About the is_gimple_min_invariant predicate
Diego Novillo
dnovillo@google.com
Thu Jul 5 00:39:00 GMT 2007
On 7/4/07 6:24 PM, Eric Botcazou wrote:
>> The problem is that in GIMPLE we only allow TREE_INVARIANT as a gimple
>> value for ADDR_EXPRs. We still require a TREE_CONSTANT as an array
>> index. So, ARRAY[TREE_INVARIANT] is not valid GIMPLE.
>
> OK, my understanding is that is_gimple_min_invariant as currently written is
> correct as "non-recursive" predicate (for the updated grammar), i.e. during
> gimplification.
>
>> FRE should refuse to substitute a TREE_INVARIANT expression as an
>> ARRAY_REF index.
>
> The problem is that FRE (and other optimization passes) uses it as an absolute
> predicate. This mostly works because it essentially encompasses terminals of
> the grammar, but fails for the non-terminal case.
>
> How would you fix that? By making it recursive, e.g.:
>
> or by uncoupling the 2 functions, i.e creating a recursive predicate for use
> outside of gimplification?
Passes ought to distinguish GIMPLE values from GIMPLE invariants
according to context. As this test case shows, a GIMPLE invariant is
not always the right value to replace as it cannot be used as a valid
ARRAY_REF index.
My proposal is that we add a predicate is_gimple_const() and
is_gimple_invariant() such that is_gimple_invariant() is implemented as:
bool
is_gimple_invariant (tree expr)
{
if (is_gimple_const (expr))
return true;
else
return TREE_INVARIANT (expr);
}
And is_gimple_const() is the current is_gimple_min_invariant() without
the ARRAY_REF case.
In the future, we will even need to distinguish local invariants from
global invariants:
foo (int i)
{
return &a[i];
}
&a[i] is an invariant within foo() but it is not invariant in IPA mode.
So my proposal is that PRE/FRE and the value propagators need to make
sure that they propagate is_gimple_invariant() except inside ARRAY_REF
indexes.
Also, we need to update the GIMPLE grammar so that ARRAY_REF and
ARRAY_RANGE_REF take the appropriate values:
inner-compref: ...
| ARRAY_REF
op0 -> inner-compref
op1 -> index-val
op2 -> val
op3 -> val
index-val: ID | CONST
val: index-val | invariant
invariant: rhs with TREE_INVARIANT set
The changes are going to be a bit invasive, but not too much, I think.
Thoughts?
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