cmd/compile/internal/gc: minor cleanup of slicing

Tidy the code up a little bit to move variable definitions closer
to uses, prefer early return to else branches and some other minor
tweaks.

I'd like to make some more changes to this code in the near future
and this CL should make those changes cleaner.

Change-Id: Ie7d7f2e4bb1e670347941e255c9cdc1703282db5
Reviewed-on: https://go-review.googlesource.com/c/go/+/170120
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -2415,7 +2415,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		if max != nil {
 			k = s.expr(max)
 		}
-		p, l, c := s.slice(n.Left.Type, v, i, j, k, n.Bounded())
+		p, l, c := s.slice(v, i, j, k, n.Bounded())
 		return s.newValue3(ssa.OpSliceMake, n.Type, p, l, c)
 
 	case OSLICESTR:
@@ -2428,7 +2428,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		if high != nil {
 			j = s.expr(high)
 		}
-		p, l, _ := s.slice(n.Left.Type, v, i, j, nil, n.Bounded())
+		p, l, _ := s.slice(v, i, j, nil, n.Bounded())
 		return s.newValue2(ssa.OpStringMake, n.Type, p, l)
 
 	case OCALLFUNC:
@@ -4359,35 +4359,25 @@ func (s *state) storeArg(n *Node, t *types.Type, off int64) {
 
 // slice computes the slice v[i:j:k] and returns ptr, len, and cap of result.
 // i,j,k may be nil, in which case they are set to their default value.
-// t is a slice, ptr to array, or string type.
-func (s *state) slice(t *types.Type, v, i, j, k *ssa.Value, bounded bool) (p, l, c *ssa.Value) {
-	var elemtype *types.Type
-	var ptrtype *types.Type
-	var ptr *ssa.Value
-	var len *ssa.Value
-	var cap *ssa.Value
-	zero := s.constInt(types.Types[TINT], 0)
+// v may be a slice, string or pointer to an array.
+func (s *state) slice(v, i, j, k *ssa.Value, bounded bool) (p, l, c *ssa.Value) {
+	t := v.Type
+	var ptr, len, cap *ssa.Value
 	switch {
 	case t.IsSlice():
-		elemtype = t.Elem()
-		ptrtype = types.NewPtr(elemtype)
-		ptr = s.newValue1(ssa.OpSlicePtr, ptrtype, v)
+		ptr = s.newValue1(ssa.OpSlicePtr, types.NewPtr(t.Elem()), v)
 		len = s.newValue1(ssa.OpSliceLen, types.Types[TINT], v)
 		cap = s.newValue1(ssa.OpSliceCap, types.Types[TINT], v)
 	case t.IsString():
-		elemtype = types.Types[TUINT8]
-		ptrtype = types.NewPtr(elemtype)
-		ptr = s.newValue1(ssa.OpStringPtr, ptrtype, v)
+		ptr = s.newValue1(ssa.OpStringPtr, types.NewPtr(types.Types[TUINT8]), v)
 		len = s.newValue1(ssa.OpStringLen, types.Types[TINT], v)
 		cap = len
 	case t.IsPtr():
 		if !t.Elem().IsArray() {
 			s.Fatalf("bad ptr to array in slice %v\n", t)
 		}
-		elemtype = t.Elem().Elem()
-		ptrtype = types.NewPtr(elemtype)
 		s.nilCheck(v)
-		ptr = v
+		ptr = s.newValue1(ssa.OpCopy, types.NewPtr(t.Elem().Elem()), v)
 		len = s.constInt(types.Types[TINT], t.Elem().NumElem())
 		cap = len
 	default:
@@ -4396,7 +4386,7 @@ func (s *state) slice(t *types.Type, v, i, j, k *ssa.Value, bounded bool) (p, l,
 
 	// Set default values
 	if i == nil {
-		i = zero
+		i = s.constInt(types.Types[TINT], 0)
 	}
 	if j == nil {
 		j = len
@@ -4433,46 +4423,52 @@ func (s *state) slice(t *types.Type, v, i, j, k *ssa.Value, bounded bool) (p, l,
 		i = s.boundsCheck(i, j, ssa.BoundsSliceB, bounded)
 	}
 
-	// Generate the following code assuming that indexes are in bounds.
-	// The masking is to make sure that we don't generate a slice
-	// that points to the next object in memory.
-	// rlen = j - i
-	// rcap = k - i
-	// delta = i * elemsize
-	// rptr = p + delta&mask(rcap)
-	// result = (SliceMake rptr rlen rcap)
-	// where mask(x) is 0 if x==0 and -1 if x>0.
+	// Word-sized integer operations.
 	subOp := s.ssaOp(OSUB, types.Types[TINT])
 	mulOp := s.ssaOp(OMUL, types.Types[TINT])
 	andOp := s.ssaOp(OAND, types.Types[TINT])
-	rlen := s.newValue2(subOp, types.Types[TINT], j, i)
-	var rcap *ssa.Value
-	switch {
-	case t.IsString():
-		// Capacity of the result is unimportant. However, we use
-		// rcap to test if we've generated a zero-length slice.
+
+	// Calculate the length (rlen) and capacity (rcap) of the new slice.
+	// For strings the capacity of the result is unimportant. However,
+	// we use rcap to test if we've generated a zero-length slice.
 	// Use length of strings for that.
-		rcap = rlen
-	case j == k:
-		rcap = rlen
-	default:
+	rlen := s.newValue2(subOp, types.Types[TINT], j, i)
+	rcap := rlen
+	if j != k && !t.IsString() {
 		rcap = s.newValue2(subOp, types.Types[TINT], k, i)
 	}
 
-	var rptr *ssa.Value
 	if (i.Op == ssa.OpConst64 || i.Op == ssa.OpConst32) && i.AuxInt == 0 {
 		// No pointer arithmetic necessary.
-		rptr = ptr
-	} else {
-		// delta = # of bytes to offset pointer by.
-		delta := s.newValue2(mulOp, types.Types[TINT], i, s.constInt(types.Types[TINT], elemtype.Width))
+		return ptr, rlen, rcap
+	}
+
+	// Calculate the base pointer (rptr) for the new slice.
+	//
+	// Generate the following code assuming that indexes are in bounds.
+	// The masking is to make sure that we don't generate a slice
+	// that points to the next object in memory. We cannot just set
+	// the pointer to nil because then we would create a nil slice or
+	// string.
+	//
+	//     rcap = k - i
+	//     rlen = j - i
+	//     rptr = ptr + (mask(rcap) & (i * stride))
+	//
+	// Where mask(x) is 0 if x==0 and -1 if x>0 and stride is the width
+	// of the element type.
+	stride := s.constInt(types.Types[TINT], ptr.Type.Elem().Width)
+
+	// The delta is the number of bytes to offset ptr by.
+	delta := s.newValue2(mulOp, types.Types[TINT], i, stride)
+
 	// If we're slicing to the point where the capacity is zero,
 	// zero out the delta.
 	mask := s.newValue1(ssa.OpSlicemask, types.Types[TINT], rcap)
 	delta = s.newValue2(andOp, types.Types[TINT], delta, mask)
-		// Compute rptr = ptr + delta
-		rptr = s.newValue2(ssa.OpAddPtr, ptrtype, ptr, delta)
-	}
+
+	// Compute rptr = ptr + delta.
+	rptr := s.newValue2(ssa.OpAddPtr, ptr.Type, ptr, delta)
 
 	return rptr, rlen, rcap
 }