[Bug rtl-optimization/63483] New: Scheduler performs Invalid move of aliased memory reference

["ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63483

            Bug ID: 63483
           Summary: Scheduler performs Invalid move of aliased memory
                    reference
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
                CC: rguenth at gcc dot gnu.org, rth at gcc dot gnu.org

The original problem is present in [1], and can be illustrated by following
test on alpha-linux-gnu:

--cut here--
static char *a;
static char *b;

void foo (void)
{
  a[1] = 1;
  b[2] = 1;
}

int bar (void)
{
  return a && b;
}
--cut here--

The compilation with -O2 produces:

$foo..ng:
        .prologue 1
        lda $1,1($31)
        lda $2,a
        ldq $3,0($2)   <-- load a
        lda $2,b
        lda $7,1($3)
        ldq_u $5,1($3)   <-- load a
        insbl $1,$7,$4
        ldq $2,0($2)   <-- load b
        mskbl $5,$7,$5
        lda $6,2($2)
        bis $4,$5,$4
        stq_u $4,1($3)  <-- store a
        insbl $1,$6,$1
        ldq_u $3,2($2)   <-- load b
        mskbl $3,$6,$3
        cpys $f31,$f31,$f31
        bis $1,$3,$1
        stq_u $1,2($2)   <-- store b
        ret $31,($26),1
        .end foo

if a and b alias to the same wide memory location, then "b"  RMW
sequence corrupts "a".

There is am early shortcut for MEM_READOLNY_P in true_dependence_1 in alias.c.
For the testcase above:

(insn 15 13 18 2 (set (reg/f:DI 78 [ b ])
        (mem/u/f/c:DI (reg/f:DI 79) [2 b+0 S8 A64])) rmw.c:7 226 {*movdi}
     (expr_list:REG_DEAD (reg/f:DI 79)
        (nil)))

is free to be scheduled before

(insn 13 12 15 2 (set (mem:DI (and:DI (plus:DI (reg/f:DI 72 [ a ])
                    (const_int 1 [0x1]))
                (const_int -8 [0xfffffffffffffff8])) [0  S8 A64])
        (reg:DI 77)) rmw.c:6 226 {*movdi}
     (expr_list:REG_DEAD (reg:DI 77)
        (expr_list:REG_DEAD (reg/f:DI 72 [ a ])
            (nil))))

rth explained situation on alpha a bit:

--quote--
It's not the loads, per se, it's the stores that get in the way.

Early alpha can't store sub-4-byte quantities.  Altivec can't store anything
but 16 byte quantities.  In order to perform smaller stores, we have to do a
read-modify-write sequence on a larger aligned chunk of memory.  Two such RMW
sequences must conflict, lest we interleave and thus bork the operation.

I don't recall how much we ever did for this, exactly, but it's certainly
possible to know that some memory operations cannot conflict with these RMW
sequence.  E.g. through size + alignment of the other memory operation.  E.g.
on Alpha, a byte RMW store can't conflict with a normal DImode memory access.
--/quote--

And:

> Btw, if the mem is MEM_READONLY_P how can it be part of
> a {un}aligned_store sequence?

This flag is copied from the original memory operand of the store by
alpha_set_memflags to all memory operands in the expanded sequence.

[1] https://gcc.gnu.org/ml/gcc-patches/2014-06/msg02251.html