[Bug lto/79061] [7 Regression][LTO][ASAN] LTO plus ASAN fails with "AddressSanitizer: initialization-order-fiasco"

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79061

--- Comment #27 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think the problem is in the vnode->dynamically_initialized handling, as well
as in get_translation_unit_decl being totally unreliable.
When LTO merges VAR_DECLs from multiple TUs, it either should clear the
dynamically_initialized flag, or should make sure that the decl that is used
for the vnode actually is going to have get_translation_unit_decl the TU that
it contained the definition.
The #c20 testcase has in the end 3 globals:
name = "__ioinit", module_name = 0x400f3c "/tmp/ccoQRMfD.ltrans0.o",
has_dynamic_init = 1
name = 0x400f5d "xptimer_clean", module_name = 0x400de0 "xptimer.cc",
has_dynamic_init = 1
name = 0x400f6b "xptimer_sweep", module_name = 0x400de0 "xptimer.cc",
has_dynamic_init = 1

xptimer_clean and xptimer_sweep are both defined in xptiming.cc and declared in
xptiming.h, xptimer.cc TU doesn't see any traces of it, yet
get_translation_unit_decl returns for it the xptimer.cc TU, because
DECL_CONTEXT of those 2 vars is a NAMESPACE_DECL and LTO merges NAMESPACE_DECLs
too.
Similarly, __ioinit has DECL_CONTEXT of std namespace and that, being prebuilt
by the compiler, doesn't have surrounding TU.
Even when considering only VAR_DECLs that have DECL_CONTEXT of
TRANSLATION_UNIT_DECL directly, I believe this doesn't work right:
1) if the VAR_DECL has more than one definition (comdat, inline var, ...?),
then I think we'd need to merge the two definitions with
dynamically_initialized 1 as dynamically_initialized 0
2) wonder if we ever merge external decl with decl definition using the
external decl's context

In short, the easiest fix is just to disable the initialization order checking
altogether for LTO (by forcing dynamically_initialized = 0 in LTO).
Or at least clear it when merging multiple varpool nodes that have
dynamically_initialized = 1, and otherwise (if there is just one TU with
vnode->dynamically_initialized == 1), make sure get_translation_unit_decl will
return that TU for it and if we can't ensure that, clear
dynamically_initialized too.  Which can be done e.g. by clearing it whenever
DECL_CONTEXT actually isn't a TRANSLATION_UNIT_DECL directly or is some other
TU, or by say adding TRANSLATION_UNIT_DECL tree into the vnode (grows memory
usage though!) and looking it up from there.