On 8/12/24 4:02 PM, Richard Sandiford wrote:
Jeff Law <jeffreya...@gmail.com> writes:
On 8/12/24 1:49 PM, Richard Sandiford wrote:
- regno = subreg_regno (x);
+ /* A paradoxical should always be REGNO (y) + 0. Using subreg_regno
+ for something like (subreg:DI (reg:SI N) 0) on a WORDS_BIG_ENDIAN
+ target will return N-1 which is catastrophic for N == 0 and just
+ wrong for other cases.
+
+ Fixing subreg_regno would be a better option, except that reload
+ depends on its current behavior. */
+ if (paradoxical_subreg_p (x))
+ regno = REGNO (y);
+ else
+ regno = subreg_regno (x);
Are you sure that's right? For a 32-bit big-endian target,
(subreg:DI (reg:SI 1) 0) really should simplify to (reg:DI 0) rather
than (reg:DI 1).
Correct, we want to get (reg:DI 0). We get "0" back from REGNO (y).
And we get 0 back from byte_lowpart_offset (remember, it's paradoxical).
The sum is 0 resulting in (reg:DI 0).
But in my example, REGNO (y) is 1 (it's a different example from the
one that prompted the patch). The simplified subreg should be the
(reg:DI 0) given by subreg_regno, rather than the (reg:DI 1) given
by using REGNO (y).
E.g.:
(set (reg:SI 1) (mem:SI ADDR))
(set (reg:DI 2) (and:DI (subreg:DI (reg:SI 1) 0)
(const_int 127)))
should (on big-endian targets) be equivalent to:
(set (reg:SI 1) (mem:SI ADDR))
(set (reg:DI 2) (and:DI (reg:DI 0)
(const_int 127)))
so that r2 is set to 0 and r3 is set to (mem:SI ADDR) & 127.
If instead we simplified it to:
(set (reg:SI 1) (mem:SI ADDR))
(set (reg:DI 2) (and:DI (reg:DI 1)
(const_int 127)))
then r2 would be set to 0 and r3 would be set to an indeterminate
value & 127.
I've gone back and forth over this before I sent that patch. I can
certainly see your logic. I'd convinced myself that the subreg should
have simplified to (reg:DI 1).
And the inconsistency was driving me bananas as my mental model is that
(reg:DI N) covers N and N+1 and all that changes in the order based on
endianness. ie, if we have (set (reg:DI 0) (...)) that changes d0/d1.
But maybe that's just 20 years of little endian thinking creeping in.
In which case (subreg:DI (reg:SI d0) 0) is actually meaningless.
Jeff
