3 ms·

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 th

by 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?
    - shakna 10mo ago
      Page 185 - 189.
      - pklausler 10mo ago
        Those pages in K&R define primary expressions through shift expressions.
  - 112233 10mo ago
    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?
    - aw1621107 10mo ago
      > 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?
      - 112233 10mo ago
        As an example, user kentonv wrote: "I patched the memory allocator used by the Cloudflare Workers runtime to overwrite all memory with a static byte pattern on free". And compiler would, like, "nah, let's leave all that data on stack". Or somebody would try to plug in mimalloc/jemalloc or a debug allocator and wonder what's going on.
        fluoridation 10mo ago
        >As an example, user kentonv wrote: "I patched the memory allocator used by the Cloudflare Workers runtime to overwrite all memory with a static byte pattern on free". And compiler would, like, "nah, let's leave all that data on stack". Such a program would continue to function as normal; the dirty data would just be left on the stack. If the developer wants to clear that data too, they'd just have to modify the compiler to overwrite the stack just before (or just after) moving the stack pointer. >Or somebody would try to plug in mimalloc/jemalloc or a debug allocator and wonder what's going on. Again, that wouldn't be broken. They would see that no dynamic allocations were performed during that particular section. Which would be correct.

by shakna 10mo ago

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?
  - shakna 10mo ago
    Page 185 - 189.
    - pklausler 10mo ago
      Those pages in K&R define primary expressions through shift expressions.
- 112233 10mo ago
  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?
  - aw1621107 10mo ago
    > 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?
    - 112233 10mo ago
      As an example, user kentonv wrote: "I patched the memory allocator used by the Cloudflare Workers runtime to overwrite all memory with a static byte pattern on free". And compiler would, like, "nah, let's leave all that data on stack". Or somebody would try to plug in mimalloc/jemalloc or a debug allocator and wonder what's going on.
      - fluoridation 10mo ago
        >As an example, user kentonv wrote: "I patched the memory allocator used by the Cloudflare Workers runtime to overwrite all memory with a static byte pattern on free". And compiler would, like, "nah, let's leave all that data on stack". Such a program would continue to function as normal; the dirty data would just be left on the stack. If the developer wants to clear that data too, they'd just have to modify the compiler to overwrite the stack just before (or just after) moving the stack pointer. >Or somebody would try to plug in mimalloc/jemalloc or a debug allocator and wonder what's going on. Again, that wouldn't be broken. They would see that no dynamic allocations were performed during that particular section. Which would be correct.