returns_twice属性

GCCとClangの独自拡張 returns_twice 属性についてメモ。

対象関数から「2回以上制御が戻ってくる可能性」*1をコンパイラに伝える属性。コンパイラによる一部の最適化処理を抑止する。

The returns_twice attribute tells the compiler that a function may return more than one time. The compiler ensures that all registers are dead before calling such a function and emits a warning about the variables that may be clobbered after the second return from the function. Examples of such functions are setjmp and vfork. The longjmp-like counterpart of such function, if any, might need to be marked with the noreturn attribute.

https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html

returns_twice
This attribute indicates that this function can return twice. The C setjmp is an example of such a function. The compiler disables some optimizations (like tail calls) in the caller of these functions.

https://llvm.org/docs/LangRef.html

関連URL

• c - What sense do these clobbered variable warnings make? - Stack Overflow
• c++ - function attribute returns_twice - Stack Overflow
• c - What is the difference between assembler generated for functions with and without returns_twice and noreturn? - Stack Overflow

*1:関数から制御が戻ってこないことを表明するnoreturn属性と対をなす。