Measure a cost on a tree that does not move when layout does

`Branch` picks which of two subtrees to draw by comparing a measured pixel
length with a threshold. That is exactly what the oracle wants -- it is how a
widget believing a measurement a cold start would not have given it becomes a
different tree -- and exactly what a rig measuring cost must not have: the
fixture's shape moves with the thing being measured.

It has been moving. Seed 1 at depth 8 draws 88 widgets and writes 2,298
primitives a frame at `5ed9e87`, and 115 and 8,209 at `bd6de71` -- three and
a half times the work -- so the handoff's "fixed point cost 3x" compared two
different workloads and is withdrawn. Measured on one tree instead, with
`Edits::fixed_branches`, `5ed9e87` is 1,761M instructions and ~699M cycles
against this head's 2,093M and ~819M, while drawing 100 widgets against 97
and writing 4,272 primitives against 3,951. Fixed point costs something like
a fifth to a quarter, not three times.

The oracle keeps measured branches: `fixed_branches` is false by default and
only the rig sets it. A branch consumes its randomness either way, so both
grow the same ids.

**Check the work counters before comparing two commits' times.** The rig
prints drawn widgets, widget draws and primitive writes for this reason;
an undrawn `leftover` child still moves them, which no flag can remove.

Checked: fmt, clippy, 105 tests, the 100-seed generated oracle.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/fixed.rs

@@ -138,6 +138,16 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
         Self(narrow(shift_round(self.0 as i64 * by.0 as i64, BY)))
     }
 
+    /// A part of a span that is often nothing: no part of nothing is
+    /// nothing, for the cost of a comparison rather than a widening
+    /// multiply and a rounding.
+    pub const fn scaled<const BY: u32>(self, by: Fixed<BY>) -> Self {
+        match self.0 == 0 {
+            true => self,
+            false => self.mul(by),
+        }
+    }
+
     /// Repeated a whole number of times, which no grid rounds.
     pub const fn mul_int(self, by: i32) -> Self {
         Self(narrow(self.0 as i64 * by as i64))
@@ -179,7 +189,7 @@ impl<const SHIFT: u32> Fixed<SHIFT> {
     /// `from` and `to` a fraction of the way apart, the fraction being the
     /// receiver -- the argument order [`crate::util::LerpUtil`] already uses.
     pub const fn lerp<const OF: u32>(self, from: Fixed<OF>, to: Fixed<OF>) -> Fixed<OF> {
-        from.add(to.sub(from).mul(self))
+        from.add(to.sub(from).scaled(self))
     }
 
     pub const fn min(self, other: Self) -> Self {

core/src/orientation/pos.rs

@@ -282,7 +282,26 @@ impl UiSpan {
         self.end += offset;
     }
 
+    /// The whole of the box it sits in: a span that composes to nothing and
+    /// a parent that changes nothing.
+    pub const fn is_full(&self) -> bool {
+        self.start.rel.raw() == Rel::ZERO.raw()
+            && self.start.px.raw() == Px::ZERO.raw()
+            && self.end.rel.raw() == Rel::ONE.raw()
+            && self.end.px.raw() == Px::ZERO.raw()
+    }
+
     pub const fn within(&self, parent: &Self) -> Self {
+        // A part that is the whole box is the box, and a box composed through
+        // the whole of its parent is itself. Both are exact -- multiplying by
+        // one rounds to what it started as -- and both are common enough to
+        // be worth four comparisons rather than four multiplies to find out.
+        if self.is_full() {
+            return *parent;
+        }
+        if parent.is_full() {
+            return *self;
+        }
         Self {
             start: self.start.within(parent),
             end: self.end.within(parent),
@@ -292,6 +311,15 @@ impl UiSpan {
     pub const fn len(&self) -> Len {
         self.end - self.start
     }
+
+    /// Both ends by the same amount, which is what moving a box without
+    /// changing its length does to every part of it.
+    pub const fn translated(self, by: Len) -> Self {
+        Self {
+            start: self.start + by,
+            end: self.end + by,
+        }
+    }
 }
 
 #[repr(C)]
@@ -302,6 +330,17 @@ pub struct UiRegion {
 }
 
 impl UiRegion {
+    /// Every part of the box by the same amount on each axis. Done to the
+    /// whole region rather than an end at a time, because that is what it is
+    /// -- and because four adds in a row are four adds, where four asked for
+    /// separately are four sequences.
+    pub const fn translated(self, x: Len, y: Len) -> Self {
+        Self {
+            x: self.x.translated(x),
+            y: self.y.translated(y),
+        }
+    }
+
     pub const FULL: Self = Self {
         x: UiSpan::FULL,
         y: UiSpan::FULL,

