 	if Arch.LinkArch.PtrSize == 4 {
 		ExtendCheckFunc[ssa.BoundsIndex] = typecheck.LookupRuntimeVar("panicExtendIndex")
@@ -3148,6 +3150,18 @@ func (s *state) expr(n ir.Node) *ssa.Value {
 		p, l, _ := s.slice(v, i, j, nil, n.Bounded())
 		return s.newValue2(ssa.OpStringMake, n.Type(), p, l)
 
+	case ir.OSLICE2ARRPTR:
+		// if arrlen > slice.len {
+		//   panic(...)
+		// }
+		// slice.ptr
+		n := n.(*ir.ConvExpr)
+		v := s.expr(n.X)
+		arrlen := s.constInt(types.Types[types.TINT], n.Type().Elem().NumElem())
+		cap := s.newValue1(ssa.OpSliceLen, types.Types[types.TINT], v)
+		s.boundsCheck(arrlen, cap, ssa.BoundsConvert, false)
+		return s.newValue1(ssa.OpSlicePtrUnchecked, types.Types[types.TINT], v)
+
 	case ir.OCALLFUNC:
 		n := n.(*ir.CallExpr)
 		if ir.IsIntrinsicCall(n) {

src/cmd/compile/internal/typecheck/builtin.go

@@ -39,6 +39,7 @@ var runtimeDecls = [...]struct {
 	{"goPanicSlice3BU", funcTag, 18},
 	{"goPanicSlice3C", funcTag, 16},
 	{"goPanicSlice3CU", funcTag, 18},
+	{"goPanicSliceConvert", funcTag, 16},
 	{"printbool", funcTag, 19},
 	{"printfloat", funcTag, 21},
 	{"printint", funcTag, 23},

src/cmd/compile/internal/typecheck/builtin/runtime.go

@@ -46,6 +46,7 @@ func goPanicSlice3B(x int, y int)
 func goPanicSlice3BU(x uint, y int)
 func goPanicSlice3C(x int, y int)
 func goPanicSlice3CU(x uint, y int)
+func goPanicSliceConvert(x int, y int)
 
 func printbool(bool)
 func printfloat(float64)

src/cmd/compile/internal/typecheck/subr.go

@@ -574,7 +574,7 @@ func Convertop(srcConstant bool, src, dst *types.Type) (ir.Op, string) {
 		return ir.OCONVNOP, ""
 	}
 
-	// src is map and dst is a pointer to corresponding hmap.
+	// 10. src is map and dst is a pointer to corresponding hmap.
 	// This rule is needed for the implementation detail that
 	// go gc maps are implemented as a pointer to a hmap struct.
 	if src.Kind() == types.TMAP && dst.IsPtr() &&
@@ -582,6 +582,16 @@ func Convertop(srcConstant bool, src, dst *types.Type) (ir.Op, string) {
 		return ir.OCONVNOP, ""
 	}
 
+	// 11. src is a slice and dst is a pointer-to-array.
+	// They must have same element type.
+	if src.IsSlice() && dst.IsPtr() && dst.Elem().IsArray() &&
+		types.Identical(src.Elem(), dst.Elem().Elem()) {
+		if !types.AllowsGoVersion(curpkg(), 1, 17) {
+			return ir.OXXX, ":\n\tconversion of slices to array pointers only supported as of -lang=go1.17"
+		}
+		return ir.OSLICE2ARRPTR, ""
+	}
+
 	return ir.OXXX, ""
 }
 

src/cmd/compile/internal/types2/api_test.go

@@ -1578,6 +1578,9 @@ func TestConvertibleTo(t *testing.T) {
 		{newDefined(new(Struct)), new(Struct), true},
 		{newDefined(Typ[Int]), new(Struct), false},
 		{Typ[UntypedInt], Typ[Int], true},
+		{NewSlice(Typ[Int]), NewPointer(NewArray(Typ[Int], 10)), true},
+		{NewSlice(Typ[Int]), NewArray(Typ[Int], 10), false},
+		{NewSlice(Typ[Int]), NewPointer(NewArray(Typ[Uint], 10)), false},
 		// Untyped string values are not permitted by the spec, so the below
 		// behavior is undefined.
 		{Typ[UntypedString], Typ[String], true},

src/cmd/compile/internal/types2/conversions.go

@@ -135,6 +135,18 @@ func (x *operand) convertibleTo(check *Checker, T Type) bool {
 		return true
 	}
 
