middle-end: Handle variable-length vector types in store_constructor
Initializes unused elements of VLAs in constructor expressions to zero.
GCC’s middle-end now correctly handles the initialization of variable-length vector (VLA) types within constructor expressions. Previously, VLAs in constructors fell back to piecewise memory stores without defined semantics for unspecified elements. This patch introduces the semantics that any unspecified elements in a VLA constructor will be initialized to zero, ensuring predictable behavior and preventing potential issues with uninitialized memory. This change impacts how constructible VLAs are generated, aligning with expected C++ behavior.
In Details
This commit modifies store_constructor in expr.cc to establish defined semantics for initializing Variable Length Array (VLA) vec_init constructs. Previously, the lack of support for VLA vec_init led to a fallback where vector elements were piecewise copied from memory, potentially leaving unspecified elements uninitialized. The new semantics dictate that for VLA constructors, the vector is first cleared to zero, and then subsequent scalar elements are used for initialization. This ensures that any elements not explicitly provided in the constructor initializer list are zero-initialized,…
- Variable Length Array (VLA)
- An array whose size is determined at runtime, rather than at compile time. In C and C++, VLAs can be local variables whose size depends on function arguments or other runtime values.
- Constructor Expression
- In C++, an expression that results in the creation and initialization of an object. This includes calling constructors, either directly or implicitly, to form temporary objects or initialize members.
- Piecewise Initialization
- A method of initializing an object by constructing its members individually. For aggregate types like vectors, this can mean copying each element from a source sequence.
- Middle-end
- The part of a compiler that performs optimizations on the intermediate representation (IR) of the code, situated between the front-end (parsing and initial IR generation) and the back-end (code generation for a specific architecture).
- vec_init
- Likely refers to a compiler internal operation or intrinsic for initializing vector types, commonly used in C++ constructors or for SIMD operations.