Sret

Sret, short for structure return, is a parameter attribute used in compiler intermediate representations (notably LLVM IR) to indicate how a function returns a value that is a structure or other large aggregate. When a function uses sret, the caller allocates memory for the return value and passes a pointer to that memory as the first argument. The callee then writes the result into that memory and returns (often as void).

In practice, sret changes the calling convention for large return types. Instead of returning the value in registers, the caller provides a memory location for the return value and the callee fills it in. This is commonly used for C/C++ functions that return structs or nontrivial aggregates on certain ABIs. The presence of the sret argument is a signal to the backend code generator to adjust parameter passing, alignment, and register usage accordingly.

Example (conceptual LLVM IR):

%Point = type { i32, i32 }

define void @makePoint(%Point* sret %ret, i32 %x, i32 %y) {

%p0 = getelementptr %Point, %Point* %ret, i32 0, i32 0

store i32 %x, i32* %p0

%p1 = getelementptr %Point, %Point* %ret, i32 0, i32 1

store i32 %y, i32* %p1

ret void

}

This function writes the Point result to the memory pointed to by %ret. A caller would allocate a %Point and pass its address as the first argument when invoking makePoint.

Sret is primarily a low-level concern of compilers and ABI compatibility, and is generally invisible to most high-level programmers. See also: structure return, LLVM IR attributes, call conventions.