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layout: avoid heap escapes in Stack and Flex

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Stack.Layout and Flex.Layout caused a lot of heap allocations / escapes. The
reason was that scratch space for dims and call and was inside Stack/FlexChild.
child.call.Add(gtx.Ops) confused the go escape analysis and caused the entired
children slice to escape to the heap, including all widgets in it. This caused a
lot of heap allocations. Now the scratch space is separate from children, and
for cases len(children) <= 32, we will allocate the scratch space on the stack.
For cases len(children) > 32, only the scratch space gets allocated from the
heap, during append.

Signed-off-by: vsariola <5684185+vsariola@users.noreply.github.com>
This commit is contained in:
5684185+vsariola@users.noreply.github.com
2025-05-26 18:44:06 +03:00
parent 0a209f7d39
commit 1a17e9ea37
2 changed files with 46 additions and 26 deletions
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layout/stack.go
+22 -12
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@@ -20,10 +20,6 @@ type Stack struct {
type StackChild struct {
expanded bool
widget Widget
// Scratch space.
call op.CallOp
dims Dimensions
}
// Stacked returns a Stack child that is laid out with no minimum
@@ -52,6 +48,20 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
// First lay out Stacked children.
cgtx := gtx
cgtx.Constraints.Min = image.Point{}
// Note: previously the scratch space was inside StackChild.
// child.call.Add(gtx.Ops) confused the go escape analysis and caused the
// entired children slice to be allocated on the heap, including all widgets
// in it. This produced a lot of object allocations. Now the scratch space
// is separate from children, and for cases len(children) <= 32, we will
// allocate the scratch space on the stack. For cases len(children) > 32,
// only the scratch space gets allocated from the heap, during append.
type scratchSpace struct {
call op.CallOp
dims Dimensions
}
var scratchArray [32]scratchSpace
scratch := scratchArray[:0]
scratch = append(scratch, make([]scratchSpace, len(children))...)
for i, w := range children {
if w.expanded {
continue
@@ -65,8 +75,8 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
if h := dims.Size.Y; h > maxSZ.Y {
maxSZ.Y = h
}
children[i].call = call
children[i].dims = dims
scratch[i].call = call
scratch[i].dims = dims
}
// Then lay out Expanded children.
for i, w := range children {
@@ -83,14 +93,14 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
if h := dims.Size.Y; h > maxSZ.Y {
maxSZ.Y = h
}
children[i].call = call
children[i].dims = dims
scratch[i].call = call
scratch[i].dims = dims
}
maxSZ = gtx.Constraints.Constrain(maxSZ)
var baseline int
for _, ch := range children {
sz := ch.dims.Size
for _, scratchChild := range scratch {
sz := scratchChild.dims.Size
var p image.Point
switch s.Alignment {
case N, S, Center:
@@ -105,10 +115,10 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
p.Y = maxSZ.Y - sz.Y
}
trans := op.Offset(p).Push(gtx.Ops)
ch.call.Add(gtx.Ops)
scratchChild.call.Add(gtx.Ops)
trans.Pop()
if baseline == 0 {
if b := ch.dims.Baseline; b != 0 {
if b := scratchChild.dims.Baseline; b != 0 {
baseline = b + maxSZ.Y - sz.Y - p.Y
}
}