> So I think Kenner's code is trying to prevent having a value in a
> SUBREG that is inconsistent with the SUBREG_PROMOTED* flag bits. But
> I think it's been unnecessary since Matz's rewrite in 2009.
I couldn't really tell what the rewrite does entirely so I tried creating
a case where we would require the SUBREG_PROMOTED_VAR but couldn't come
up with any. At least for the most common path through expr I believe
I know why:
So our case is when we have an SI subreg from a DI reg that is originally
a sign-extended SI. Then we NOP-convert the SI subreg from signed to
unsigned. We only perform implicit sign extensions therefore we can
omit the implicit zero-extension case here.
The way the result of the signed->unsigned conversion is used determines
whether we can use SUBREG_PROMOTED_VAR. There are two possibilities
(1) and (2).
void foo (int bar)
{
unsigned int u = (unsigned int) bar;
(1) unsigned long long ul = (unsigned long long) u;
As long as the result is used unsigned, we will always perform a zero
extension no matter the "Kenner hunk" (because whether the subreg has
SRP_SIGNED or !SUBREG_PROMOTED_VAR does not change the need for a
zero_extend).
(2) long long l = (long long) u;
SUBREG_PROMOTED is checked by the following in convert_move:
scalar_int_mode to_int_mode;
if (GET_CODE (from) == SUBREG
&& SUBREG_PROMOTED_VAR_P (from)
&& is_a <scalar_int_mode> (to_mode, &to_int_mode)
&& (GET_MODE_PRECISION (subreg_promoted_mode (from))
>= GET_MODE_PRECISION (to_int_mode))
&& SUBREG_CHECK_PROMOTED_SIGN (from, unsignedp))
The SUBREG_CHECK_PROMOTED_SIGN (from, unsignedp) is decisive
as far as I can tell. unsignedp = 1 comes from treeop0 so our
"from" (i.e. unsigned int u).
With the "Kenner hunk" SUBREG_PROMOTED_VAR is unset, so we don't
strip the extension. Without it, SUBREG_PROMOTED_VAR () == SRP_SIGNED
which is != unsignedp, so no stripping either.
Now there are several other paths that would need auditing as well
but at least this one is safe. An interesting test target would be
a backend that does implicit zero extensions but as we haven't seen
fallout so far chances to find a trigger are slim.
Does that make sense?
Regards
Robin
