RFR: Fix incorrect base offset handling in generated array field accessors

[Maurizio Cimadamore]

On Fri, 7 Nov 2025 01:15:32 GMT, devjeonghwan <duke at openjdk.org> wrote:

> #### 1. Status Quo
> 
> Array field accessors generated by jextract (e.g., for `int arr[3]`) currently use a fixed base offset `0L` in `VarHandle.get/set` or `MethodHandle.invokeExact`.
> This causes all array accesses to start from the beginning of the struct, ignoring the field’s actual offset.
> 
> #### 2. Problem
> 
> Because of the missing base offset, writing to array elements in nested structs or unions can overwrite unrelated fields in the same structure.
> For example, updating `maxBlockDimension[1]` could corrupt `computeCapabilityMajor` or other preceding fields.
> 
> typedef struct CudaDeviceInfo_st
> {
>     int id;
> 
>     int computeCapabilityMajor;
>     int computeCapabilityMinor;
>     int multiprocessorCount;
> 
>     int warpSize;
>     int maxThreadsPerBlock;
>     int maxThreadsPerMultiprocessor;
>     int maxBlockDimension[3];
>     int maxGridDimension[3];
> 
>     bool unifiedAddressing;
>     bool integratedMemory;
> } CudaDeviceInfo;
> 
> 
> #### 3. Fix
> 
> This patch updates `emitFieldArrayGetter` and `emitFieldArraySetter` to pass the correct field offset (`offsetField`) to all generated array accessors.
> This ensures correct addressing for both primitive and struct/union element arrays.
> 
> **Example:**
> 
> 
> // Before
> varHandle.get(struct, 0L, index);
> 
> // After
> varHandle.get(struct, fieldOffset, index);
> 
> 
> #### 4. Additional Notes
> 
> While the more idiomatic FFM approach of generating 'path-based' `VarHandle`s (e.g., `LAYOUT.varHandle(groupElement(...), ...)`) was considered, this patch takes a minimal-change approach.  So It retains the existing handle and fixes the bug by passing the `fieldOffset` as the base offset (replacing `0L`).  
> 
> If the path-based `VarHandle` approach is preferred, I can rework this.
> 
> #### 5. Testing
> 
> New test cases were added to verify correct offset handling for:
> 
> * Primitive arrays in structs
> * Struct arrays in structs
> * Struct arrays inside unions
> 
> All tests pass locally.

Good fix, and well spotted! This bug probably sneaked in when we passed from path-based var handles to using offsets.

Marked as reviewed by mcimadamore (Reviewer).

test/jtreg/generator/testStructArrayFields/TestStructArrayFields.java line 125:

> 123:         assertTrue(offTail < offFlag);
> 124: 
> 125:         assertEquals(3L * Integer.BYTES, Record_st.arr$layout().byteSize());

Maybe better to use C_INT * 3 here?

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

PR Review: https://git.openjdk.org/jextract/pull/294#pullrequestreview-3452824766
PR Review: https://git.openjdk.org/jextract/pull/294#pullrequestreview-3452829281
PR Review Comment: https://git.openjdk.org/jextract/pull/294#discussion_r2517820621