core/src/ui/render_state.rs

@@ -3,7 +3,7 @@ use crate::layout_diagnostics::{self as diag, Counter, ReuseOutcome, TimerKind};
 use crate::ui::painter::{declared_box, declared_lens, placed_box};
 use crate::{
     ActiveData, Axis, DrawLayers, Holds, IdLike, LayoutLen, Len, MaskIdx, MoveIdx, Moves, Painter,
-    PixelRegion, PxVec2, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan, Weight,
+    PixelRegion, Px, PxVec2, RegionAlign, Rel, Size, StrongWidget, UiRegion, UiRsc, UiSpan, Weight,
     WidgetId, Widgets,
     util::{HashMap, Vec2},
 };
@@ -632,7 +632,7 @@ impl UiRenderState {
                 self.moves.set(slot, region);
             } else {
                 let remap = RegionRemap::new(old_region, region)?;
-                self.remap_subtree(id, remap, info.parent_move, mask, rsc);
+                self.remap_subtree(id, &remap, info.parent_move, mask, rsc);
             }
         }
         let active = self.active.get_mut(&id).unwrap();
@@ -668,7 +668,7 @@ impl UiRenderState {
     fn remap_subtree(
         &mut self,
         id: WidgetId,
-        remap: RegionRemap,
+        remap: &RegionRemap,
         parent_move: MoveIdx,
         inherited_mask: MaskIdx,
         rsc: &mut dyn UiRsc,
@@ -1045,55 +1045,110 @@ fn same_pixel_region(a: PixelRegion, b: PixelRegion) -> bool {
 /// source extent can be translated but cannot recover fractions for a resize.
 #[derive(Clone, Copy)]
 struct RegionRemap {
-    from: UiRegion,
-    to: UiRegion,
+    axes: [AxisRemap; 2],
+}
+
+/// Moving one axis of a box into another, worked out once for the whole
+/// subtree that moves with it. Every part of that subtree is divided by the
+/// same extent and placed between the same two ends, so the ends and the
+/// divisor belong here rather than in each part's arithmetic.
+#[derive(Clone, Copy)]
+enum AxisRemap {
+    /// A box that kept its length carries its parts by moving them, which is
+    /// exact. Dividing to find the fraction each sits at and multiplying to
+    /// place it again are two roundings, and they land a step from where
+    /// growing the tree that way does.
+    Translate(Len),
+    /// A box that changed length has to re-express each part as a fraction of
+    /// the new one, which is what a part of a box means.
+    Scale(AxisScale),
+}
+
+#[derive(Clone, Copy)]
+struct AxisScale {
+    /// What the fraction is measured from, and what divides it. `whole` is
+    /// the common case of a box spanning the whole of its parent's, where
+    /// dividing by one is the expensive way to write a subtraction.
+    start_rel: Rel,
+    extent: Rel,
+    whole: bool,
+    /// `lerp` is `a + (b - a) * fraction`, and both ends are the same for
+    /// every part, so each is kept as its near end and its span.
+    from_px: Px,
+    from_px_span: Px,
+    to_rel: Rel,
+    to_rel_span: Rel,
+    to_px: Px,
+    to_px_span: Px,
 }
 
 impl RegionRemap {
     fn new(from: UiRegion, to: UiRegion) -> Option<Self> {
-        AXES.into_iter()
-            .all(|axis| {
-                let from = from.axis(axis);
-                from.start.rel != from.end.rel || from.len() == to.axis(axis).len()
-            })
-            .then_some(Self { from, to })
+        Some(Self {
+            axes: [AxisRemap::new(from.x, to.x)?, AxisRemap::new(from.y, to.y)?],
+        })
     }
 
-    fn apply(self, region: UiRegion) -> UiRegion {
+    fn apply(&self, region: UiRegion) -> UiRegion {
+        // A box that only moved carries every part of itself by the same two
+        // amounts, and that is the common move. Asking it once for the whole
+        // region is what lets it be eight adds in a row rather than four
+        // sequences with a branch each -- measured, it is where the time in a
+        // move goes.
+        if let [AxisRemap::Translate(x), AxisRemap::Translate(y)] = self.axes {
+            return region.translated(x, y);
+        }
         UiRegion {
-            x: self.apply_span(region.x, self.from.x, self.to.x),
-            y: self.apply_span(region.y, self.from.y, self.to.y),
+            x: self.axes[0].apply_span(region.x),
+            y: self.axes[1].apply_span(region.y),
         }
     }
+}
 
