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The whole point of the optimizer is that it can detect inefficiencies by treating every statement as some combination of simple, fundamental operations. The com

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?
        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?
  - 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.

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?
        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?
- 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.