3 ms·

Optimizing out a write to (example) an array on the stack seems fine to me. Optimizing out a function call to a heap pointer (especially memset) seems wrong to

Optimizing out a write to (example) an array on the stack seems fine to me. Optimizing out a function call to a heap pointer (especially memset) seems wrong to me. You called the function, it should call the function! But it's again the C language saving time not wearing a seatbelt or checking the tire pressure for saving 10s on a 2h trip

fluoridation 10mo ago
The whole point of the optimizer is that it can detect inefficiencies by treating every statement as some combination of simple, fundamental operations. The compiler is not seeing "call memset() on pointer to heap", it's seeing "write of variable size" just before "deallocation". For some, optimizing that will be a problem, for others, not optimizing it will leave performance on the table. There are still ways to obtain the desired behavior. Just put a call to a DLL or SO that implements what you need. The compiler cannot inspect the behavior of functions across module boundaries, so it cannot tell whether removing the call preserves semantics or not (for example, it could be that the external function sends the contents of the buffer to a file), so it will not remove it.
- uecker 10mo ago
  A modern compiler may also completely remove malloc / free pairs and move the computation to the stack. And I do not see what this has to do with C, it should be the same for most languages. C gives you tools to express low-level intent such as "volatile", but one has to use them.
  - 112233 10mo ago
    Strong disagree. In C, malloc and free are functions, and I expect no magic to happen when calling a function. If malloc and free were keywords like sizeof, it would have been different.
    - shakna 10mo ago
      In C, malloc and free used to often be macros. As they're freely replaceable through loading, and designed for that, I would strongly suggest that are among the most magical areas of the C standard. We get a whole section for those in the standard: 7.24.3 Memory management functions Hell, malloc is allowed to return you _less than you asked for_: > The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and size less than or equal to the size requested
      - 112233 10mo ago
        I read the text as saying the object size can be less or equal to returned memory size. Anyway, section 7 is library. As you say, replacing through loading is a common thing to do — surely compiler is not free to simply elide external library function at will? This is not C++ after all, it must be sensible
        shakna 10mo 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.
        pklausler 10mo ago
        Where exactly does K&R specify the buddy system?
        10mo ago

fluoridation 10mo ago

The whole point of the optimizer is that it can detect inefficiencies by treating every statement as some combination of simple, fundamental operations. The compiler is not seeing "call memset() on pointer to heap", it's seeing "write of variable size" just before "deallocation". For some, optimizing that will be a problem, for others, not optimizing it will leave performance on the table. There are still ways to obtain the desired behavior. Just put a call to a DLL or SO that implements what you need. The compiler cannot inspect the behavior of functions across module boundaries, so it cannot tell whether removing the call preserves semantics or not (for example, it could be that the external function sends the contents of the buffer to a file), so it will not remove it.

uecker 10mo ago
A modern compiler may also completely remove malloc / free pairs and move the computation to the stack. And I do not see what this has to do with C, it should be the same for most languages. C gives you tools to express low-level intent such as "volatile", but one has to use them.
- 112233 10mo ago
  Strong disagree. In C, malloc and free are functions, and I expect no magic to happen when calling a function. If malloc and free were keywords like sizeof, it would have been different.
  - shakna 10mo ago
    In C, malloc and free used to often be macros. As they're freely replaceable through loading, and designed for that, I would strongly suggest that are among the most magical areas of the C standard. We get a whole section for those in the standard: 7.24.3 Memory management functions Hell, malloc is allowed to return you _less than you asked for_: > The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object with a fundamental alignment requirement and size less than or equal to the size requested
    - 112233 10mo ago
      I read the text as saying the object size can be less or equal to returned memory size. Anyway, section 7 is library. As you say, replacing through loading is a common thing to do — surely compiler is not free to simply elide external library function at will? This is not C++ after all, it must be sensible
      - shakna 10mo 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.
        pklausler 10mo ago
        Where exactly does K&R specify the buddy system?
        10mo ago