4 ms·

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

by 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?
      - shakna 9mo ago
        Page 185 - 189.
        pklausler 9mo 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
Your problem is that you're treating words such as "function" and "call" as if they had meaning outside of the language itself (or, more specifically, outside of the C abstract machine), when the point of the compiler is precisely to melt away the language parts of the specified program and be left with a concrete program that matches its behavior. If you view a binary in a disassembler, you will not find any "functions" or "calls". Maybe that particular architecture happens to have a "call" instruction to jump to "functions", but these words are merely homophones with what C refers to as "functions" and "calls". When you "call" a "function" in the source you're not specifying to the compiler that you want a specific opcode in the generated executable, you're merely specifying a particular observable behavior. This is why optimizations such as inlining and TCO are valid. If the compiler can prove that a heap allocation can be turned into a stack allocation, or even removed altogether (e.g. free(malloc(1ULL << 50))), the fact that these are exposed to the programmer as "functions" he can "call" poses no obstacle.
- 112233 10mo ago
  Closest to what you say that I can find is 5.1.2.3 §4 of N3096 In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or through volatile access to an object) Problem is, calling external library function has a needed side effect of calling that library function. I do not see language that allows simply not doing that, based on assumed but unknown function behaviour.
  - uecker 10mo ago
    You should read "7.1.4 1 Use of library functions". Also "calling a function" is not a side effect.
    - 112233 10mo ago
      Thanks, I did read it! Things like footnote 236: "This means that an implementation is required to provide an actual function for each library function, even if it also provides a macro for that function", where macro is shown to use compiler builtin as an example. Again, could you please explain how compiler can decide to remove call to a function in an external dynamically loaded library, that is not known at compile time, simply based on the name of the function (i.e. not because the call is unreachable)? I do not see any such language in the standard. And yes, calling unknown function from a dynamically loaded library totally is a side effect.

by 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?
    - shakna 9mo ago
      Page 185 - 189.
      - pklausler 9mo 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

Your problem is that you're treating words such as "function" and "call" as if they had meaning outside of the language itself (or, more specifically, outside of the C abstract machine), when the point of the compiler is precisely to melt away the language parts of the specified program and be left with a concrete program that matches its behavior. If you view a binary in a disassembler, you will not find any "functions" or "calls". Maybe that particular architecture happens to have a "call" instruction to jump to "functions", but these words are merely homophones with what C refers to as "functions" and "calls". When you "call" a "function" in the source you're not specifying to the compiler that you want a specific opcode in the generated executable, you're merely specifying a particular observable behavior. This is why optimizations such as inlining and TCO are valid. If the compiler can prove that a heap allocation can be turned into a stack allocation, or even removed altogether (e.g. free(malloc(1ULL << 50))), the fact that these are exposed to the programmer as "functions" he can "call" poses no obstacle.

112233 10mo ago
Closest to what you say that I can find is 5.1.2.3 §4 of N3096 In the abstract machine, all expressions are evaluated as specified by the semantics. An actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no needed side effects are produced (including any caused by calling a function or through volatile access to an object) Problem is, calling external library function has a needed side effect of calling that library function. I do not see language that allows simply not doing that, based on assumed but unknown function behaviour.
- uecker 10mo ago
  You should read "7.1.4 1 Use of library functions". Also "calling a function" is not a side effect.
  - 112233 10mo ago
    Thanks, I did read it! Things like footnote 236: "This means that an implementation is required to provide an actual function for each library function, even if it also provides a macro for that function", where macro is shown to use compiler builtin as an example. Again, could you please explain how compiler can decide to remove call to a function in an external dynamically loaded library, that is not known at compile time, simply based on the name of the function (i.e. not because the call is unreachable)? I do not see any such language in the standard. And yes, calling unknown function from a dynamically loaded library totally is a side effect.