On Mon, Mar 20, 2023 at 8:47 PM Joseph Myers <jos...@codesourcery.com> wrote: > > I've observed an LTO wrong-code bug with a large testcase in GCC 12, > that results from TYPE_TYPELESS_STORAGE not being set consistently on > type variants. > > Specifically, in the LTO stage of compilation, there is an aggregate > type passed to get_alias_set, whose TYPE_MAIN_VARIANT does not have > TYPE_TYPELESS_STORAGE set. However, the TYPE_CANONICAL of that main > variant *does* have have TYPE_TYPELESS_STORAGE set; note that the use > of TYPE_CANONICAL in get_alias_set comes after the check of > TYPE_TYPELESS_STORAGE. The effect is that when (one-argument) > record_component_aliases is called, the recursive call to > get_alias_set gives alias set 0, and the aggregate type ends up not > being considered to alias its members, with wrong-code consequences. > > I haven't managed to produce a self-contained executable testcase to > demonstrate this, but it clearly seems appropriate for > TYPE_TYPELESS_STORAGE to be consistent on type variants, so this patch > makes it so, which appears to be sufficient to resolve the bug. I've > attached a reduced test that does at least demonstrate main-variant > versions of a type (SB in this test) being written out to LTO IR both > with and without TYPE_TYPELESS_STORAGE, although not the subsequent > consequences of a type without TYPE_TYPELESS_STORAGE with a > TYPE_CANONICAL (as constructed after LTO type merging) with > TYPE_TYPELESS_STORAGE and following wrong-code. > > Bootstrapped with no regressions for x86_64-pc-linux-gnu. OK to commit?
OK for trunk and branches. Thanks, Richard. > * stor-layout.cc (finalize_type_size): Copy TYPE_TYPELESS_STORAGE > to variants. > > diff --git a/gcc/stor-layout.cc b/gcc/stor-layout.cc > index 45bf2d18639..023de8c37db 100644 > --- a/gcc/stor-layout.cc > +++ b/gcc/stor-layout.cc > @@ -1996,6 +1996,7 @@ finalize_type_size (tree type) > unsigned int user_align = TYPE_USER_ALIGN (type); > machine_mode mode = TYPE_MODE (type); > bool empty_p = TYPE_EMPTY_P (type); > + bool typeless = AGGREGATE_TYPE_P (type) && TYPE_TYPELESS_STORAGE > (type); > > /* Copy it into all variants. */ > for (variant = TYPE_MAIN_VARIANT (type); > @@ -2020,6 +2021,8 @@ finalize_type_size (tree type) > TYPE_PRECISION (variant) = precision; > SET_TYPE_MODE (variant, mode); > TYPE_EMPTY_P (variant) = empty_p; > + if (AGGREGATE_TYPE_P (variant)) > + TYPE_TYPELESS_STORAGE (variant) = typeless; > } > } > } > > -- > Joseph S. Myers > jos...@codesourcery.com
