Messages in this thread | | | Date | Wed, 30 Mar 2005 20:14:32 +0100 | From | Paulo Marques <> | Subject | Re: Do not misuse Coverity please |
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Shankar Unni wrote: > Jean Delvare wrote: > >> v = p->field; >> if (!p) return; >> >> can be seen as equivalent to >> >> if (!p) return; >> v = p->field; > > > Heck, no. > > You're missing the side-effect of a null pointer dereference crash (for > p->field) (even though v is unused before the return). The optimizer is > not allowed to make exceptions go away as a result of the hoisting.
I just had to try this out :)
Using gcc 3.3.2 this code sample:
> struct test { > int code; > }; > > int test_func(struct test *a) > { > int ret; > if (!a) return -1; > ret = a->code; > return ret; > }
is compiled into:
> 0: 8b 54 24 04 mov 0x4(%esp,1),%edx > 4: 83 c8 ff or $0xffffffff,%eax > 7: 85 d2 test %edx,%edx > 9: 74 02 je d <test_func+0xd> > b: 8b 02 mov (%edx),%eax > d: c3 ret
whereas this one:
> int test_func(struct test *a) > { > int ret; > ret = a->code; > if (!a) return -1; > return ret; > }
is simply compiled into:
> 0: 8b 44 24 04 mov 0x4(%esp,1),%eax > 4: 8b 00 mov (%eax),%eax > 6: c3 ret
It seems that gcc is smart enough to know that after we've dereferenced a pointer, if it was NULL, it doesn't matter any more. So it just assumes that if execution reaches that "if" statement then the pointer can not be NULL at all.
So the 2 versions aren't equivalent, and gcc doesn't treat them as such either.
Just a minor nitpick, though: wouldn't it be possible for an application to catch the SIGSEGV and let the code proceed, making invalid the assumption made by gcc?
-- Paulo Marques - www.grupopie.com
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