RFR: 8323582: C2 SuperWord AlignVector: misaligned vector memory access with unaligned native memory

[Emanuel Peter]

On Tue, 18 Feb 2025 09:09:15 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Note: the approach with Predicates and Multiversioning prepares us well for Runtime Checks for Aliasing Analysis, see more below.
>> 
>> **Background**
>> 
>> With `-XX:+AlignVector`, all vector loads/stores must be aligned. We try to statically determine if we can always align the vectors. One condition is that the address `base` is already aligned. For arrays, we know that this always holds, because they are `ObjectAlignmentInBytes` aligned. But with native memory, the `base` is just some arbitrarily aligned pointer.
>> 
>> **Problem**
>> 
>> So far, we have just naively assumed that the `base` is always `ObjectAlignmentInBytes` aligned. But that does not hold for `native` memory segments: the `base` can also be unaligned. I had constructed such an example, and with `-XX:+AlignVector -XX:+VerifyAlignVector` this example hits the verification code.
>> 
>> 
>> MemorySegment nativeAligned = Arena.ofAuto().allocate(RANGE * 4 + 1);
>> MemorySegment nativeUnaligned = nativeAligned.asSlice(1);
>> test3(nativeUnaligned);
>> 
>> 
>> When compiling the test method, we assume that the `nativeUnaligned.address()` is aligned - but it is not!
>> 
>>     static void test3(MemorySegment ms) {
>>         for (int i = 0; i < RANGE; i++) {
>>             long adr = i * 4L;
>>             int v = ms.get(ELEMENT_LAYOUT, adr);
>>             ms.set(ELEMENT_LAYOUT, adr, (int)(v + 1));
>>         }
>>     }
>> 
>> 
>> **Solution: Runtime Checks - Predicate and Multiversioning**
>> 
>> Of course we could just forbid cases where we have a `native` base from vectorizing. But that would lead to regressions currently - in most cases we do get aligned `base`s, and we currently vectorize those. We cannot statically determine if the `base` is aligned, we need a runtime check.
>> 
>> I came up with 2 options where to place the runtime checks:
>> - A new "auto vectorization" Parse Predicate:
>>   - This only works when predicates are available.
>>   - If we fail the predicate, then we recompile without the predicate. That means we cannot add a check to the predicate any more, and we would have to do multiversioning at that point if we still want to have a vectorized loop.
>> - Multiversion the loop:
>>   - Create 2 copies of the loop (fast and slow loops).
>>   - The `fast_loop` can make speculative alignment assumptions, and add the corresponding check to the `multiversion_if` which decides which loop we take
>>   - In the `slow_loop`, we make no assumption which means we can not vectorize, but we still compile - so even ...
>
> src/hotspot/share/opto/loopTransform.cpp line 751:
> 
>> 749:         // Peeling also destroys the connection of the main loop
>> 750:         // to the multiversion_if.
>> 751:         cl->set_no_multiversion();
> 
> Would we want to change the multiversion guard at this point so it constant folds and the slow version is removed?

I suppose we can probably do that. Otherwise, we just have to wait until the `OpaqueMultiversioningNode` constant folds after loop-opts.

> src/hotspot/share/opto/loopUnswitch.cpp line 513:
> 
>> 511: 
>> 512:   // Create new Region.
>> 513:   RegionNode* region = new RegionNode(1);
> 
> So we create a new `Region` every time a new condition is added?

Yes. Are you ok with that? Or would you prefer if we extended an existing region (is that possible?) and then we'd have 2 cases, one where there is none yet, and one where we'd extend. I think adding one each time is easier, and it would get commoned anyway, right?

> src/hotspot/share/opto/traceAutoVectorizationTag.hpp line 32:
> 
>> 30: 
>> 31: #define COMPILER_TRACE_AUTO_VECTORIZATION_TAG(flags) \
>> 32:   flags(POINTER_PARSING,            "Trace VPointer/MemPointer parsing") \
> 
> Has anything changed here? I stared at it a few times and couldn't figure out what has.

I added the tag `SPECULATIVE_RUNTIME_CHECKS`. And then had to change alignment for all others ;)

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/22016#discussion_r1959397988
PR Review Comment: https://git.openjdk.org/jdk/pull/22016#discussion_r1959392450
PR Review Comment: https://git.openjdk.org/jdk/pull/22016#discussion_r1959394676