L4 SYCL replay via a resolved --gcc-path icpx/dpcpp override — flag vocabulary fixed, the two-pass JSON-document crash fixed, real host+device dual-context replay still not implemented

Index: Known gaps

Status (re-verified 2026-10-09 at 456989f): PARTIAL. Fixed: -fsycl flag vocabulary (buildsource/adapters/adapter_contract.py:160) and the two-pass JSON crash. Still open, as the entry says: host+device dual-context L4 replay.

Fixing L4 clang_bin resolution to honor --gcc-path (an earlier PR) meant L4 could for the first time actually invoke a SYCL-capable compiler (icpx/dpcpp) instead of always a bare clang, which surfaced a narrower, real gap: -fsycl/-fsycl-* wasn't in adapters.base.ABI_RELEVANT_FLAG_PREFIXES, so it never reached the reconstructed L4 replay command even when the real build recorded it (Codex review) — fixed, since the existing abi_relevant_flags carry-through (replay_extra_flags) already handles this class of flag correctly for every other case (-std=, -fvisibility, …), so this was a one-line vocabulary gap, not a design gap. That fix's own consequence #1 — "whether replaying without an explicit -fsycl-host-only pin causes icpx to attempt a device pass this pipeline can't consume" — was later confirmed against a real toolchain: a real -fsycl bazel compile unit (oneDAL, 2.8GB AST) hit exactly this, json.load() failing with Extra data: line 26076735 column 2 at the byte offset where the device pass's document begins, i.e. consequence #2 also confirmed (legacy dpcpp/icpx both emit multi-document host+device AST output for a plain -fsycl, the identical shape sycl_context.py already handles for the L2 header-AST backend). Now fixed by mirroring the L2 backend's own fix (dumper_ast_config._build_clang_header_command/ dumper_clang._needs_sycl_host_only) rather than adopting sycl_context.py's full host/device dual-context decoder: L4 replay parses ONE compile unit at a time and has no --frontend-context device concept to select against (unlike L2's header-AST dump, which can be asked for either context), so there is nothing for a second document to serve here — _clang_context_args (shared by both the AST pass and the macro pass) now appends -fsycl-host-only whenever dumper_clang._needs_sycl_host_only says the resolved compiler is an Intel oneAPI driver with SYCL effectively enabled and no single pass already pinned, collapsing the compile back to the one host-side pass that actually links into the scanned .so — the device pass's SPIR-V kernel code never does. Reuses _needs_sycl_host_only directly (not a reimplementation) so the last-flag-wins -fsycl/-fno-sycl scan and the legacy dpcpp/dpcpp-cl SYCL-implied-by-default handling stay a single source of truth with the L2 fix. The remaining "Extra data" json.load() path now also emits an actionable hint (mirroring dumper_clang_errors._parse_clang_ast_result's) for any other, not-yet-special-cased offload flag that still produces a multi-document stream, rather than a bare byte offset. Still not implemented, and deliberately out of scope: a genuine host+device dual-context L4 replay (an L4 counterpart to --frontend-context device) — nothing in L4's SourceAbiTu/CompileUnit model has a notion of "this TU's device pass," so adding one is a real, separate design (schema + linker + cross-check changes), not a follow-up to this crash fix.