[patch] Fix PR tree-optimization/51624

[Richard Guenther]

On Mon, Jan 2, 2012 at 1:06 PM, Eric Botcazou <ebotcazou@adacore.com> wrote:
>> Note that you'll get ICEs whenever TYPE_CANONICAL of some
>> aggregate type is NULL (thus, TYPE_STRUCTURAL_EQUALITY), so this
>> seems inherently fragile (similar to using TYPE_CANONICAL == TYPE_CANONICAL
>> here, we'd break the TYPE_STRUCTURAL_EQUALITY case again).
>
> Could you elaborate?  The patch was successfully regtested.

If you have nested structure types that are TYPE_STRUCTURAL_EQUALITY
then they have TYPE_CANONICAL == NULL_TREE and types_compatible_p
will return false (it ICEd in previous releases I believe) and thus you won't
stop attaching COMPONENT_REFs.

So the fix isn't complete.

>> I think that either the original fix needs to be re-thought or we need to
>> pass the base FIELD_DECL to build_ref_for_model.
>
>> That is, why not stop extracting the component-refs in
>>
>>       do {
>>         tree field = TREE_OPERAND (expr, 1);
>>         tree cr_offset = component_ref_field_offset (expr);
>>         gcc_assert (cr_offset && host_integerp (cr_offset, 1));
>>
>>         offset -= TREE_INT_CST_LOW (cr_offset) * BITS_PER_UNIT;
>>         offset -= TREE_INT_CST_LOW (DECL_FIELD_BIT_OFFSET (field));
>>
>>         VEC_safe_push (tree, stack, cr_stack, expr);
>>
>>         expr = TREE_OPERAND (expr, 0);
>>         type = TREE_TYPE (expr);
>>       } while (TREE_CODE (expr) == COMPONENT_REF);
>>
>> when expr == base?
>
> Because SRA uses the model of the LHS to build the access on the RHS, so this
> kind of equalitty condition cannot work.

Ugh.

>> The whole point of course was to never need something like
>> build_ref_for_model given that we have MEM_REFs.
>
> MEM_REF is nice, but SRA should stop rewriting component accesses that it
> doesn't scalarize, this is waste of time and memory, and introduces bugs.

I agree - I didn't know it does that.  Do you have an example?

Richard.

> --
> Eric Botcazou