7 ms·

I don't believe the spec ever calls out what is exactly supposed to happen for multiplication overflow, and this was left up to the implementation to decide wha

I don't believe the spec ever calls out what is exactly supposed to happen for multiplication overflow, and this was left up to the implementation to decide what to do. That being said, libmusl implemented a short but sweet check [1]. Fun stuff. [1]: https://github.com/rofl0r/musl/blob/d05aaedaabd4f5472c233dbbd1ff4bb9c9c99794/src/malloc/calloc.c#L8-L11 https://github.com/rofl0r/musl/blob/d05aaedaabd4f5472c233dbb...

haberman 4y ago
Even more satisfying, I tried this out in godbolt and it appears that compilers are able to optimize that check into a single `mul, jo` (jump on overflow) sequence, which is how you would write it if you were writing assembly directly. It avoids a division instruction (which would be far more expensive) even though the code is written as a division: https://godbolt.org/z/r6cxo8vc7 https://godbolt.org/z/r6cxo8vc7
- sltkr 4y ago
  But less satisfying, it looks like clang optimizes out the entire zero-initialization logic, presumably because it assumes the expression ((size_t *)p)[-1] is undefined behavior. Makes me wonder what kind of compiler flags muslibc uses to prevent this.
  - rightbyte 4y ago
    That is not undefined right? As long as p - 1 points into a valid object. Edit: Maybe Clang abuses the bitand with an int 7 to remove the code?
    - Someone 4y ago
      It is undefined behavior. The undefined behavior rules for pointers boil down to (https://stackoverflow.com/questions/56360316/c-standard-regarding-pointer-arithmetic-outside-arrays https://stackoverflow.com/questions/56360316/c-standard-rega...) “Pointer arithmetic is well defined only on array type and between array[0] and array[array_size+1]” And it’s even stricter: - you can create a pointer to array[array_size+1], but dereferencing it invokes undefined behavior. - changing a pointer that points into an object so that it points into another object invokes undefined behavior. Modern compilers track “pointer provenance” to sometimes speed up code (and occasionally perplex the programmer)
      - twoodfin 4y ago
        Those rules apply to access to the underlying array, they’re not a constraint on syntax for array expressions. malloc just returns a pointer, which could be in the middle of an array if malloc were an ordinary user-defined function. It’s only special knowledge of the behavior of standard malloc that allows the compiler to assume that pointer is to offset 0 of an allocated array.
        wizeman 4y ago
        > malloc just returns a pointer, which could be in the middle of an array if malloc were an ordinary user-defined function. Yes, but even in that case, it would have to be an array of `size_t` objects for the pointer cast `(size_t *)` in the musl code not to trigger undefined behavior (especially if malloc would get inlined), right? Which means calloc() would trigger undefined behavior as soon as one of its callers used it for allocating a non-size_t array (especially if calloc() would get inlined with link-time optimization)?
        Dylan16807 4y ago
        You just need certain parts of the returned allocation to be castable to size_t, and there's no way for the compiler to prove that wrong if it doesn't know the internals of the allocator. The underlying allocation could be as big as you want, and can be composed of whatever type you like.
        wizeman 4y ago
        > You just need certain parts of the returned allocation to be castable to size_t, and there's no way for the compiler to prove that wrong if it doesn't know the internals of the allocator. The underlying allocation could be as big as you want, and can be composed of whatever type you like. Hmm, I don't think this is true. Even if you don't assume special semantics for malloc() and friends (by using the `-ffreestanding` and the `-fno-builtin` flags) and even if the compiler knows nothing about the malloc() implementation, I think if you're doing pointer arithmetic (with a pointer type other than `void *` or `[unsigned] char *`) then the compiler can assume that the pointer is pointing to either: 1) an actual array of objects of the underlying type (or a compatible one), or 2) a pointer to a single object, which is treated as an array with one element. So if you do `((size_t *) p)[-1]` within `calloc()`, then the compiler can assume that `*p`, if used, must either be a `size_t` (or a compatible integer) or `p` must be pointing to one past the end of a single-element array (which, since `*p` is a dereference, would trigger undefined behavior). Which means that if you call `calloc()` and it returns `p`, then you can only use it to dereference `size_t` objects or compatible ones (otherwise it would trigger undefined behavior). The effects of the undefined behavior would likely be observed if `calloc()` gets inlined into the caller (due to link-time optimization, for instance). Or am I wrong?
  - Sharlin 4y ago
    In the real world, musl would call its own `malloc` and can presumably trust that `((size_t *)p)[-1]` is valid and contains the block metadata (and the compiler can't assume it isn't valid), whereas the godbolt snippet calls libc's `malloc` which is a black box to outside callers. Or maybe LLVM can even deduce that the low bits are always zero on `libc`?
  - leoetlino 4y ago
    Tried -fno-builtin-malloc on a hunch and it does seem to prevent the zero-initialisation logic from getting optimised away: https://godbolt.org/z/xWbEqcenn https://godbolt.org/z/xWbEqcenn musl probably uses -fno-builtin to disable any special handling of "builtin" library functions like malloc and memcpy?
  - wizeman 4y ago
