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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/flex.go
+24 -14
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@@ -30,10 +30,6 @@ type FlexChild struct {
weight float32
widget Widget
// Scratch space.
call op.CallOp
dims Dimensions
}
// Spacing determine the spacing mode for a Flex.
@@ -88,6 +84,20 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
remaining := mainMax
var totalWeight float32
cgtx := gtx
// Note: previously the scratch space was inside FlexChild.
// 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))...)
// Lay out Rigid children.
for i, child := range children {
if child.flex {
@@ -104,8 +114,8 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
if remaining < 0 {
remaining = 0
}
children[i].call = c
children[i].dims = dims
scratch[i].call = c
scratch[i].dims = dims
}
if w := f.WeightSum; w != 0 {
totalWeight = w
@@ -139,16 +149,16 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
if remaining < 0 {
remaining = 0
}
children[i].call = c
children[i].dims = dims
scratch[i].call = c
scratch[i].dims = dims
}
maxCross := crossMin
var maxBaseline int
for _, child := range children {
if c := f.Axis.Convert(child.dims.Size).Y; c > maxCross {
for _, scratchChild := range scratch {
if c := f.Axis.Convert(scratchChild.dims.Size).Y; c > maxCross {
maxCross = c
}
if b := child.dims.Size.Y - child.dims.Baseline; b > maxBaseline {
if b := scratchChild.dims.Size.Y - scratchChild.dims.Baseline; b > maxBaseline {
maxBaseline = b
}
}
@@ -169,8 +179,8 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
mainSize += space / (len(children) * 2)
}
}
for i, child := range children {
dims := child.dims
for i, scratchChild := range scratch {
dims := scratchChild.dims
b := dims.Size.Y - dims.Baseline
var cross int
switch f.Alignment {
@@ -185,7 +195,7 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
}
pt := f.Axis.Convert(image.Pt(mainSize, cross))
trans := op.Offset(pt).Push(gtx.Ops)
child.call.Add(gtx.Ops)
scratchChild.call.Add(gtx.Ops)
trans.Pop()
mainSize += f.Axis.Convert(dims.Size).X
if i < len(children)-1 {