[cfe-commits] r63242 - in /cfe/trunk: Driver/RewriteObjC.cpp include/clang/AST/Decl.h include/clang/AST/Expr.h include/clang/AST/ExprCXX.h include/clang/Basic/DiagnosticSemaKinds.def lib/AST/Expr.cpp lib/AST/StmtPrinter.cpp lib/CodeGen/CGExprAgg.

[Douglas Gregor]

On Jan 29, 2009, at 11:50 AM, Daniel Dunbar wrote:

> Can you clarify this? I thought that it wasn't actually a
> miscompilation, rather a different choice in undefined behavior.
>
> Did I misunderstand, or is this just an area where we feel we need to
> be compatible with gcc?

The error is for something slightly different than what you're  
probably thinking about. This error refers specifically to GNU's array- 
range designators, where in GCC

	int array[] = { [1 ... 99] = something_with_side_effects() };

the side effects from something_with_side_effects() will happen only  
once. They actually document that this is the intended behavior in:

	http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Designated-Inits.html#Designated-Inits

In our codegen, the side effect would happen 99 times. That's in  
violation of GCC's specification, so we error out rather than produce  
wrong code (at Chris's request).

You're probably thinking about the multiple-initializations rule,  
e.g., if one has:

	int array[] = { f1(), 2, 3, [0] = f2() };

and both f1() and f2() have side effects, it is unspecified whether  
the side effects of f1() ever actually occur (see C99 6.7.8p19) or, if  
they do, in what order the side effects of f1() and f2() occur (see  
C99 6.7.8p23) Here, we warn that the later initializer overrides the  
former initializer [*], and then do the same thing that GCC does: we  
drop the call to f1(). Also, just like GCC, our side effects happen in  
subobject initialization order.

	- Doug

[*] The warning itself is meant to give more information when we think  
that the initializer being dropped has side effects, but we don't yet  
have the ability to determine whether an expression has side effects.


> - Daniel
>
> On Thu, Jan 29, 2009 at 10:58 AM, Douglas Gregor <dgregor at apple.com>  
> wrote:
>>
>> On Jan 29, 2009, at 10:48 AM, Chris Lattner wrote:
>>
>>>
>>> On Jan 29, 2009, at 8:09 AM, Douglas Gregor wrote:
>>>
>>>>> Very nice Doug, please make these errors though.  Some code bases
>>>>> build with many warnings, so this can be easy to miss.  Thanks for
>>>>> working on this!
>>>>
>>>>
>>>> FWIW, follow-on patches (one still to-be-finished) get this down to
>>>> only a very minor difference that we warn about (different behavior
>>>> when GNU array-range designated initializers have side effects).
>>>
>>> I'm still not very comfortable about only warning about cases where
>>> we miscompile the code.  The "different behavior" really is a
>>> miscompilation in my opinion, do you agree?
>>
>>
>> Yes, it's a miscompilation. I'll make this an "unsupported" error in
>> CodeGen, so that semantic clients aren't affected.
>>
>>       - Doug
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