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

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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>
This commit is contained in:
Michael Munday 2019-03-30 14:11:05 +00:00
parent 5fc55b3180
commit 726a9398f7
1 changed files with 46 additions and 50 deletions
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96
src/cmd/compile/internal/gc/ssa.go
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@ -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
@ -4433,47 +4423,53 @@ 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])
// 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.
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.
// Use length of strings for that.
rcap = rlen
case j == k:
rcap = rlen
default:
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))
// 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)
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, ptr.Type, ptr, delta)
return rptr, rlen, rcap
}