Re: [Open64-devel] optimizing extern const

[Christian B. <chr...@st...>]

you might also want to test for SCLASS_AUTO since a const local variable 
might be modified. the front end would warn about this.

Regards,

Christian

Mike Murphy wrote:
> "extern const" is probably not a common usage,
> but I ran into it in one of our tests, and discovered
> that we don't optimize it very well.  E.g. consider the code:
> 
> extern const int ec;
> extern void dummy(void);
> 
> int test (int x, int y)
> {
>         if (ec * 3 == x)
>                 return 0;
>         dummy();
>         if (ec * 3 == y)
>                 return 1;
>         return 2;
> }
> Ideally we would cse the ec*3,
> which should be safe since the value of ec
> will not change across the call.
> But open64 doesn't do this, because gccfe is only
> marking initialized constants as ST_is_const_var.
> This is a case of a const_var with unknown initialization.
> 
> The fix I came up with is to add to
> gccfe/tree_symtab.cxx:Create_ST_For_Tree:
>        if (TREE_CODE(decl_node) == VAR_DECL && TREE_READONLY(decl_node)) {
>                Set_ST_is_const_var(st);
>        }
> and then in be/com/be_symtab.cxx:ST_is_const_initialized:
>    // if is extern const, then is same as unknown const
>    if (ST_sclass(st) == SCLASS_EXTERN)
>        return FALSE;
> 
> If anyone sees a problem with this, please let me know.
> 
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