Re[2]: [GSoC: DDG export][RFC] Current status

[Dmitry Melnik <dm at ispras dot ru>]

Hello Ian,

Tuesday, July 3, 2007, 6:23:02 PM, you wrote:

> Adding a new field to mem_attrs could be a problem.  You'll have to
> measure to see how much the memory usage changes.

We have tested that previously with alias-export patch, which uses the
same mechanism for saving original exprs for rtl.
It showed that the change in cc1-i-files compile time was less than 1%,
memory usage change was negligible.  We'll do the test again with the
new patch, though I don't think the memory consumption will increase.

>> +/* If EXPR contains conversions at the root of the tree, all of them
>> +   will be removed.  */
>> +
>> +static tree
>> +skip_conversions (tree expr)
>> +{
>> +  tree inner = expr;
>> +  /* Remove any conversions: they don't change what the underlying
>> +     object is.  Likewise for SAVE_EXPR.  */
>> +  while (TREE_CODE (inner) == NOP_EXPR || TREE_CODE (inner) == CONVERT_EXPR
>> +        || TREE_CODE (inner) == NON_LVALUE_EXPR
>> +        || TREE_CODE (inner) == VIEW_CONVERT_EXPR
>> +        || TREE_CODE (inner) == SAVE_EXPR)
>> +    inner = TREE_OPERAND (inner, 0);
>> +  return inner;
>> +}

> I don't understand why it's OK to throw all these away.  Some of them
> change the meaning of the expression.  I also don't understand why you
> want to throw them away.

This piece of code was actually derived from alias-export branch, and
is not needed for this project.
In alias-export we also used original expression trees to disambiguate
memory references.  E.g. two mems with orig_exprs (t1) x and (t2 *) p
can be disambiguated if points-to for p doesn't contain x.  Here it
doesn't matter what types x and p have, so we used skip_conversions to
simplify disambiguation routines logic.  The cleanup from conversions
worked for alias-export, because when we later searched for the base
variable to take it's points-to, we never went through INDIRECT_REFs.


>> @@ -1401,27 +1456,36 @@ component_ref_for_mem_expr (tree ref)
...
>>  }

> The handling of INDIRECT_REF here seems clearly wrong.  You definitely
> can't discard conversions for an INDIRECT_REF.

I agree.  This code should have been removed in early versions of
alias-export.  We don't modify MEM_EXPR at all since MEM_ORIG_EXPR
was introduced.


-- 
Best regards,
 Dmitry                            mailto:dm@ispras.ru