The section claimed what was left of fixed point's cost was "an integer
multiply with a rounding against a float fused multiply-add". Both halves are
wrong and the evidence is in the binaries.
`5ed9e87` contains no FMA instruction, and rebuilding it with
`-C target-feature=+fma,+avx2` still produces none: Rust does not contract
`a + b * c`, so `LerpUtil::lerp` is `subss`, `mulss`, `addss` on any target.
The float side never had the instruction the grid was being compared against.
Nor is the multiply the cost. Replacing `shift_round` in `Fixed::mul` with a
truncating shift -- all twenty-seven work counters diffed identical -- takes
43M instructions off and leaves cycles inside the noise. What moves cycles is
whether `UiSpan::within` inlines: the rounding and both short-circuits removed
together are worth 118M instructions and 57M cycles where separately they are
worth 61M and 8M, and `<UiSpan>::within` stops being a symbol. None of that is
shippable, but it is where to push.
Three other entries corrected against measurement rather than argument. The
division is not 0.00% of cycles -- that was `apply_scalar`; `Holds::through`
runs 2.23M `i64` divisions a 500-frame `many` and holds the divider busy 21.3M
cycles. `d75a1e2` and `4cbb242`'s short-circuits no longer earn anything now
that `Fixed` wraps. Branchless `shift_round` still loses, for the opposite
reason to the one recorded: it is the rounding's size, not its branch.
And the measurement posture: cycles spread 1-3% across nine runs of one
binary rather than 0.23%, `instructions:u` returns a garbage value in roughly
a quarter of readings, and an instruction count moves 0.5% across a rebuild of
identical source. Quote deltas against a baseline built beside the thing
measured.
Checked: `/home/bob/repos/iris-float-cmp` carries `5ed9e87` with
`Edits::fixed_branches` applied, and is recorded under "Rigs and reproduction"
so the comparison can be re-run.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
iris-aiandClaude Opus 5
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