[Bug tree-optimization/88709] New: Improve store-merging

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88709

            Bug ID: 88709
           Summary: Improve store-merging
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

As shown in:
struct S { char buf[8]; };
void bar (struct S *);

void
foo (void)
{
  struct S s = {};
  s.buf[1] = 1;
  s.buf[3] = 2;
  bar (&s);
}

or

struct val_t
{
  char data[16];
};

void optimize_me (val_t);
void optimize_me3 (val_t, val_t, val_t);

void
good ()
{
  optimize_me ({ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 });
}

void
bad ()
{
  optimize_me ({ 1, 2, 3, 4, 5 });
}

void
why ()
{
  optimize_me ({ 1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 });
}

void
srsly ()
{
  optimize_me3 ({ 1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
                { 11, 12, 13, 14, 15, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
10 },
                { 21, 22, 23, 24, 25, 20, 20, 20, 10, 20, 20, 20, 20, 20, 20
});
}

void
srsly_not_one_missing ()
{
  optimize_me3 ({ 1, 2, 3, 4, 5, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
                { 11, 12, 13, 14, 15, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
10 },
                { 21, 22, 23, 24, 25, 20, 20, 20, 10, 20, 20, 20, 20, 20, 20,
11 });
}

there is room for improvement in store-merging.  In the first testcase, we
ignore the clearing because !lhs_valid_for_store_merging_p, the lhs is in that
case the whole VAR_DECL rather than a component of it.  And in the second
testcase, we sometimes punt because of the same reason, sometimes because
rhs_valid_for_store_merging_p is false.  Handling these = {} storage clearings
(or perhaps even __builtin_memset calls) is something we could handle, though
with extra care, we don't want to take apart those clears if it doesn't reduce
the amount of needed stores.