[SYCL RTC] Introduce --auto-pch support

[aelovikov-intel]

Compilation of #include <sycl/sycl.hpp> is slow and that's especially problematic for SYCL RTC (run-time compilation). One way to overcome this is fine-grained includes that are being pursued separately. Another way is to employ clang's precompiled headers support which this PR is doing. Those two approaches can be combined, and this PR adds test-e2e/PerformanceTests/KernelCompiler/auto-pch.cpp that gives some idea of the PCH impact. The test shows PCH benefits when compiling some of the fine-grained includes on top of absolute minimum required to compiled SYCL RTC's "Hello world". From one of the CI runs:

Extra Headers	Without PCH	With auto-PCH
	176ms 137ms 136ms 136ms 136ms	226ms 64ms 64ms 64ms 64ms
sycl/half_type.hpp	165ms 165ms 165ms 165ms 165ms	267ms 71ms 72ms 72ms 72ms
sycl/ext/oneapi/bfloat16.hpp	174ms 173ms 173ms 173ms 173ms	279ms 76ms 73ms 73ms 74ms
sycl/marray.hpp	142ms 143ms 142ms 142ms 143ms	235ms 66ms 66ms 66ms 66ms
sycl/vector.hpp	296ms 290ms 290ms 290ms 290ms	487ms 124ms 125ms 125ms 125ms
sycl/multi_ptr.hpp	278ms 278ms 276ms 275ms 274ms	441ms 125ms 125ms 125ms 125ms
sycl/builtins.hpp	537ms 533ms 531ms 531ms 531ms	883ms 218ms 218ms 219ms 218ms

It misses sycl/sycl.hpp line because that currently crashes FE when reading the generated PCH, the crash is being investigated/fixed separately.

Implementation-wise I'm reusing existing upstream clang::PrecompiledPreamble with one minor modification. It seems that PrecompiledPreamble's main usage is for things like clangd so it ignores errors in the code. I've modified it so that those errors would break pch-generation the same way normal compilation would break. I'm also not sure if we'd want that long-term, because it seems that making such "auto-pch" persistent would deviate from the upstream version of PrecompiledPreamble even more. I can imagine that in some near future we'd need to "fork" it into a separate utility. Still, seems to be fine for the first step.

Driver modifications are for the --auto-pch option support that should only be present on the SYCL RTC path and not for the regular clang invocations from the command line. I'm relatively confident those will stay in future.

[srividya-sundaram]

Driver changes LGTM!

[cperkinsintel]

rocking!

[elizabethandrews]

LGTM

cc @tahonermann for awareness.

[aelovikov-intel]

I'm also not sure if we'd want that long-term, because it seems that making such "auto-pch" persistent would deviate from the upstream version of PrecompiledPreamble even more. I can imagine that in some near future we'd need to "fork" it into a separate utility. Still, seems to be fine for the first step.

I've sketched the changes to make the cache persistent, PrecompiledPreamble would only need this additional member (in both .hpp/.cpp for declaration/definition):

llvm::StringRef PrecompiledPreamble::memoryContents() const {
  return Storage->memoryContents();
}

and the SYCL RTC code would do something like this:

if (!llvm::sys::fs::exists(PCHPath)) {
  auto PCHPreamble = PrecompiledPreamble::Build(...);
  raw_fd_ostream{PCHPath, EC} << PCHPreamble->memoryContents();
}

adjustInvocation(PCHPath); // Impl mostly copy-pasted from PrecompiledPreamble.

[aelovikov-intel]

@gmlueck , your approval is the only remaining blocker.

[tahonermann]

@premanandrao should review this.

This seems like useful functionality we should consider upstreaming if we can demonstrate it to be a robust solution.

[tahonermann]

This looks nice and clean. I noted one FIXME comment that should ideally be addressed/removed. Otherwise, this looks good to me.

[tahonermann]

This isn't an immediate concern, but future support for C++20 modules might require changes to include module imports in the preamble. Module imports don't look like preprocessor control lines, but they are. The language has rules that prevent use of macros that expand to import declarations so that recognition of them isn't dependent on preprocessing.

import std;      // Named module import
import <vector>; // Header unit import

[gmlueck]

Cool! This seems like a great feature. I have some comments / questions on the documentation and exact behavior, though. See below.

[gmlueck]

I think we need some more description here in order for people to understand what this does. I suggest something like:

Automatically create and use a precompiled header (PCH) to speed up compilation. The first time this option is passed, the compiler finds the initial set of #include directives in the compiled source string (the preamble) and creates a PCH from these header files. On subsequent compilations, if the compiled source string has the same preamble, the PCH is used instead of the header files, which speeds up compilation. If the compiled source string has a different preamble, a new PCH is generated, and that PCH can also be used to speed up subsequent compilations. These PCH files are stored internally in memory, so they do not persist from one execution of the application to the next.

The preamble ends with the first statement that is not a preprocessing directive. For example, in the code below, the preamble ends immediately before the namespace syclext = statement because this is the first statement that is not a preprocessing directive.

[Example here. I suggest an example that has a #define before the #include to illustrate that this is legal. For example, you could #define SYCL_SIMPLE_SWIZZLES.]

The compiler uses the following factors when deciding whether a previously generated PCH can be used:

• The preamble must exactly match (including whitespace and comments).
• The compilation options must match (including the same order).

There are also certain restrictions that the user must avoid:

• The content of each header file in the preamble must not change from one compilation to another.
• The header files in the preamble must not use the __DATE__ or __TIME__ macros.

I guessed at some of the details above like whitespace and the order of options. If I guessed wrong, please correct.

[gmlueck]

This limitation isn't clear to me. Does the limitation exist whenever there is a #if / #endif? Or, is it a limitation because the preamble ends inside the body of an #if / #endif?

How is this handled? Does the user get an obvious error, so they know it's related to --auto-pch? Can we instead make this work by ending the preamble at the #if that encloses the non-directive statement?

[gmlueck]

What does "no support" mean? Does this generate an error? If so, does the error message tell the user it's related to --auto-pch? Does it generate wrong code? Does it cache the value of the date / time in the PCH and use the cached values?

[aelovikov-intel]

I planned that as UB at this stage.

[gmlueck]

Silently generating bad code is not a good option. I think any of these would be acceptable:

• Issue an error and abort compilation with a clear message.
• Cache the value of the date / time in the PCH and use that in subsequent compilations that use the PCH.
• Do not generate the PCH if it contains these macros, optionally issuing a warning message.

[gmlueck]

It misses sycl/sycl.hpp line because that currently crashes FE when reading the generated PCH, the crash is being investigated/fixed separately.

@tahonermann I assume your team is the one investigating this? I think this needs to be fixed before we can make a release with the --auto-pch option because the main use case (and the example in the documentation) does #include <sycl/sycl.hpp>. How should we handle this dependency? Is it safe to merge this PR with the expectation that the FE will be fixed before the next release? Or, should we hold this PR until there is a fix?