XLS FAQ

Q: How do I select bubble strategies in pipeline generation?

Tags: codegen, pipeline, configuration

I/O behavior is described in the codegen options documentation. ("codegen" for XLS is the concept of "exactly what verilog should be produced?") There are currently options for controlling queueing behavior at the inputs or outputs of the block, but not internal to the pipeline, ability to stall with queueing that is embedded in the generated pipeline is tracked in issue #255.

Q: What is the granularity of ready/valid signaling?

Tags: codegen, pipeline, configuration

Ready/valid signals are associated with a (streaming) channel, and in general I/O signaling is configured on a per-channel basis. Users are expected to send things "broadside" (all together at once) if they should share the same ready/valid signaling; e.g. by sending a struct or array over the channel.

See the --streaming_channel_* options within the codegen options documentation.

Q: How do I call my XLS functions from C++?

Tags: native, simulation, cpp

The steps are:

1. Wrap up your XLS so it can be called from C++ (using a utility).
2. Include the created header.
3. Call the "Run" API with XLS-understood values.

The cc_xls_ir_wrapper rule in the Bazel rule set invokes a tool (the JIT wrapper generator) that makes a shim that provides an API for running the wrapped code. Despite the name the code the wrapper invokes is compiled ahead of time and directly linked into the resulting binary.

XLS fn, proc and block targets can all be simulated in this way with minimal or no overhead.

As an example, see the float32 multiply test, which calls Run() on the float32 multiplier which is written in XLS and wrapped in the float32_mul_jit_wrapper build target.

Note that for some APIs, e.g. those taking a XLS single precision float, the created interface will be able to accept a native C++ float directly, and similar for types like uint32_t, uint64_t, etc.