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   John Regehr (regehr@mastodon.social)'s status on Saturday, 06-Jul-2024 07:38:44 JST John Regehr

   why are we assuming malloc can't fail?

   https://gcc.godbolt.org/z/d4rs4hfGE

   • Embed this notice
     Rich Felker (dalias@hachyderm.io)'s status on Saturday, 06-Jul-2024 07:38:41 JST Rich Felker

     in reply to

     • Josh Simmons
     • Per Vognsen

     @dotstdy @pervognsen @regehr You presume I don't already have one...

     Alexandre Oliva likes this.
   • Embed this notice
     Rich Felker (dalias@hachyderm.io)'s status on Saturday, 06-Jul-2024 07:38:42 JST Rich Felker

     in reply to

     • Josh Simmons
     • Per Vognsen

     @dotstdy @pervognsen @regehr Yes, that's a bug.

   • Embed this notice
     Josh Simmons (dotstdy@mastodon.social)'s status on Saturday, 06-Jul-2024 07:38:42 JST Josh Simmons

     in reply to

     • Rich Felker
     • Per Vognsen

     @dalias @pervognsen @regehr i think you might need to start a small war with clang, gcc and msvc developers if you want to argue that line of thought :')

   • Embed this notice
     Rich Felker (dalias@hachyderm.io)'s status on Saturday, 06-Jul-2024 07:38:43 JST Rich Felker

     in reply to

     • Per Vognsen

     @pervognsen @regehr Only because you dereferenced it without checking the return value, so the case where it failed has UB and can be assumed not to happen.

     For small n where the "as-if" allocation can surely happen on the stack, you could also assume it doesn't fail *if* you make the transformation not to really call malloc. But in general, you can't do that.

   • Embed this notice
     Josh Simmons (dotstdy@mastodon.social)'s status on Saturday, 06-Jul-2024 07:38:43 JST Josh Simmons

     in reply to

     • Rich Felker
     • Per Vognsen

     @dalias @pervognsen @regehr even if you check the return value it can elide it (it's also irrelevant how large the allocation is, you can bloat it to allocate an extremely large amount of memory and it'll still elide the whole thing) https://gcc.godbolt.org/z/doEfnac61

   • Embed this notice
     Per Vognsen (pervognsen@mastodon.social)'s status on Saturday, 06-Jul-2024 07:38:44 JST Per Vognsen

     in reply to

     @regehr This must be related to the fact that malloc can be optimized away, right? If I write something like

     int f(void) {
     int *p = malloc(sizeof(int));
     *p = 42;
     int i = *p;
     free(p);
     return i;
     }

     then it's valid to optimize it to

     int f(void) {
     return 42;
     }