https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101641
--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
Wow, and this time it's even combine coming into play!
(insn 10 9 11 2 (set (reg/v:DI 82 [ xy ])
(mem/j:DI (reg/v/f:DI 86 [ pu ]) [2 pu_6(D)->y+0 S8 A64])) "t.i":12:8
76 {*movdi_internal}
(nil))
(insn 11 10 12 2 (set (mem/j:DI (reg/v/f:DI 86 [ pu ]) [2 pu_6(D)->x+0 S8 A64])
(reg/v:DI 82 [ xy ])) "t.i":13:9 76 {*movdi_internal}
(expr_list:REG_DEAD (reg/v/f:DI 86 [ pu ])
(expr_list:REG_DEAD (reg/v:DI 82 [ xy ])
(nil))))
Trying 10 -> 11:
10: r82:DI=[r86:DI]
11: [r86:DI]=r82:DI
REG_DEAD r86:DI
REG_DEAD r82:DI
Failed to match this instruction:
(set (mem/j:DI (reg/v/f:DI 86 [ pu ]) [2 pu_6(D)->x+0 S8 A64])
(mem/j:DI (reg/v/f:DI 86 [ pu ]) [2 pu_6(D)->y+0 S8 A64]))
allowing combination of insns 10 and 11
original costs 4 + 4 = 8
replacement cost 4
deferring deletion of insn with uid = 10.
modifying insn i3 11: [r86:DI]=[r86:DI]
REG_DEAD r86:DI
deferring rescan insn with uid = 11.
Trying 4 -> 11:
4: r86:DI=r91:DI
REG_DEAD r91:DI
11: [r86:DI]=[r86:DI]
REG_DEAD r86:DI
Failed to match this instruction:
(set (mem/j:DI (reg:DI 91) [2 pu_6(D)->x+0 S8 A64])
(mem/j:DI (reg:DI 91) [2 pu_6(D)->y+0 S8 A64]))
allowing combination of insns 4 and 11
original costs 4 + 4 = 8
replacement cost 4
deferring deletion of insn with uid = 4.
modifying insn i3 11: [r91:DI]=[r91:DI]
REG_DEAD r91:DI
deferring rescan insn with uid = 11.
deleting noop move 11
somewhere inside combine we'd have to realize that this isn't a noop move
and then maybe not allow the combination in the first place since it
isn't recognizable? That is, somehow we must anticipate the removal,
I suppose it is via
/* Recognize all noop sets, these will be killed by followup pass. */
if (insn_code_number < 0 && GET_CODE (pat) == SET && set_noop_p (pat))
insn_code_number = NOOP_MOVE_INSN_CODE, num_clobbers_to_add = 0;
where set_noop_p for two MEMs simply dispatches to
rtx_equal_p && !side_effects_p.
Note on RTL we see that we cannot rely on MEM_ALIAS_SET but have to
use MEM_EXPR to conservatively assess that the access is _not_ through
a union ... (or as said we could annotate the alias set entry as to
belonging to a union).
In the end how we handle TBAA and unions might not be the very best way
(but I can't offer something better yet).
