[cfe-commits] Patch to change how const arrays/structs are handled

[Douglas Gregor]

On Oct 22, 2009, at 8:46 PM, Eli Friedman wrote:

> On Thu, Oct 22, 2009 at 3:08 PM, Tanya Lattner <lattner at apple.com>  
> wrote:
>> Ok, I don't understand why address taken should matter if it is  
>> declared
>> const. Am I missing something?
>
> It's sort of an edge case, but it's something like gcc's
> -fmerge-all-constants, which isn't completely standard-compliant.  In
> essence, if there are multiple instances of the given function on the
> stack, the addresses of each version of the constant are supposed to
> be distinct, and this is broken by making the variable a global.


Right. If the address of the constant was never taken (even implicitly  
via a reference binding), I think we would be free to perform this  
optimization. Otherwise, we'll probably have to put it behind -fmerge- 
all-constants, like GCC does, so that we don't break conforming  
programs with this optimization.

If this code is only enabled with -fmerge-all-constants, the patch  
looks fine to me, but please also add some tests that make sure we  
only perform the optimization when it's safe.

	- Doug