[cfe-commits] [PATCH] Model C++11 rules for odr-use for variables correctly

[Eli Friedman]

On Thu, Jan 26, 2012 at 7:29 PM, John McCall <rjmccall at apple.com> wrote:
> On Jan 24, 2012, at 9:34 PM, Eli Friedman wrote:
>> Patch attached.  Basically, this is implementing the C++11 rules in
>> [basic.def.odr]p2 for variables allowed in constant expressions.
>> (Those rules don't matter so much for most code given how we do code
>> generation, but we have to get the rules exactly right to get implicit
>> capture in lambda expressions working correctly.)
>>
>> I don't really like this patch, but it seems like it is necessary.
>
> Yeah, I'm really not happy with this.  Obvious alternative:  mark ODR use
> at the DRE creation site unless the object is not already marked and it
> meets the constant-loading criteria.  If it does, flag that this full-expression
> contains a potential reference, then walk back over the completed
> full-expression looking for PE DREs not direct operands of L2R casts.

Is there any existing hook that actually triggers where I need it to?
MaybeCreateExprWithCleanups seems close... but doesn't quite cover
everything.

> +static bool DelayVarODRUsedForType(QualType T) {
> +  // C++11 [basic.def.odr]p1: A variable or non-overloaded function whose name
> +  // appears as a potentially-evaluated expression is odr-used unless it is
> +  // an object that satisfies the requirements for appearing in a
> +  // constant expression (5.19) and the lvalue-to-rvalue conversion (4.1)
> +  // is immediately applied.
> +
> +  // We don't try to check the constant expression bit here because it gives
> +  // our assertions better coverage.
> +
> +  // Check for an "object".
> +  if (!T->isObjectType())
> +    return false;
> +  // The lvalue-to-rvalue conversion can't be applied to arrays.
> +  if (T->isArrayType())
> +    return false;
> +  // lvalue-to-rvalue conversion doesn't apply to classes.
> +  // FIXME: That isn't completely true... but I doubt anyone cares about the
> +  // one edge case where we actually do apply lvalue-to-rvalue conversion.
> +  if (T->isRecordType())
> +    return false;
> +  // FIXME: Need to extend MarkUseAsLValue to misc vector expressions.
> +  if (T->isVectorType())
> +    return false;
> +  return true;
> +}
>
> This can and should be done on canonical types using direct tests.
> You can leave the other checks as asserts for equivalence.

You mean isa<ArrayType>(T) etc.?

-Eli