    > But less satisfying, it looks like clang optimizes out the entire zero-initialization logic, presumably because it assumes the expression ((size_t *)p)[-1] is undefined behavior. Yeah, I also noticed that is undefined behavior! > Makes me wonder what kind of compiler flags muslibc uses to prevent this. As far as I can see, the only related flags are `-ffreestanding`, `-fno-builtin` and `-fno-strict-aliasing` but I think none of these flags prevent the undefined behavior if 1) either the malloc() implementation gets inlined into calloc(), or 2) calloc() gets inlined into the caller, so that the compiler realizes that `p` is not actually an array of `size_t` or compatible integers. So perhaps musl also avoids undefined behavior by forcing `malloc()` and `calloc()` to not be inlined into the caller? Although I'm not entirely sure that actually completely prevents undefined behavior, I think it would just avoid it assuming how compilers usually/currently work? Edit: On further investigation, I think what I said above is wrong. I think `-ffreestanding` and `-fno-builtin` should be enough to prevent undefined behavior. I wasn't considering the case where `((size_t) *p)[-1]` points to a struct like this: struct alloc { size_t something; unsigned char buffer[]; // flexible array member } In this case, then I think the code is not triggering undefined behavior when used with those compiler flags (which prevent the compiler from using its knowledge about malloc's special semantics). Edit 2: On second thought, even then I think it might trigger undefined behavior because the C standard doesn't guarantee that there's no padding at the end of such a struct?
    - wizeman 4y ago
      So, I just checked and it seems that the parent pointed to a musl repository that contains old code. The current code doesn't contain that undefined behavior-triggering expression: https://git.musl-libc.org/cgit/musl/tree/src/malloc/calloc.c https://git.musl-libc.org/cgit/musl/tree/src/malloc/calloc.c
  - tpolzer 4y ago
    Try with -ffreestanding. Otherwise clang is allowed to make assumptions about the behavior of "malloc". It then does the typical unsatisfying clang thing of unrolling and vectorizing a _lot_ with unclear benefits: https://godbolt.org/z/eG33YM7qs https://godbolt.org/z/eG33YM7qs
noncoml 4y ago
> if (n && m > (size_t)-1/n) Am I the only one that doesn’t think this is “sweet”? I mean I know what it does and why and how it works, but IMHO a good language should let you express your intention and algorithm in human terms and logic in which there is no -1 value for an unsigned integer type.
- haberman 4y ago
  Agreed that it would be better if C had standardized versions of GCC's overflow checking functions: https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Integer-Overflow-Builtins.html https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Integer-Overflo... With these functions, it could be written as: if (__builtin_mul_overflow(n, m, &n))
  - saagarjha 4y ago
    C23 has ckd_mul.
    - jwilk 4y ago
      https://thephd.dev/c-the-improvements-june-september-virtual-c-meeting#n2683---towards-integer-safety https://thephd.dev/c-the-improvements-june-september-virtual...
  - raverbashing 4y ago
    True. It seems the C committee at some point makes things harder than they should. And true, the C committee is not preventing anyone from using something else (Another reason why we should retire C for most projects)
    - klodolph 4y ago
      > It seems the C committee at some point makes things harder than they should This seems like entirely the wrong way of looking at it to me. The C committee is not trying to make things harder than they should be, but they’re really just a committee in charge of C. Their goal isn’t to make C into some kind of beautiful safe language, it’s just to provide incremental improvements to C for the benefit of people stuck using it for whatever reason. Like, the C committee isn’t trying to stop you from retiring C. They’re just trying to help out the people who aren’t retiring C, for whatever reason—which could be toolchain availability (lots of architectures out there, you may only have a C compiler for some), could be so they can work with a legacy codebase, could be that some other tooling or process they have works with C (the various “safe C” / analyzable subsets or formal verification processes).
ainar-g 4y ago
The current C2x (aka C23) draft finally spells the behaviour out explicitly: > 7.24.3.2 The calloc function > […] > Returns > The calloc function returns either a pointer to the allocated space or a null pointer if the space cannot be allocated or if the product nmemb * size would wraparound size_t. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf
- kelnos 4y ago
  > if the product nmemb * size would wraparound size_t. Gah! "Wraparound" is not a verb! You'd think someone writing a computer language specification would try to use (human) language correctly.
  - jwilk 4y ago
    FWIW, it's a technical term defined as: > the process by which a value is reduced modulo 2ᴺ, where N is the width of the resulting type
    - Dylan16807 4y ago
      That's not what the objection is. Your quote is clearly defining a noun.
  - someweirdperson 4y ago
    If it is like overflow, it should indeed be "aroundwrap".
  - Someone 4y ago
    Verbing weirds language (https://www.gocomics.com/calvinandhobbes/1993/01/25 https://www.gocomics.com/calvinandhobbes/1993/01/25), but in English, you can verb (https://en.wikipedia.org/wiki/Conversion_(word_formation)#Verbing https://en.wikipedia.org/wiki/Conversion_(word_formation)#Ve...) a lot of phrases.
  - gnubison 4y ago
    Prescriptivism much? :p
bornfreddy 4y ago
From my (admittedly very limited, and very dated) knowledge of x86 asm, I remember that multiplication sets some flags if there was an overflow. Is there no way to multiply in C so that such an overflow was detected? Or do some CPU architectures not support such flags? It seems inefficient (to me) to do this check before each multiplication.
- SAI_Peregrinus 4y ago
  The C abstract machine does not have such flags. C therefore does not have any support for checking them (from code, the compiler can add checks if the target supports them but doesn't have to).
deleted 4y ago
[deleted]

