tree-optimization: Preserve reverse storage order during reference translation
tree-optimization: Fixes incorrect bit numbering by punting reference translation through aggregate copies when storage order is reversed.
This commit enhances the vn_reference_lookup_3 function in the tree-optimization pass to correctly handle references within aggregate copies that have a reversed storage order. Previously, if the lookup reduced to a base plus offset and consumed all original operands, the reverse storage order information was lost. This led to incorrect interpretation of stored bytes with wrong bit numbering on assignment. The fix makes the function punt in such cases, preserving the correct storage order and preventing value misinterpretation.
In Details
When vn_reference_lookup_3 translates a reference through an aggregate copy, it may reduce the lookup to the aggregate's RHS base plus offset, discarding original operands. If either the original access or the RHS access has reverse_storage_order, this reduction loses the flag. The fix makes vn_reference_lookup_3 punt if all operands are consumed and reverse storage order is involved in either access, preserving the flag through the outermost operand.
- tree-optimization
- A phase in GCC that manipulates the compiler's intermediate representation (the 'tree') to perform various optimizations.
- reverse storage order
- A property of data types or components where the byte order is reversed compared to the natural endianness of the system, affecting how bits within bytes are interpreted.
- aggregate copy
- A copy operation involving a structure or array, where multiple elements are copied together.
- VN (Value Numbering)
- A compiler optimization technique that identifies and represents equivalent expressions with the same value number.