------- Comment #7 from rguenther at suse dot de 2010-02-26 19:56 ------- Subject: Re: [4.5 regression] miscompilation of ACATS c37105a at -O2
On Fri, 26 Feb 2010, ebotcazou at gcc dot gnu dot org wrote: > > > ------- Comment #6 from ebotcazou at gcc dot gnu dot org 2010-02-26 17:46 > ------- > > We might be able to save the day with the help of TYPE_CANONICAL in this > > case > > since the size is fixed. > > TYPE_CANONICAL is too strong, it will cause useless_type_conversion_p to > return > true for conversions between type and subtypes and, even with fixed size, > these > conversions aren't useless. So we need this for Ada: > > Index: tree-ssa-alias.c > =================================================================== > --- tree-ssa-alias.c (revision 156989) > +++ tree-ssa-alias.c (working copy) > @@ -544,13 +544,15 @@ same_type_for_tbaa (tree type1, tree typ > && TREE_CODE (type2) == ARRAY_TYPE) > return -1; > > - /* In Ada, an lvalue of unconstrained type can be used to access an object > - of one of its constrained subtypes, for example when a function with an > - unconstrained parameter passed by reference is called on a constrained > - object and inlined. In this case, the types have the same alias set. > */ > - if (TYPE_SIZE (type1) && TYPE_SIZE (type2) > - && TREE_CONSTANT (TYPE_SIZE (type1)) != TREE_CONSTANT (TYPE_SIZE > (type2)) > - && get_alias_set (type1) == get_alias_set (type2)) > + /* ??? In Ada, an lvalue of an unconstrained type can be used to access an > + object of one of its constrained subtypes, e.g. when a function with an > + unconstrained parameter passed by reference is called on an object and > + inlined. But, even in the case of a fixed size, type and subtypes are > + not equivalent enough as to share the same TYPE_CANONICAL, since this > + would mean that conversions between them are useless, whereas they are > + not (e.g. type and subtypes can have different modes). So, in the end, > + they are only guaranteed to have the same alias set. */ > + if (get_alias_set (type1) == get_alias_set (type2)) > return -1; > > /* The types are known to be not equal. */ > > > The change doesn't introduce any regressions in the C/C++ testsuite on x86 so > it might not be a big pessimization. > > Richard, what do you think? Looks reasonable. Richard. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43096