haberman 4y ago

Even more satisfying, I tried this out in godbolt and it appears that compilers are able to optimize that check into a single `mul, jo` (jump on overflow) sequence, which is how you would write it if you were writing assembly directly. It avoids a division instruction (which would be far more expensive) even though the code is written as a division: https://godbolt.org/z/r6cxo8vc7 https://godbolt.org/z/r6cxo8vc7

sltkr 4y ago
But less satisfying, it looks like clang optimizes out the entire zero-initialization logic, presumably because it assumes the expression ((size_t *)p)[-1] is undefined behavior. Makes me wonder what kind of compiler flags muslibc uses to prevent this.
- rightbyte 4y ago
  That is not undefined right? As long as p - 1 points into a valid object. Edit: Maybe Clang abuses the bitand with an int 7 to remove the code?
  - Someone 4y ago
    It is undefined behavior. The undefined behavior rules for pointers boil down to (https://stackoverflow.com/questions/56360316/c-standard-regarding-pointer-arithmetic-outside-arrays https://stackoverflow.com/questions/56360316/c-standard-rega...) “Pointer arithmetic is well defined only on array type and between array[0] and array[array_size+1]” And it’s even stricter: - you can create a pointer to array[array_size+1], but dereferencing it invokes undefined behavior. - changing a pointer that points into an object so that it points into another object invokes undefined behavior. Modern compilers track “pointer provenance” to sometimes speed up code (and occasionally perplex the programmer)
    - twoodfin 4y ago
      Those rules apply to access to the underlying array, they’re not a constraint on syntax for array expressions. malloc just returns a pointer, which could be in the middle of an array if malloc were an ordinary user-defined function. It’s only special knowledge of the behavior of standard malloc that allows the compiler to assume that pointer is to offset 0 of an allocated array.
      - wizeman 4y ago
        > malloc just returns a pointer, which could be in the middle of an array if malloc were an ordinary user-defined function. Yes, but even in that case, it would have to be an array of `size_t` objects for the pointer cast `(size_t *)` in the musl code not to trigger undefined behavior (especially if malloc would get inlined), right? Which means calloc() would trigger undefined behavior as soon as one of its callers used it for allocating a non-size_t array (especially if calloc() would get inlined with link-time optimization)?
        Dylan16807 4y ago
        You just need certain parts of the returned allocation to be castable to size_t, and there's no way for the compiler to prove that wrong if it doesn't know the internals of the allocator. The underlying allocation could be as big as you want, and can be composed of whatever type you like.
        wizeman 4y ago
        > You just need certain parts of the returned allocation to be castable to size_t, and there's no way for the compiler to prove that wrong if it doesn't know the internals of the allocator. The underlying allocation could be as big as you want, and can be composed of whatever type you like. Hmm, I don't think this is true. Even if you don't assume special semantics for malloc() and friends (by using the `-ffreestanding` and the `-fno-builtin` flags) and even if the compiler knows nothing about the malloc() implementation, I think if you're doing pointer arithmetic (with a pointer type other than `void *` or `[unsigned] char *`) then the compiler can assume that the pointer is pointing to either: 1) an actual array of objects of the underlying type (or a compatible one), or 2) a pointer to a single