+	// "x is a slice, T is a pointer-to-array type,
+	// and the slice and array types have identical element types."
+	if s := asSlice(V); s != nil {
+		if p := asPointer(T); p != nil {
+			if a := asArray(p.Elem()); a != nil {
+				if check.identical(s.Elem(), a.Elem()) {
+					return true
+				}
+			}
+		}
+	}
+
 	return false
 }
 

src/cmd/compile/internal/walk/expr.go

@@ -205,6 +205,11 @@ func walkExpr1(n ir.Node, init *ir.Nodes) ir.Node {
 		n := n.(*ir.ConvExpr)
 		return walkConv(n, init)
 
+	case ir.OSLICE2ARRPTR:
+		n := n.(*ir.ConvExpr)
+		n.X = walkExpr(n.X, init)
+		return n
+
 	case ir.ODIV, ir.OMOD:
 		n := n.(*ir.BinaryExpr)
 		return walkDivMod(n, init)

src/go/types/stdlib_test.go

@@ -163,6 +163,10 @@ func TestStdTest(t *testing.T) {
 		"embedfunc.go",   // tests //go:embed
 		"embedvers.go",   // tests //go:embed
 		"linkname2.go",   // go/types doesn't check validity of //go:xxx directives
+
+		"convert2.go",     // temporary: go/types doesn't know yet about converting from slices to array pointers
+		"convert4.go",     // temporary: go/types doesn't know yet about converting from slices to array pointers
+		"escape_slice.go", // temporary: go/types doesn't know yet about converting from slices to array pointers
 	)
 }
 

src/runtime/asm_386.s

@@ -1473,6 +1473,10 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-8
 	MOVL	AX, x+0(FP)
 	MOVL	CX, y+4(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-8
+	MOVL	DX, x+0(FP)
+	MOVL	BX, y+4(FP)
+	JMP	runtime·goPanicSliceConvert(SB)
 
 // Extended versions for 64-bit indexes.
 TEXT runtime·panicExtendIndex(SB),NOSPLIT,$0-12

src/runtime/asm_amd64.s

@@ -2091,6 +2091,14 @@ TEXT runtime·panicSlice3CU<ABIInternal>(SB),NOSPLIT,$0-16
 	MOVQ	CX, y+8(FP)
 #endif
 	JMP	runtime·goPanicSlice3CU<ABIInternal>(SB)
+TEXT runtime·panicSliceConvert<ABIInternal>(SB),NOSPLIT,$0-16
+#ifdef GOEXPERIMENT_regabiargs
+	MOVQ	DX, AX
+#else
+	MOVQ	DX, x+0(FP)
+	MOVQ	BX, y+8(FP)
+#endif
+	JMP	runtime·goPanicSliceConvert<ABIInternal>(SB)
 
 #ifdef GOOS_android
 // Use the free TLS_SLOT_APP slot #2 on Android Q.

src/runtime/asm_arm.s

@@ -992,6 +992,10 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-8
 	MOVW	R0, x+0(FP)
 	MOVW	R1, y+4(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-8
+	MOVW	R2, x+0(FP)
+	MOVW	R3, y+4(FP)
+	JMP	runtime·goPanicSliceConvert(SB)
 
