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Re: Question about bitsizetype


On Wed, 2012-05-09 at 13:47 -0700, Andrew Pinski wrote:
> On Wed, May 9, 2012 at 1:36 PM, William J. Schmidt
> <wschmidt@linux.vnet.ibm.com> wrote:
> > Greetings,
> >
> > I've been debugging a Fedora 17 build problem on ppc64-redhat-linux, and
> > ran into an issue with bitsizetype.  I have a patch that fixes the
> > problem, but I'm not yet convinced it's the right fix.  I'm hoping
> > someone here can help me sort it out.
> >
> > The problem occurs when compiling some Java code at -O3.  The symptom is
> > a segv during predictive commoning.  The problem comes when analyzing a
> > data dependence between two field references.  The access functions for
> > the data refs are determined in tree-data-ref.c: dr_analyze_indices ():
> >
> >      else if (TREE_CODE (ref) == COMPONENT_REF
> >               && TREE_CODE (TREE_TYPE (TREE_OPERAND (ref, 0))) == RECORD_TYPE)
> >        {
> >          /* For COMPONENT_REFs of records (but not unions!) use the
> >             FIELD_DECL offset as constant access function so we can
> >             disambiguate a[i].f1 and a[i].f2.  */
> >          tree off = component_ref_field_offset (ref);
> >          off = size_binop (PLUS_EXPR,
> >                            size_binop (MULT_EXPR,
> >                                        fold_convert (bitsizetype, off),
> >                                        bitsize_int (BITS_PER_UNIT)),
> >                            DECL_FIELD_BIT_OFFSET (TREE_OPERAND (ref, 1)));
> >          VEC_safe_push (tree, heap, access_fns, off);
> >        }
> >
> > Note the use of bitsizetype.  On a 64-bit target that defines TImode,
> > this is apparently set to a 128-bit unsigned type, verified in gdb:
> >
> > (gdb) ptr bitsizetype
> >  <integer_type 0xfffb5d700a8 bitsizetype public unsigned sizetype TI
> >    size <integer_cst 0xfffb5c82380 type <integer_type 0xfffb5d700a8
> > bitsizetype> constant 128>
> >    unit size <integer_cst 0xfffb5c823a0 type <integer_type
> > 0xfffb5d70000 sizetype> constant 16>
> >    align 128 symtab 0 alias set -1 canonical type 0xfffb5d700a8
> > precision 128 min <integer_cst 0xfffb5c823c0 0> max <integer_cst
> > 0xfffb5c82360 -1>>
> >
> > The problem arises in tree-data-ref.c: analyze_ziv_subscript:
> >
> >  type = signed_type_for_types (TREE_TYPE (chrec_a), TREE_TYPE (chrec_b));
> >  chrec_a = chrec_convert (type, chrec_a, NULL);
> >  chrec_b = chrec_convert (type, chrec_b, NULL);
> >  difference = chrec_fold_minus (type, chrec_a, chrec_b);
> >
> > Both input types are bitsizetype of mode TImode.  This call reduces to a
> > call to tree.c: signed_or_unsigned_type_for ():
> >
> >  return lang_hooks.types.type_for_size (TYPE_PRECISION (t), unsignedp);
> 
> And that was fixed by not calling type_for_size with the following patch:
> r185226 | rguenth | 2012-03-12 06:04:43 -0700 (Mon, 12 Mar 2012) | 9 lines
> 
> 2012-03-12  Richard Guenther  <rguenther@suse.de>
> 
>         * tree.c (signed_or_unsigned_type_for): Use
>         build_nonstandard_integer_type.
>         (signed_type_for): Adjust documentation.
>         (unsigned_type_for): Likewise.
>         * tree-pretty-print.c (dump_generic_node): Use standard names
>         for non-standard integer types if available.
> Thanks,
> Andrew Pinski
> 
> 
Ah, Andrew, you're a life-saver.  Thanks!

Bill

> 
> >
> > So this is the interesting point.  We are calling back to the front end
> > to find a type having the same precision as bitsizetype, in this case
> > 128.  The C lang hook handles this fine, but the Java one does not:
> >
> > tree
> > java_type_for_size (unsigned bits, int unsignedp)
> > {
> >  if (bits <= TYPE_PRECISION (byte_type_node))
> >    return unsignedp ? unsigned_byte_type_node : byte_type_node;
> >  if (bits <= TYPE_PRECISION (short_type_node))
> >    return unsignedp ? unsigned_short_type_node : short_type_node;
> >  if (bits <= TYPE_PRECISION (int_type_node))
> >    return unsignedp ? unsigned_int_type_node : int_type_node;
> >  if (bits <= TYPE_PRECISION (long_type_node))
> >    return unsignedp ? unsigned_long_type_node : long_type_node;
> >  return 0;
> > }
> >
> > This returns zero, causing the first call to chrec_convert in
> > analyze_ziv_subscript to segfault.
> >
> > I can cause the build to succeed with the following patch...
> >
> > Index: gcc/java/typeck.c
> > ===================================================================
> > --- gcc/java/typeck.c   (revision 187158)
> > +++ gcc/java/typeck.c   (working copy)
> > @@ -189,6 +189,12 @@ java_type_for_size (unsigned bits, int unsignedp)
> >     return unsignedp ? unsigned_int_type_node : int_type_node;
> >   if (bits <= TYPE_PRECISION (long_type_node))
> >     return unsignedp ? unsigned_long_type_node : long_type_node;
> > +  /* A 64-bit target with TImode requires 128-bit type definitions
> > +     for bitsizetype.  */
> > +  if (int128_integer_type_node
> > +      && bits == TYPE_PRECISION (int128_integer_type_node))
> > +    return (unsignedp ? int128_unsigned_type_node
> > +           : int128_integer_type_node);
> >   return 0;
> >  }
> >
> > ...but I wonder whether this is the correct approach.  Is the problem
> > really that the lang hook is missing handling for bitsizetype for
> > certain targets, or is the problem that bitsizetype is 128 bits?  All of
> > the other front ends seem to get along fine with a 128-bit bitsizetype;
> > it's just kind of an odd choice on a 64-bit machine.  Or is the problem
> > in the dr_analyze_indices code that's using bitsizetype?
> >
> > The thing that gives me pause here is that other machines would likely
> > have the same problem.  Any machine using a 128-bit bitsizetype would
> > hit this problem sooner or later when optimizing Java code.  Perhaps
> > it's just that few people compile Java statically anymore -- certainly
> > we don't even build it during normal development.
> >
> > I had myself convinced that all 64-bit machines with a TImode would have
> > a 128-bit bitsizetype, but I'm having trouble connecting the dots on
> > that at the moment, so that may or may not be true.  If it is, though,
> > then this would seemingly come up periodically on Intel building Java.
> > That makes me suspicious that I don't understand this well enough yet.
> >
> > Thanks in advance for any help!  I'd like to get this resolved quickly
> > for the Fedora folks, but I want to do it properly.
> >
> > Thanks,
> > Bill
> >
> >
> 


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