object, which is treated as an array with one element. So if you do `((size_t *) p)[-1]` within `calloc()`, then the compiler can assume that `*p`, if used, must either be a `size_t` (or a compatible integer) or `p` must be pointing to one past the end of a single-element array (which, since `*p` is a dereference, would trigger undefined behavior). Which means that if you call `calloc()` and it returns `p`, then you can only use it to dereference `size_t` objects or compatible ones (otherwise it would trigger undefined behavior). The effects of the undefined behavior would likely be observed if `calloc()` gets inlined into the caller (due to link-time optimization, for instance). Or am I wrong?
- Sharlin 4y ago
  In the real world, musl would call its own `malloc` and can presumably trust that `((size_t *)p)[-1]` is valid and contains the block metadata (and the compiler can't assume it isn't valid), whereas the godbolt snippet calls libc's `malloc` which is a black box to outside callers. Or maybe LLVM can even deduce that the low bits are always zero on `libc`?
- leoetlino 4y ago
  Tried -fno-builtin-malloc on a hunch and it does seem to prevent the zero-initialisation logic from getting optimised away: https://godbolt.org/z/xWbEqcenn https://godbolt.org/z/xWbEqcenn musl probably uses -fno-builtin to disable any special handling of "builtin" library functions like malloc and memcpy?
- wizeman 4y ago
  > But less satisfying, it looks like clang optimizes out the entire zero-initialization logic, presumably because it assumes the expression ((size_t *)p)[-1] is undefined behavior. Yeah, I also noticed that is undefined behavior! > Makes me wonder what kind of compiler flags muslibc uses to prevent this. As far as I can see, the only related flags are `-ffreestanding`, `-fno-builtin` and `-fno-strict-aliasing` but I think none of these flags prevent the undefined behavior if 1) either the malloc() implementation gets inlined into calloc(), or 2) calloc() gets inlined into the caller, so that the compiler realizes that `p` is not actually an array of `size_t` or compatible integers. So perhaps musl also avoids undefined behavior by forcing `malloc()` and `calloc()` to not be inlined into the caller? Although I'm not entirely sure that actually completely prevents undefined behavior, I think it would just avoid it assuming how compilers usually/currently work? Edit: On further investigation, I think what I said above is wrong. I think `-ffreestanding` and `-fno-builtin` should be enough to prevent undefined behavior. I wasn't considering the case where `((size_t) *p)[-1]` points to a struct like this: struct alloc { size_t something; unsigned char buffer[]; // flexible array member } In this case, then I think the code is not triggering undefined behavior when used with those compiler flags (which prevent the compiler from using its knowledge about malloc's special semantics). Edit 2: On second thought, even then I think it might trigger undefined behavior because the C standard doesn't guarantee that there's no padding at the end of such a struct?
  - wizeman 4y ago
    So, I just checked and it seems that the parent pointed to a musl repository that contains old code. The current code doesn't contain that undefined behavior-triggering expression: https://git.musl-libc.org/cgit/musl/tree/src/malloc/calloc.c https://git.musl-libc.org/cgit/musl/tree/src/malloc/calloc.c
- tpolzer 4y ago
  Try with -ffreestanding. Otherwise clang is allowed to make assumptions about the behavior of "malloc". It then does the typical unsatisfying clang thing of unrolling and vectorizing a _lot_ with unclear benefits: https://godbolt.org/z/eG33YM7qs https://godbolt.org/z/eG33YM7qs