 // Extended versions for 64-bit indexes.
 TEXT runtime·panicExtendIndex(SB),NOSPLIT,$0-12

src/runtime/asm_arm64.s

@@ -1314,3 +1314,7 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	MOVD	R0, x+0(FP)
 	MOVD	R1, y+8(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
+	MOVD	R2, x+0(FP)
+	MOVD	R3, y+8(FP)
+	JMP	runtime·goPanicSliceConvert(SB)

src/runtime/asm_mips64x.s

@@ -805,3 +805,7 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	MOVV	R1, x+0(FP)
 	MOVV	R2, y+8(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
+	MOVV	R3, x+0(FP)
+	MOVV	R4, y+8(FP)
+	JMP	runtime·goPanicSliceConvert(SB)

src/runtime/asm_mipsx.s

@@ -801,6 +801,10 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-8
 	MOVW	R1, x+0(FP)
 	MOVW	R2, y+4(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-8
+	MOVW	R3, x+0(FP)
+	MOVW	R4, y+4(FP)
+	JMP	runtime·goPanicSliceConvert(SB)
 
 // Extended versions for 64-bit indexes.
 TEXT runtime·panicExtendIndex(SB),NOSPLIT,$0-12

src/runtime/asm_ppc64x.s

@@ -1022,3 +1022,7 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	MOVD	R3, x+0(FP)
 	MOVD	R4, y+8(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
+	MOVD	R5, x+0(FP)
+	MOVD	R6, y+8(FP)
+	JMP	runtime·goPanicSliceConvert(SB)

src/runtime/asm_riscv64.s

@@ -806,6 +806,10 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	MOV	T0, x+0(FP)
 	MOV	T1, y+8(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
+	MOV	T2, x+0(FP)
+	MOV	T3, y+8(FP)
+	JMP	runtime·goPanicSliceConvert(SB)
 
 DATA	runtime·mainPC+0(SB)/8,$runtime·main(SB)
 GLOBL	runtime·mainPC(SB),RODATA,$8

src/runtime/asm_s390x.s

@@ -906,3 +906,7 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	MOVD	R0, x+0(FP)
 	MOVD	R1, y+8(FP)
 	JMP	runtime·goPanicSlice3CU(SB)
+TEXT runtime·panicSliceConvert(SB),NOSPLIT,$0-16
+	MOVD	R2, x+0(FP)
+	MOVD	R3, y+8(FP)
+	JMP	runtime·goPanicSliceConvert(SB)

src/runtime/error.go

@@ -134,6 +134,7 @@ const (
 	boundsSlice3B    // s[?:x:y], 0 <= x <= y failed (but boundsSlice3A didn't happen)
 	boundsSlice3C    // s[x:y:?], 0 <= x <= y failed (but boundsSlice3A/B didn't happen)
 
+	boundsConvert // (*[x]T)(s), 0 <= x <= len(s) failed
 	// Note: in the above, len(s) and cap(s) are stored in y
 )
 
@@ -149,6 +150,7 @@ var boundsErrorFmts = [...]string{
 	boundsSlice3Acap: "slice bounds out of range [::%x] with capacity %y",
 	boundsSlice3B:    "slice bounds out of range [:%x:%y]",
 	boundsSlice3C:    "slice bounds out of range [%x:%y:]",
+	boundsConvert:    "cannot convert slice with length %y to pointer to array with length %x",
 }
 
 // boundsNegErrorFmts are overriding formats if x is negative. In this case there's no need to report y.

src/runtime/panic.go

@@ -160,6 +160,12 @@ func goPanicSlice3CU(x uint, y int) {
 	panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
 }
 
