[RFC/RFA] Const variable initializer folding

[H.J. Lu]

On Thu, Sep 9, 2010 at 9:29 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Aug 31, 2010 at 4:08 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>> Hi.
>> at various places we need to fold const static variable via knowledge of its
>> initializer.  The precise conditions when we do so hoever vary. For example
>>
>> const int a=5;
>> const int b[2]={0,1};
>> main()
>> {
>>  return a+b[0];
>> }
>>
>> compiled with -O2 -fpic gets a folded to 5, but b[0] is not folded to 0.
>>
>> In order to fix darwin WHOPR partitioning I need variables with extern linkage
>> but known initializer.  Even this is supported for scalars, but not for arrays,
>> see i.e. g++.dg/opt/static3.C but not for arrays or structures.
>>
>> I am quite convinced that with WHOPR partitioning, we can't derive the fact
>> from current DECL flags.
>>
>> static const int a;
>>
>> Allows folding referenced of A to 0. However with partitioning it might become:
>> __attribute__ ((visibility ("hidden"))) extern const int a;
>> ... and we would lose knowledge that A is 0.
>>
>> For this reason patch adds const_value_known into varpool structure and makes
>> existing folders to use it.  Note that there is other place folding initialiers
>> in expand.c I would like to remove (tree optimizers can do the same) but that
>> would need some extra work.
>>
>> There is one failure with the patch, gfortran.dg/cray_pointers_5.f90.  It is
>> because fortran adds into varpool variable that is !TREE_STATIC &&
>> !DECL_EXTERNAL that seems like a bug to me, but I am not familiar enough with
>> this area of FE to know what it is supposed to do.
>>
>> Does this seem to make sense?  Patch was bootstrapped/regtested x86_64-linux and
>> darwin.
>>
>> Honza
>>
>>        * cgraph.h (struct varpool_node): Add const_value_known.
>>        (varpool_decide_const_value_known): Declare.
>>        * tree-ssa-ccp.c (fold_const_aggregate_ref): Update initializer folding.
>>        * lto-cgraph.c (lto_output_varpool_node): Store const_value_known.
>>        (input_varpool_node): Restore const_value_known.
>>        * tree-ssa-loop-ivcanon (constant_after_peeling): Check varpool for
>>        initializer folding.
>>        * ipa.c (ipa_discover_readonly_nonaddressable_var,
>>        function_and_variable_visibility): Compute const_value_known.
>>        * gimple-fold.c (get_symbol_constant_value): Use varpool for initializer
>>        folding.
>>        * varpool.c (varpool_decide_const_value_known): New function.
>
> This caused:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45626
>

This also caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45712


-- 
H.J.