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

[Emanuel Peter]

On Tue, 18 Feb 2025 09:14:28 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/loopnode.cpp line 1097:
> 
>> 1095:     // PhaseIdealLoop::add_parse_predicate only checks trap limits per method, so
>> 1096:     // we do a custom check here.
>> 1097:     if (!C->too_many_traps(cloned_sfpt->jvms()->method(), cloned_sfpt->jvms()->bci(), Deoptimization::Reason_auto_vectorization_check)) {
> 
> Isn't that done by `add_parse_predicate`?

@rwestrel I only see `if (!C->too_many_traps(reason)) {` in `PhaseIdealLoop::add_parse_predicate`. And as the comment I put here that only checks the `reason` per `method`, and not per `bci`. Do you see anything else?

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PR Review Comment: https://git.openjdk.org/jdk/pull/22016#discussion_r1959403871