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Matthew Garrett (mjg59@nondeterministic.computer)'s status on Thursday, 12-Dec-2024 07:01:52 JST Matthew Garrett

Today's discovery: gcc will remove calls to memcpy() with a len of 0 (good), knows it's UB to call memcpy() with a dest of NULL (spec compliant), will assume that dest can't be NULL (fucking compilers, man), will combine both to engage in this UTTERLY PATHOLOGICAL BEHAVIOUR https://godbolt.org/z/bGqM6arGT
In conversation about 5 months ago from nondeterministic.computer permalink
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  Compiler Explorer - C++ (x86-64 gcc 14.2)
  
  from Matt Godbolt
  
  int do_thing1(); int do_thing2(); int test(char *dest, const char *src, size_t len) { memcpy(dest, src, len); if (dest == NULL) { // This branch will be removed by GCC due to undefined behavior. return do_thing1(); } else { return do_thing2(); } } int main() { // Will not call memcpy() // (because len is 0), will call // do_thing2() (because memcpy with // a dest of NULL is UB so simply // don't) return test(NULL, NULL, 0); }
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  waffle (wffl@im-in.space)'s status on Thursday, 12-Dec-2024 07:02:31 JST waffle
  in reply to
  
  @mjg59 I don't think this is pathological or gcc's fault. this is natural UB => anything can happen.
  the culprit here is the standard that says memcpy(NULL, NULL, 0) is UB IMO. (and as https://tech.lgbt/@pierogiburo/113634964183471754 suggests, this is being fixed :3)
  In conversation about 5 months ago permalink
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    Evvy :neofox_floof: (@pierogiburo@tech.lgbt)
    
    from Evvy :neofox_floof:
    
    @mjg59@nondeterministic.computer good news? https://developers.redhat.com/articles/2024/12/11/making-memcpynull-null-0-well-defined

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