+// failures in the conversion (*[x]T)s, 0 <= x <= y, x == cap(s)
+func goPanicSliceConvert(x int, y int) {
+	panicCheck1(getcallerpc(), "slice length too short to convert to pointer to array")
+	panic(boundsError{x: int64(x), signed: true, y: y, code: boundsConvert})
+}
+
 // Implemented in assembly, as they take arguments in registers.
 // Declared here to mark them as ABIInternal.
 func panicIndex(x int, y int)
@@ -178,6 +184,7 @@ func panicSlice3B(x int, y int)
 func panicSlice3BU(x uint, y int)
 func panicSlice3C(x int, y int)
 func panicSlice3CU(x uint, y int)
+func panicSliceConvert(x int, y int)
 
 var shiftError = error(errorString("negative shift amount"))
 

test/convert2.go

@@ -313,3 +313,17 @@ func _() {
 	t = u       // ERROR "cannot use .* in assignment|incompatible type"
 	t = (*T)(u) // ERROR "cannot convert"
 }
+
+func _() {
+	var s []byte
+	_ = ([4]byte)(s) // ERROR "cannot convert"
+	_ = (*[4]byte)(s)
+
+	type A [4]byte
+	_ = (A)(s) // ERROR "cannot convert"
+	_ = (*A)(s)
+
+	type P *[4]byte
+	_ = (P)(s)
+	_ = (*P)(s) // ERROR "cannot convert"
+}

test/convert4.go

@@ -0,0 +1,86 @@
+// run
+
+// Copyright 2020 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.
+
+// Test conversion from slice to array pointer.
+
+package main
+
+func wantPanic(fn func(), s string) {
+	defer func() {
+		err := recover()
+		if err == nil {
+			panic("expected panic")
+		}
+		if got := err.(error).Error(); got != s {
+			panic("expected panic " + s + " got " + got)
+		}
+	}()
+	fn()
+}
+
+func main() {
+	s := make([]byte, 8, 10)
+	if p := (*[8]byte)(s); &p[0] != &s[0] {
+		panic("*[8]byte conversion failed")
+	}
+	wantPanic(
+		func() {
+			_ = (*[9]byte)(s)
+		},
+		"runtime error: cannot convert slice with length 8 to pointer to array with length 9",
+	)
+
+	var n []byte
+	if p := (*[0]byte)(n); p != nil {
+		panic("nil slice converted to *[0]byte should be nil")
+	}
+
+	z := make([]byte, 0)
+	if p := (*[0]byte)(z); p == nil {
+		panic("empty slice converted to *[0]byte should be non-nil")
+	}
+
+	// Test with named types
+	type Slice []int
+	type Int4 [4]int
+	type PInt4 *[4]int
+	ii := make(Slice, 4)
+	if p := (*Int4)(ii); &p[0] != &ii[0] {
+		panic("*Int4 conversion failed")
+	}
+	if p := PInt4(ii); &p[0] != &ii[0] {
+		panic("PInt4 conversion failed")
+	}
+}
+
+// test static variable conversion
+
+var (
+	ss  = make([]string, 10)
+	s5  = (*[5]string)(ss)
+	s10 = (*[10]string)(ss)
+
+	ns  []string
+	ns0 = (*[0]string)(ns)
+
+	zs  = make([]string, 0)
+	zs0 = (*[0]string)(zs)
+)
+
+func init() {
+	if &ss[0] != &s5[0] {
+		panic("s5 conversion failed")
+	}
+	if &ss[0] != &s10[0] {
+		panic("s5 conversion failed")
+	}
+	if ns0 != nil {
+		panic("ns0 should be nil")
+	}
+	if zs0 == nil {
+		panic("zs0 should not be nil")
+	}
+}

test/escape_slice.go

@@ -101,6 +101,10 @@ func slice11() {
 	_ = s
 }
 
+func slice12(x []int) *[1]int { // ERROR "leaking param: x to result ~r1 level=0$"
+	return (*[1]int)(x)
+}
+
 func envForDir(dir string) []string { // ERROR "dir does not escape"
 	env := os.Environ()
 	return mergeEnvLists([]string{"PWD=" + dir}, env) // ERROR ".PWD=. \+ dir escapes to heap" "\[\]string{...} does not escape"