Comment by shakna
16 hours ago
If the function is equivalent to a no-op, and not explicitly marked as volatile for side-effects, it absolutely can elide it. If there is a side-effect in hardware or wider systems like the OS, then it must be marked as volatile. If the code is just code, then a function call that does effectively nothing, will probably become nothing.
That was one of the first optimisations we had, back with Fortran and COBOL. Before C existed - and as B started life as a stripped down Fortran compiler, the history carried through.
The K&R book describes the buddy system for malloc, and how its design makes it suitable for compiler optimisations - including ignoring a write to a pointer that does nothing, because the pointer will no longer be valid.
Where exactly does K&R specify the buddy system?
You are literally scaring me now. I'd understand such things being done when statically linking or running JIT, but for "normal" program which function implementation malloc() will link against is not known during compilation. How can compiler go, like, "eh, I'll assume free(malloc(x)) is NOP and drop it" and not break most existing code?
> but for "normal" program which function implementation malloc() will link against is not known during compilation. How can compiler go, like, "eh, I'll assume free(malloc(x)) is NOP and drop it" and not break most existing code?
I'd suspect that eliding suitable malloc/free pairs would not break most existing code because most existing code simply does not depend on malloc/free doing anything other than and/or beyond what the C standard requires.
How would you propose that eliding free(malloc(x)) would break "most" existing code, anyways?