[michalharakal]

Closes #1494. Resolves Open Question 2 of SKEEP-008.

Problem

Two adjacent mixed-precision boundaries each cast back to the network dtype, so the tensor between them is widened only to be narrowed again. A BF16 dense followed by an FP16 stage exported:

%v13 = stablehlo.convert %v12 : (tensor<1x3xbf16>) -> tensor<1x3xf32>
%v14 = stablehlo.convert %v13 : (tensor<1x3xf32>) -> tensor<1x3xf16>

Change

CastFoldingPass (skainet-compile-opt) applies two value-preserving rewrites:

• A cast after an exact widening (FP16 or BF16 to FP32, any of them to FP64) reads from the original tensor. bf16 -> f32 -> f16 becomes bf16 -> f16. The widening is removed once nothing else reads it; if it has another consumer it stays and only the second cast is rewired.
• A cast to the dtype its input already has is removed. The first rewrite leaves this behind for f16 -> f32 -> f16, and DtypeForwardPropagationPass(targetFloatDtype = …) can produce it.

Never folded: the narrowing round trip f32 -> bf16 -> f32 (the rounding the layer asked for), narrow-to-narrow casts, and integer casts (Int32 to FP32 is not exact).

The pass has no options, as assessed in the issue: the exactness rule follows from the dtypes, and per-target variation already has TargetOptimizers.registerDagPasses. It runs right after DTypeConstraintResolutionPass in the default, aggressive and LLM pipelines of GraphOptimizationPipeline.

Beyond the issue

ComputeGraphExecutor could not run convert nodes, so "bit-identical before and after" could not be tested by execution. It now dispatches convert / cast / to, taking the target from the node's output dtype.

Tests

• CastFoldingPassTest (12): each rule, the adjacent-boundary chain, a widening with a second consumer, cast aliases and dtype spellings, diagnostics, an untouched graph returned as the same instance.
• MixedPrecisionExportTest (+2): the traced network exports three stablehlo.convert instead of four; the folded and unfolded cast chain executed on the CPU backend give identical raw bits.

Review notes

• Public API: CastFoldingPass added; skainet-compile-opt.api regenerated.
• Adding the pass to the default pipelines changes exports of existing mixed-precision graphs (fewer casts, same values).
• Docs: explanation/mixed-precision.adoc, SKEEP-008 Open Questions (2 resolved, 4 links [Feature]: Decide whether a PrecisionBoundary carries DTypePolicy metadata into the compiled graph (SKEEP-008 follow-up) #1495), CHANGELOG.

Test plan

• ./gradlew apiCheck checkUnmarkedStubs
• jvmTest for skainet-compile-opt, skainet-compile-hlo, skainet-compile-dag
• linuxX64 compile of skainet-compile-opt and skainet-compile-dag
• Native and wasm test targets (CI; not run locally)

🤖 Generated with Claude Code

[github-actions]

📖 Documentation Preview

The documentation has been built successfully for this PR.

Generated Files:

• Operator documentation: docs/modules/operators/_generated_/
• JSON schema output: operators.json

Artifacts:

• Download the documentation-preview-1496 artifact to view the complete documentation locally.

This comment will be updated automatically when the PR is updated.