Out-of-SSA uses spill predicate for partition declarations
Fixes memory corruption from overlapping partitions in out-of-SSA form by using the correct spill predicate.
The out-of-SSA pass sometimes leaves multiple partitions sharing a base variable declaration when their live ranges overlap. This commit corrects how these overlapping partitions are spilled to memory. Previously, approximations were used, leading to distinct memory slots being combined and causing corruption. The change now uses the same spill predicate as used during expansion, ensuring that distinct memory slots are properly allocated and accessed, preventing data corruption.
In Details
The out-of-ssa pass can create partitions of a VAR_DECL to represent non-overlapping live ranges. When these live ranges overlap, the partitions are not split. Existing code used BLKmode to approximate memory decisions for spilling these partitions, leading to MEM_EXPRs being created at offset zero for all spilled slots. This change modifies remove_ssa_form to call split_overlapping_partition_decls, which uses use_register_for_decl to determine if a partition should be spilled to memory. Overlapping partitions are split after TER, with the original decl retaining its metadata and new ar…
- out-of-SSA
- A compiler optimization pass that converts Static Single Assignment (SSA) form back into a more traditional three-address code representation. This is often done to facilitate optimizations that are difficult to express in SSA form or to reduce pressure on the compiler's internal representation.
- partition decls
- In the context of the out-of-SSA pass, these are temporary declarations created to represent portions of a variable's lifetime or memory location. When a variable's SSA versions have overlapping live ranges, partitions are used to manage the storage for these overlapping parts.
- spill predicate
- A condition or function used by the compiler to determine whether a variable or temporary value needs to be stored (spilled) from a CPU register to memory. This is typically done when there are not enough registers available to hold all live values.
- TER
- Tree Edge Reordering. A phase in GCC's optimization pipeline that reorders basic blocks. It aims to improve instruction locality and reduce control flow overhead. This commit splits overlapping partitions after TER.
- VAR_DECL
- A GCC internal tree node representing a variable declaration. It stores information about a variable, such as its type, scope, and location.
- MEM_EXPR
- A GCC internal tree expression representing a memory access. It can encapsulate addressing modes and offsets for reading from or writing to memory.