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From: "dizhao at os dot amperecomputing.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/114674] New: [aarch64] ldp_fusion fails to
 merge 2 strs due to imprecise alignment info
Date: Wed, 10 Apr 2024 07:41:23 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 14.0
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X-Bugzilla-Who: dizhao at os dot amperecomputing.com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114674

            Bug ID: 114674
           Summary: [aarch64] ldp_fusion fails to merge 2 strs due to
                    imprecise alignment info
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dizhao at os dot amperecomputing.com
  Target Milestone: ---

For the case below:

        typedef struct {
                unsigned int f1;
                unsigned int f2;
        } test_struct;

        static test_struct ts =3D {
                123, 456
        };

        void foo(void)
        {
                ts.f2 =3D 36969 * (ts.f2 & 65535) + (ts.f1 >> 16);
                ts.f1 =3D 18000 * (ts.f2 & 65535) + (ts.f2 >> 16);
        }

When compiled with "-O3 --param=3Daarch64-stp-policy=3Daligned", gcc failed=
 to fuse
the memory access instructions into stp/ldp, the dump file 312r.ldp_fusion1
says "ldp/stp policy says no". However, the accessing to ts.f1 is aligned t=
o 64
bit:

(insn 26 23 33 2 (set (mem/c:SI (lo_sum:DI (reg/f:DI 113)
                (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])) [1 ts.f1+0 S4
A64])
...

So it looks like the 2 str instructions should be fused with
aarch64-stp-policy=3Daligned.

After debugged this a bit, it seems the problem is ldp_bb_info::fuse_pair
changed the alignment info when calling adjust_address_nv, to rewrite the b=
ase
of ts.f1.=