noncoml 4y ago

> if (n && m > (size_t)-1/n) Am I the only one that doesn’t think this is “sweet”? I mean I know what it does and why and how it works, but IMHO a good language should let you express your intention and algorithm in human terms and logic in which there is no -1 value for an unsigned integer type.

haberman 4y ago
Agreed that it would be better if C had standardized versions of GCC's overflow checking functions: https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Integer-Overflow-Builtins.html https://gcc.gnu.org/onlinedocs/gcc-9.1.0/gcc/Integer-Overflo... With these functions, it could be written as: if (__builtin_mul_overflow(n, m, &n))
- saagarjha 4y ago
  C23 has ckd_mul.
  - jwilk 4y ago
    https://thephd.dev/c-the-improvements-june-september-virtual-c-meeting#n2683---towards-integer-safety https://thephd.dev/c-the-improvements-june-september-virtual...
- raverbashing 4y ago
  True. It seems the C committee at some point makes things harder than they should. And true, the C committee is not preventing anyone from using something else (Another reason why we should retire C for most projects)
  - klodolph 4y ago
    > It seems the C committee at some point makes things harder than they should This seems like entirely the wrong way of looking at it to me. The C committee is not trying to make things harder than they should be, but they’re really just a committee in charge of C. Their goal isn’t to make C into some kind of beautiful safe language, it’s just to provide incremental improvements to C for the benefit of people stuck using it for whatever reason. Like, the C committee isn’t trying to stop you from retiring C. They’re just trying to help out the people who aren’t retiring C, for whatever reason—which could be toolchain availability (lots of architectures out there, you may only have a C compiler for some), could be so they can work with a legacy codebase, could be that some other tooling or process they have works with C (the various “safe C” / analyzable subsets or formal verification processes).

ainar-g 4y ago

The current C2x (aka C23) draft finally spells the behaviour out explicitly: > 7.24.3.2 The calloc function > […] > Returns > The calloc function returns either a pointer to the allocated space or a null pointer if the space cannot be allocated or if the product nmemb * size would wraparound size_t. https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf

kelnos 4y ago
> if the product nmemb * size would wraparound size_t. Gah! "Wraparound" is not a verb! You'd think someone writing a computer language specification would try to use (human) language correctly.
- jwilk 4y ago
  FWIW, it's a technical term defined as: > the process by which a value is reduced modulo 2ᴺ, where N is the width of the resulting type
  - Dylan16807 4y ago
    That's not what the objection is. Your quote is clearly defining a noun.
- someweirdperson 4y ago
  If it is like overflow, it should indeed be "aroundwrap".
- Someone 4y ago
  Verbing weirds language (https://www.gocomics.com/calvinandhobbes/1993/01/25 https://www.gocomics.com/calvinandhobbes/1993/01/25), but in English, you can verb (https://en.wikipedia.org/wiki/Conversion_(word_formation)#Verbing https://en.wikipedia.org/wiki/Conversion_(word_formation)#Ve...) a lot of phrases.
- gnubison 4y ago
  Prescriptivism much? :p

bornfreddy 4y ago

From my (admittedly very limited, and very dated) knowledge of x86 asm, I remember that multiplication sets some flags if there was an overflow. Is there no way to multiply in C so that such an overflow was detected? Or do some CPU architectures not support such flags? It seems inefficient (to me) to do this check before each multiplication.

SAI_Peregrinus 4y ago
The C abstract machine does not have such flags. C therefore does not have any support for checking them (from code, the compiler can add checks if the target supports them but doesn't have to).

deleted 4y ago

[deleted]