[cfe-commits] r64994 - in /cfe/trunk/lib: AST/ExprConstant.cpp CodeGen/CGBlocks.cpp CodeGen/CGExprConstant.cpp CodeGen/CGExprScalar.cpp CodeGen/CodeGenModule.h

[Eli Friedman]

On Wed, Feb 18, 2009 at 7:22 PM, Mike Stump <mrs at apple.com> wrote:
> On Feb 18, 2009, at 7:12 PM, Eli Friedman wrote:
>>
>> On Wed, Feb 18, 2009 at 5:01 PM, Mike Stump <mrs at apple.com> wrote:
>>>
>>> --- cfe/trunk/lib/AST/ExprConstant.cpp (original)
>>> +++ cfe/trunk/lib/AST/ExprConstant.cpp Wed Feb 18 19:01:04 2009
>>> @@ -260,6 +260,7 @@
>>>  APValue VisitAddrLabelExpr(AddrLabelExpr *E)
>>>     { return APValue(E, 0); }
>>>  APValue VisitCallExpr(CallExpr *E);
>>> +  APValue VisitBlockExpr(BlockExpr *E) { return APValue(E, 0); }
>>
>> Aren't there some missing checks here?
>
> Work in progress.  There is all sorts of checking, semantics and codegen
> missing.

That's fine, but it would be nice to at least put in a FIXME for the
missing checks.

>> Block expressions can't possibly be constant in general.
>
> They are?!  That is why they are called block literal expressions.  In the
> same way that 1 is a literal and unchanging, ^{} is a literal, constant and
> unchanging.  Now, I could easily have it wrong, it you could point out the
> part you thought would vary, I can contemplate it.

Hmm, I might be confused about something, but take something like the
following (I haven't checked if this particular example compiles, but
this sort of thing is legal):
int a(int x) {
static int (^y)(void) = ^{x;};
return y();
}

This initializer for y must be illegal because it can't be evaluated
at compile-time: the returned value changes!  Therefore, blocks aren't
constant in the general case.

I believe that file-scope block expressions are constant, but you need
some way to check that; the use of isFileScope for compund literals
would be a good example to follow.

-Eli