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>

6 files changed, 32 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/ssa/expand_calls.go b/src/cmd/compile/internal/ssa/expand_calls.go
index be460457a8..46c2388e7b 100644
--- a/src/cmd/compile/internal/ssa/expand_calls.go
+++ b/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]
diff --git a/src/cmd/compile/internal/ssa/gen/dec.rules b/src/cmd/compile/internal/ssa/gen/dec.rules
index 4c677f8418..b19489870d 100644
--- a/src/cmd/compile/internal/ssa/gen/dec.rules
+++ b/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
diff --git a/src/cmd/compile/internal/ssa/gen/genericOps.go b/src/cmd/compile/internal/ssa/gen/genericOps.go
index c38d22e07f..9f6664386c 100644
--- a/src/cmd/compile/internal/ssa/gen/genericOps.go
+++ b/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
diff --git a/src/cmd/compile/internal/ssa/op.go b/src/cmd/compile/internal/ssa/op.go
index b99a7a6646..f09a08abcf 100644
--- a/src/cmd/compile/internal/ssa/op.go
+++ b/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,
diff --git a/src/cmd/compile/internal/ssa/opGen.go b/src/cmd/compile/internal/ssa/opGen.go
index 8c753ea2a3..2f56625397 100644
--- a/src/cmd/compile/internal/ssa/opGen.go
+++ b/src/cmd/compile/internal/ssa/opGen.go
@@ -2834,6 +2834,7 @@ const (
 	OpSlicePtr
 	OpSliceLen
 	OpSliceCap
+	OpSlicePtrUnchecked
 	OpComplexMake
 	OpComplexReal
 	OpComplexImag
@@ -35899,6 +35900,11 @@ var opcodeTable = [...]opInfo{
 		generic: true,
 	},
 	{
+		name:    "SlicePtrUnchecked",
+		argLen:  1,
+		generic: true,
+	},
+	{
 		name:    "ComplexMake",
 		argLen:  2,
 		generic: true,
diff --git a/src/cmd/compile/internal/ssa/rewritedec.go b/src/cmd/compile/internal/ssa/rewritedec.go
index 4b7db60551..2a73a5ddc8 100644
--- a/src/cmd/compile/internal/ssa/rewritedec.go
+++ b/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]