https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65270
--- Comment #21 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 4 Mar 2015, hubicka at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65270 > > --- Comment #16 from Jan Hubicka <hubicka at gcc dot gnu.org> --- > Richard, > thanks, I also think alias trick makes gloal vars safe for merging across > RESTRICT flags. > > One however needs to consider merging of items referring restricted vars. > > const restrict int *a=&var; > const int *b = &var; > > const int **ptrs1={&a}; > const int **ptrs2=[&b}; > > with -fmerge-all-constants we may merge ptrs1 and ptrs2 and, in the late > compilation, in turn fold expression "ptrs2[0]" into a restricted pointer to > var? So we merge a and b with introducing an alias which is why we can merge ptrs1 and ptrs2, correct? But still with introducing an alias. But folding ptrs1[0] and ptrs2[0] will now return the same (but random?) value. Note that it's not folding that can introduce issues but points-to analysis and what it computes for the globals ptrs1 and ptrs2 and thus for code that reads from them. We are not really parsing constructors fully in PTA - at least I see ptrs1 = NONLOCAL ptrs1.0_2 = ptrs1 _3 = *ptrs1.0_2 _4 = *_3 only for int var; const int * restrict a=&var; const int *b = &var; const int * const *ptrs1={&a}; const int * const *ptrs2={&b}; int main() { return *(ptrs1[0]); } IPA PTA does sth funny though: ptrs2 = NONLOCAL b = NONLOCAL var = NONLOCAL b = &var ESCAPED = &var ptrs2 = &b ESCAPED = &b ptrs1 = NONLOCAL a = &GLOBAL_RESTRICT GLOBAL_RESTRICT = NONLOCAL ptrs1 = &a ESCAPED = &a but obviously we don't seem to merge ptrs1/ptrs2. But IPA PTA needs quite some thoughts with respect to aliases I think (and in other ways as well...). > If this case is legit, the correct place to match RESTRICT flags is > compare_cgraph_references. We can also go with your patch that will make A and > B considered to be different and thus prevent merging PTRS1&PTRS2. That would certainly be a safe thing to do. Even with -flto -fmerge-all-constants we don't get ptrs1 and ptrs2 merged it seems (with -fwhole-program we fold stuff too early). Richard.
