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[SYCL RTC] Introduce --persistent-auto-pch support #20374

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Built on top of --auto-pch (in-memory) introduced in #20226.

The most significant technical decision was how to implement the filesystem cache. I've looked into the following options:

  • sycl/source/detail/persistent_device_code_cache.hpp Also, see sycl/doc/design/KernelProgramCache.md Seems to be tailored for the very specific usage scenarios, would be very resource consuming to split into a generic data structure that would then be used for two different use cases.

    This cache is disabled by default and I'm not sure how well-tested it is. Also, using plain ".lock" files for "advisory locking" instead of the native filesystem mechanisms (e.g., locking APIs in fcntl/flock/CreateFile/LockFileEx) made me question if it's worth generalizing and how much work would be necessary there.

  • llvm/include/llvm/Support/Caching.hpp Originally implemented as part of ThinLTO implementation, moved into LLVMSupport later with the following commit message:

    We would like to move ThinLTO’s battle-tested file caching
    mechanism to the LLVM Support library so that we can use it
    elsewhere in LLVM.

    API is rather unexpected, so my research hasn't stopped here.

  • lldb/include/lldb/Core/DataFileCache.h Uses LLVMSupport's caching from the previous bullet under the hood, but provides an easier to grasp API. If we were developing upstream I think uplifting that abstraction into LLVMSupport library and then using in both lldb and libsycl would probably be the choice I'd vote for. However, doing that downstream was too much efforts so I ultimately decided not to go with this approach.

    That cache also has a std::mutex on the "hot" DataFileCache::GetCachedData path, I presume to avoid creating the same entry from multiple threads.

In the end, I've chosen to use LLVMSupport's quirky (or maybe I just hasn't grown enough to appreciate it) caching API directly and that's what is done in this PR. Unlike lldb's cache, I decided to trade possible duplicate work of building the preamble on a cache miss from concurrent threads in favor of no inter-thread synchronization (not profiled/measured though) on the cache hit path and implementation simplicity.

@aelovikov-intel
[SYCL RTC] Introduce --persistent-auto-pch support
2858c85 
Built on top of `--auto-pch` (in-memory) introduced in
intel#20226.

The most significant technical decision was how to implement the
filesystem cache. I've looked into the following options:

* `sycl/source/detail/persistent_device_code_cache.hpp`
  Also, see `sycl/doc/design/KernelProgramCache.md` Seems to be tailored
  for the very specific usage scenarios, would be very resource
  consuming to split into a generic data structure that would then be
  used for two different use cases.

  This cache is disabled by default and I'm not sure how well-tested it
  is. Also, using plain ".lock" files for "advisory locking" instead of
  the native filesystem mechanisms (e.g., locking APIs in
  `fcntl`/`flock`/`CreateFile`/`LockFileEx`) made me question if it's
  worth generalizing and how much work would be necessary there.

* `llvm/include/llvm/Support/Caching.hpp`
  Originally implemented as part of ThinLTO implementation, moved into
  `LLVMSupport` later with the following commit message:

    > We would like to move ThinLTO’s battle-tested file caching
    > mechanism to the LLVM Support library so that we can use it
    > elsewhere in LLVM.

  API is rather unexpected, so my research hasn't stopped here.

* `lldb/include/lldb/Core/DataFileCache.h`
  Uses `LLVMSupport`'s caching from the previous bullet under the hood,
  but provides an easier to grasp API. If we were developing upstream I
  think uplifting that abstraction into `LLVMSupport` library and then
  using in both `lldb` and `libsycl` would probably be the choice I'd
  vote for. However, doing that downstream was too much efforts so I
  ultimately decided not to go with this approach.

  That cache also has a `std::mutex` on the "hot"
  `DataFileCache::GetCachedData` path, I presume to avoid creating the
  same entry from multiple threads.

In the end, I've chosen to use `LLVMSupport`'s quirky (or maybe I just
hasn't grown enough to appreciate it) caching API directly and that's
what is done in this PR. Unlike `lldb`'s cache, I decided to trade
possible duplicate work of building the preamble on a cache miss from
concurrent threads in favor of no inter-thread synchronization (not
profiled/measured though) on the cache hit path and implementation simplicity.
@aelovikov-intel aelovikov-intel requested review from a team and cperkinsintel as code owners October 15, 2025 21:47
@aelovikov-intel aelovikov-intel marked this pull request as draft October 15, 2025 22:34
@aelovikov-intel
Remove multi_threaded_persistent_auto_pch.cpp
b937fa6 
persistent_auto_pch_stress_deletion.cpp triggers failed `commit` better
gmlueck
gmlueck approved these changes Oct 16, 2025
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Spec change LGTM.

@aelovikov-intel aelovikov-intel marked this pull request as ready for review October 16, 2025 20:55
srividya-sundaram
srividya-sundaram reviewed Oct 17, 2025
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clang/include/clang/Driver/Options.td
let Group = sycl_rtc_only_Group in {
def auto_pch : Flag<["--"], "auto-pch">,
HelpText<"Enable Auto-PCH for SYCL RTC Compilation">;
def persistent_auto_pch_EQ
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Could you add a Driver test similar to this?

srividya-sundaram
srividya-sundaram approved these changes Oct 17, 2025
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Driver changes LGTM.

@aelovikov-intel
Handle error creating tmp file in the cache
9e2a209 
Uncovered by `persistent_auto_pch_stress_deletion.cpp`
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@gmlueck gmlueck gmlueck approved these changes

@srividya-sundaram srividya-sundaram srividya-sundaram approved these changes

@cperkinsintel cperkinsintel Awaiting requested review from cperkinsintel cperkinsintel is a code owner

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@aelovikov-intel @gmlueck @srividya-sundaram