[Bug rtl-optimization/104498] New: Alias attribute being ignored by scheduler

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104498

            Bug ID: 104498
           Summary: Alias attribute being ignored by scheduler
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: avieira at gcc dot gnu.org
  Target Milestone: ---

Whilst working on a tuning structure I saw a correctness regression that I
believe is a result of the alias attribute not working properly.

You can reproduce it using an existing tuning for AArch64 using:
gcc -O2 src/gcc/gcc/testsuite/gcc.c-torture/execute/alias-2.c -S
-mtune=cortex-a34

This will lead to the 'a[off] = 2' store being moved after the b load in
'b[off] != 2'.

In RTL:

(insn 23 18 19 2 (set (reg:SI 110 [ b[off.0_1] ])
        (mem:SI (plus:DI (mult:DI (reg:DI 99 [ off.0_1 ])
                    (const_int 4 [0x4]))
                (reg/f:DI 97)) [1 b[off.0_1]+0 S4 A32]))
"gcc/gcc/testsuite/gcc.c-torture/execute/alias-2.c":10:6 52 {*movsi_aarch64}
     (expr_list:REG_DEAD (reg:DI 99 [ off.0_1 ])
        (expr_list:REG_DEAD (reg/f:DI 97)
            (nil))))
(insn 19 23 24 2 (set (mem:SI (plus:DI (mult:DI (reg:DI 99 [ off.0_1 ])
                    (const_int 4 [0x4]))
                (reg/f:DI 104)) [1 a[off.0_1]+0 S4 A32])
        (reg:SI 106)) "gcc/gcc/testsuite/gcc.c-torture/execute/alias-2.c":9:9
52 {*movsi_aarch64}
     (expr_list:REG_DEAD (reg:SI 106)
        (expr_list:REG_DEAD (reg/f:DI 104)
            (nil))))

After some debugging I found that true_dependence returns false for these two
memory accesses because base_alias_check sees they have different base objects
('a' and 'b') and deduces they can't alias based on that, without realising 'b'
isn't an actual base object but an alias to 'a'. I think we should make it so
that at expand pointers to 'b' get 'a' as a base object.