Re: Questions about a constant array reference in the C++ front end

[Mark Mitchell <mark at codesourcery dot com>]

Kazu Hirata wrote:
> Hi,
>
> I have two questions about the C++ front end.
>
> Consider a C++ program
>
> static const int array[] = { 0, 0, 0, 1, 1, 1, 2, 2, 2 };
>
> int
> foo (int a)
> {
>   return array[7];
> }
>
> I am trying to fold array[7] into 2.  It turns out that if T is an
> ARRAY_REF,
>
>   TREE_READONLY (TREE_OPERAND (T, 0))
>
> is 0.  Why?  This would be 1 if the program is fed into the C front
> end, which is needed to safely fold a constant array reference.

That's a bug, or, rather, a place where the C++ front-end is failing to 
give full information to the optimizer.  It should be TREE_READONLY. 
There are some tricky bits, in that if we're doing a dynamic 
initialization, we presently cannot mark it TREE_READONLY, but this is a 
static initialization, so it should be fine.  Isn't TREE_OPERAND (T, 0) 
the VAR_DECL for array itself?  If so, waht's probably going wrong is 
that it's not being marked TREE_READONLY because we're afraid of the 
dynamic initialization case.  We're missing a call to 
c_apply_quals_to_decl (sp?) somewhere.

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com
(916) 791-8304