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Update the layout measurements to the current head

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Three more roundings and divisions taken out after measuring each, one
optimisation tried and reverted with its number kept so it is not tried
again.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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iris-aiandClaude Opus 5 committed 2026-09-16 04:14:18 -04:00
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@@ -8,7 +8,7 @@ log.
Canonical Iris `main` is **`ca2b4b2`** (#17, the headless rig). **#18
`split/18-position-chain`** is open in `/home/bob/repos/iris-pr18`; its local
head is **`f11f5f4`**, sixty commits. Built-in alignment is complete there;
head is **`4f5e27c`**, sixty-three commits. Built-in alignment is complete there;
see "Built-in alignment" below for the retained-layout details. No PR reviews
were present when checked on 2026-09-15.
@@ -72,7 +72,7 @@ rewrite:
`core/src/ui/holds.rs`, the retained tests, and the generated cold-layout oracle
pin those rules. Seeds 10 and 86 are now in the ordinary generated set.
## Verification at `f11f5f4`
## Verification at `4f5e27c`
- `cargo fmt --all --check`
- `cargo build --workspace --all-features`
@@ -124,12 +124,12 @@ Measured 2026-09-16 on the release `layout_diagnostics` fixture, seed 1,
depth 8, 500 frames, as median milliseconds a frame. `5ed9e87` is the commit
before fixed point; `39e4ca2` is fixed point complete.
| phase | `5ed9e87` | `39e4ca2` | `f11f5f4` |
| phase | `5ed9e87` | `39e4ca2` | `4f5e27c` |
| --- | ---: | ---: | ---: |
| many | 0.179 ms | 0.544 | 0.278 |
| resize | 0.020 | 0.035 | 0.041 |
| scroll | 0.011 | 0.030 | 0.020 |
| repaint | 0.012 | 0.031 | 0.028 |
| many | 0.179 ms | 0.544 | 0.274 |
| resize | 0.020 | 0.035 | 0.033 |
| scroll | 0.011 | 0.030 | 0.019 |
| repaint | 0.012 | 0.031 | 0.021 |
**Fixed point cost 3x, and two thirds of that was not the grid.** The
counters said eight more "placed by redrawing" a frame in `scroll`, all of
@@ -139,14 +139,19 @@ drawing it again on the child layer. `97cc8b3` measures on the layer the
child ends up on, which puts the scroll phase's counters back exactly where
they were -- 4 widget draws, 12 draw requests.
What is left is per-operation cost, not more work: at `f11f5f4` the `many`
phase does 125 widget draws against 121 before, and takes 1.6x as long. The
arithmetic is the difference -- an `i64` multiply and a rounding branch where
there was an `f32` multiply. The profile's shape, in order: `InstanceList::
push`, `Painter::glyphs`, `RegionRemap::apply_span`, `UiRegion::within`. The
one obvious thing left is `glyphs`, which converts four `f32`s per glyph:
a `GlyphEntry` holding `Px` would convert once when the glyph is rasterised
instead of on every frame that draws it.
What is left is per-operation cost, not more work: the `many` phase does 125
widget draws against 121 before, and takes 1.5x as long. The arithmetic is
the difference -- an `i64` multiply and a rounding branch where there was an
`f32` multiply, and an `i64` division in the remap. Three were taken out
after measuring (`11c55bc`, `4f5e27c`, and halving the divisions in
`Holds::through`); `RegionRemap::apply_span` is still a fifth of the phase,
and the rest is spread thin. Measure with `perf stat -e instructions:u`
rather than the clock, which varies 2x here.
One thing tried and reverted, recorded so it is not tried again: short-
circuiting `apply_scalar` where the fraction is nought or one. Those are not
the common cases, and the comparisons cost 17% more than the divisions they
saved.
Earlier, against #18's own history: the retained rewrite took `many` from
25.17M instructions a frame at `691e3eb` to 6.14M at `29c7881`, and `resize`