-    fn apply_span(self, span: UiSpan, from: UiSpan, to: UiSpan) -> UiSpan {
-        UiSpan {
-            start: self.apply_scalar(span.start, from, to),
-            end: self.apply_scalar(span.end, from, to),
+impl AxisRemap {
+    fn new(from: UiSpan, to: UiSpan) -> Option<Self> {
+        if from.len() == to.len() {
+            return Some(Self::Translate(to.start - from.start));
         }
-    }
-
-    fn apply_scalar(self, scalar: Len, from: UiSpan, to: UiSpan) -> Len {
         let extent = from.end.rel - from.start.rel;
-        // A box that only moved, or that has no relative extent to divide,
-        // carries its parts by moving them, which is exact. Dividing to find
-        // the fraction each sits at and multiplying to place it again are two
-        // roundings, and they land a step from where growing the tree that
-        // way does. Where the box changed length there is nothing else to do,
-        // and the fraction is what a part means.
-        if from.len() == to.len() || extent == Rel::ZERO {
-            return scalar + to.start - from.start;
+        // Without a relative extent there is no fraction to re-express: a box
+        // of fixed length cannot say where its parts sit in a different one.
+        if extent == Rel::ZERO {
+            return None;
         }
-        // A box that spans the whole of its parent's is the common one, and
-        // dividing by one is the expensive way to write a subtraction.
-        let offset = scalar.rel - from.start.rel;
-        let fraction = match extent == Rel::ONE {
-            true => offset,
-            false => offset / extent,
+        Some(Self::Scale(AxisScale {
+            start_rel: from.start.rel,
+            extent,
+            whole: extent == Rel::ONE,
+            from_px: from.start.px,
+            from_px_span: from.end.px - from.start.px,
+            to_rel: to.start.rel,
+            to_rel_span: to.end.rel - to.start.rel,
+            to_px: to.start.px,
+            to_px_span: to.end.px - to.start.px,
+        }))
+    }
+
+    fn apply_span(&self, span: UiSpan) -> UiSpan {
+        UiSpan {
+            start: self.apply_scalar(span.start),
+            end: self.apply_scalar(span.end),
+        }
+    }
+
+    fn apply_scalar(&self, scalar: Len) -> Len {
+        let scale = match self {
+            Self::Translate(by) => return scalar + *by,
+            Self::Scale(scale) => scale,
         };
-        let from_px = fraction.lerp(from.start.px, from.end.px);
-        let to_rel = fraction.lerp(to.start.rel, to.end.rel);
-        let to_px = fraction.lerp(to.start.px, to.end.px);
+        let offset = scalar.rel - scale.start_rel;
+        let fraction = match scale.whole {
+            true => offset,
+            false => offset / scale.extent,
+        };
+        let from_px = scale.from_px + scale.from_px_span.scaled(fraction);
+        let to_rel = scale.to_rel + scale.to_rel_span.scaled(fraction);
+        let to_px = scale.to_px + scale.to_px_span.scaled(fraction);
         Len::from_parts(to_rel, scalar.px - from_px + to_px)
     }
 }

src/random.rs

@@ -29,6 +29,17 @@ pub struct Edits {
     /// one. Region nodes change what a move writes and how deep a primitive's
     /// chain is, so a tree that never grows one leaves both untested.
     pub nodes: HashMap<usize, bool>,
+    /// Whether a [`Branch`] takes the side it would take at any measurement,
+    /// rather than the side the one it made says. The oracle wants the
+    /// measured side -- that is the whole point of a branch, and how a widget
+    /// believing a measurement a cold start would not have given it becomes a
+    /// different tree. A rig measuring cost wants this instead: a fixture
+    /// whose shape moves with the thing being measured cannot be compared
+    /// with itself across a change to it, and seed 1 at depth 8 went from 88
+    /// drawn widgets and 2,298 primitive writes a frame to 115 and 8,209
+    /// across fixed point, which is three and a half times the work behind a
+    /// number read as three and a half times the cost.
+    pub fixed_branches: bool,
 }
 
 #[derive(Default, Clone)]
@@ -286,7 +297,14 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
             let probe = self.node(depth - 1);
             let wide = self.node(depth - 1);
             let narrow = self.node(depth - 1);
-            let threshold = self.rng.below(500) as f32;
+            // Drawn either way, so the side a fixed branch takes is still a
+            // side the generator chose -- and it consumes the same randomness
+            // as a measured one, so the two grow the same ids.
+            let measured = self.rng.below(500) as f32;
+            let threshold = match self.edits.fixed_branches {
+                true => f32::MIN,
+                false => measured,
+            };
             let id = Branch {
                 probe,
                 wide,

tests/layout_diagnostics.rs

@@ -92,9 +92,18 @@ fn trace_selected(tree: &Tree) {
 #[cfg(not(feature = "layout-diagnostics"))]
 fn trace_selected(_: &Tree) {}
 
+/// The shape a cost is measured on must not depend on what layout measured,
+/// or two commits are compared on two different trees. See `Edits`.
+fn rig_edits() -> Edits {
+    Edits {
+        fixed_branches: true,
+        ..Default::default()
+    }
+}
+
 fn warm(seed: u64, depth: usize) -> (Harness, Tree) {
     let mut harness = Harness::new(OUTPUT);
-    let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+    let (root, tree) = grow(&mut harness.rsc, seed, depth, &rig_edits());
     harness.state.root = Some(root);
     harness.frame();
     println!(
@@ -173,7 +182,7 @@ fn layout_cost() {
 
     if selected("cold") {
         let mut harness = Harness::new(OUTPUT);
-        let (root, tree) = grow(&mut harness.rsc, seed, depth, &Edits::default());
+        let (root, tree) = grow(&mut harness.rsc, seed, depth, &rig_edits());
         harness.state.root = Some(root);
         println!(
             "fixture: seed {seed}, depth {depth}, {} widgets",