Load and store trace packets should respect access predicates #9504

This text was written by a machine (GitHub Copilot), not by the PR author.

Description

Tracking the bug Andrew Adams requested while reviewing PR #9476: load and store trace packets ignore the access predicate. A trace should describe the accesses actually performed, not report inactive accesses or lanes.

Originating review comment

Source: #9476 (comment)

Issues to open:
bug: load and store trace packets ignore the predicate
enhancement: use predicated calls for zero boundary conditions

Reproducing case / code evidence

In PR #9476, src/Tracing.cpp, InjectTracing::visit(const Call *) builds a halide_trace_load event and wraps the load in Let(value, call, return_second(trace, value)). The trace does not inherit the load Call's predicate: a skipped Call produces zero, but still gets traced. The store visitor applies a predicate to the trace Call, but trace packets also need to faithfully represent active lanes after vectorization rather than advertising every lane as accessed.

This is a review-requested tracking issue; a standalone runtime reproducer has not been independently validated here. Regression coverage should include scalar and vector loads and stores with false/mixed predicates, checking emitted events and coordinates against actual memory accesses.

Expected resolution

Preserve access predicates through tracing and vectorization, and represent/filter inactive lanes appropriately in the emitted packets. Keep trace value evaluation ordered correctly and avoid re-evaluating side effects.

Additional context

Built-from-source PR #9476, reviewed at commit 7217e690970301666ba5dbe03eb84548b6c0dff6. This follow-up is separate from the predicated-Calls implementation review fixes.