XLS: support all hls4ml rounding modes.

[vasdommes]

Description

Currently, XLS backend supports only rounding modes that exist (under different names) in DSLX stdlib: TRN, TRN_ZERO, RND_INF, RND_CONV.

hls4ml/hls4ml/templates/xls/firmware/ap_types/fixed_point_util.x

Lines 582 to 596 in f982546

	fn convert_rounding_mode<rm: RoundingMode>() -> round::RoundingMode {
	match rm {
	RoundingMode::TRN => round::RoundingMode::RTN,
	RoundingMode::TRN_ZERO => round::RoundingMode::RTZ,
	// RoundingMode::RND => TODO,
	// RoundingMode::RND_ZERO => TODO,
	RoundingMode::RND_INF => round::RoundingMode::RNA,
	// RoundingMode::RND_MIN_INF => TODO,
	RoundingMode::RND_CONV => round::RoundingMode::RNE,
	_ => {
	assert_fmt!(false, "unsupported_RoundingMode_{}", (rm as RoundingModeIntegerType));
	round::RoundingMode::RTN
	}
	}
	}

This PR adds missing rounding modes.

Implementation:

• Copied round.x from stdlib, changed enum RoundingMode to the one from hls4ml
• Added missing cases to the function round().
• Added DSLX tests

Type of change

• New feature (non-breaking change which adds functionality)

Tests

Unit tests in DSLX code can be run e.g. as (for xls build from sources in$HOME/xls):

$HOME/xls/bazel-bin/xls/dslx/interpreter_main --dslx_stdlib_path=$HOME/xls/xls/dslx/stdlib ap_round.x

All existing hls4ml tests pass.

I also ran softmax tests based on the commit e5bf4af0 from PR #1476. These tests originally failed with

[Assert] Assertion failure via assert_fmt! @ /builds/fastmachinelearning/hls4ml/test/pytest/test_softmax_test_softmax_16,6-input_shape0-auto-io_parallel-False-stable-XLS-activation/firmware/ap_types/fixed_point_util.x:592:24-592:95: unsupported_RoundingMode_3

(which means RoundingMode::RND)

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.

[Copilot]

Pull request overview

This PR extends the XLS backend’s fixed-point rounding support to cover all hls4ml rounding modes by introducing an hls4ml-compatible DSLX rounding module and wiring it into existing fixed-point utilities.

Changes:

• Added a new DSLX rounding implementation (ap_round.x) adapted from XLS stdlib to support the full hls4ml RoundingMode set.
• Updated fixed_point_util.x to use the new ap_round module directly (removing the previous partial adapter/conversion logic).
• Added DSLX unit tests to validate all rounding modes and verify equivalence for modes that map to XLS stdlib rounding.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
hls4ml/templates/xls/firmware/ap_types/fixed_point_util.x	Switches rounding to use ap_types.ap_round and removes the old partial rounding-mode conversion layer.
hls4ml/templates/xls/firmware/ap_types/ap_round.x	New rounding implementation and tests supporting all hls4ml rounding modes (with conversions for stdlib-equivalent modes).

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[jmitrevs]

Is it worth adding explicit pytests for the rounding modes?

[JanFSchulte]

Probably a good idea to add those.