https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94600
--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're going through
rtx
store_expr (tree exp, rtx target, int call_param_p,
bool nontemporal, bool reverse)
{
...
normal_expr:
/* If we want to use a nontemporal or a reverse order store, force the
value into a register first. */
tmp_target = nontemporal || reverse ? NULL_RTX : target;
temp = expand_expr_real (exp, tmp_target, GET_MODE (target),
(call_param_p
? EXPAND_STACK_PARM : EXPAND_NORMAL),
&alt_rtl, false);
which expands directly into target via
else if (optimize >= 1
&& modifier != EXPAND_CONST_ADDRESS
&& modifier != EXPAND_INITIALIZER
&& modifier != EXPAND_MEMORY
&& TREE_READONLY (array) && ! TREE_SIDE_EFFECTS (array)
&& TREE_CODE (index) == INTEGER_CST
&& (VAR_P (array) || TREE_CODE (array) == CONST_DECL)
&& (init = ctor_for_folding (array)) != error_mark_node)
{
...
/* If VALUE is a CONSTRUCTOR, this
optimization is only useful if
this doesn't store the CONSTRUCTOR
into memory. If it does, it is more
efficient to just load the data from
the array directly. */
rtx ret = expand_constructor (value, target,
modifier, true);
and then store_constructor (exp, target, 0, int_expr_size (exp), false);
store_constructor cannot anymore fail and since we're invoking
expand_constructor with avoid_temp_mem == true that's the place to
fixup I think, possibly by amending
/* Handle calls that pass values in multiple non-contiguous
locations. The Irix 6 ABI has examples of this. */
if (target == 0 || ! safe_from_p (target, exp, 1)
|| GET_CODE (target) == PARALLEL || modifier == EXPAND_STACK_PARM)
{
if (avoid_temp_mem)
return NULL_RTX;
with the case for volatile target. Or of course at the very toplevel
by passing down a different expand modifier or by skipping ctor folding.
Note that an aggregate volatile assignment is not very well specified
when bitfields are involved - C says the copy is elementwise and then
strict volatile bitfield semantics would suggest that N stores happen
together with the required reads for RMW cycles.
So I'm not sure if this is really a bug or rather wrong user expectation.
