RFR: 8371603: C2: Missing Ideal optimizations for load and store vectors on SVE [v2]

[Xiaohong Gong]

On Fri, 5 Dec 2025 09:37:22 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> **Problem:**
>> 
>> This issue occurs on a 256-bit SVE machine, caused by the following problematic pattern in `LoadVectorNode::Ideal()`:
>> 
>> 
>> Node* LoadVectorNode::Ideal(PhaseGVN* phase, bool can_reshape) {
>>   const TypeVect* vt = vect_type();
>>   if (Matcher::vector_needs_partial_operations(this, vt)) {
>>     return VectorNode::try_to_gen_masked_vector(phase, this, vt);
>>   }
>>   return LoadNode::Ideal(phase, can_reshape);
>> }
>> 
>> 
>> The condition `Matcher::vector_needs_partial_operations(this, vt)` returns true for `LoadVectorNode` with 256-bit vector size even when the vector size equals the maximum vector size on SVE. In such cases, when `VectorNode::try_to_gen_masked_vector()` returns `nullptr`, the method exits early without calling `LoadNode::Ideal()`. This results in missing crucial optimizations that would normally be applied by the superclass.
>> 
>> This code was introduced by https://bugs.openjdk.org/browse/JDK-8286941 to generate vector masks for partial vector operations, but it failed to ensure that the superclass `Ideal()` method is always invoked when no transformation is applied.
>> 
>> **Solution:**
>> 
>> This patch addresses the issue through two changes:
>> 
>> 1. Refine `Matcher::vector_needs_partial_operations()` to return true only when the vector node genuinely represents a partial vector operation that requires masking.
>> 2. Modify `VectorNode::try_to_gen_masked_vector()` to never return `nullptr`, ensuring the superclass `Ideal()` method is always invoked when no transformation is applied.
>> 
>> **Testing:**
>> 
>> - Verified on different SVE platforms with different vector sizes (128|256|512 bits).
>> - Verified on X86 platforms with different avx options (-XX:UseAVX=1|2|3).
>> - Added two new IR tests to verify 1) previously missing optimizations for `LoadVector/StoreVector` are now applied, and 2) that mask and the correct IR patterns are generated for partial vector operations.
>
> Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Combine the condition check and IR transformation to a method

Hi @erifan @shqking , could you please help take a look at this PR? Thanks a lot!

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PR Comment: https://git.openjdk.org/jdk/pull/28651#issuecomment-3624159509