Re: [PATCH, rs6000] (v2) Fold vector shifts in GIMPLE

[Will Schmidt <will_schmidt at vnet dot ibm dot com>]

On Tue, 2017-06-13 at 10:03 +0200, Richard Biener wrote:
> On Mon, Jun 12, 2017 at 11:56 PM, Will Schmidt
> <will_schmidt@vnet.ibm.com> wrote:
> > Hi,
> >
> >
> > diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
> > index 63ca2d1..55592fb 100644
> > --- a/gcc/config/rs6000/rs6000.c
> > +++ b/gcc/config/rs6000/rs6000.c
> > @@ -16588,6 +16588,83 @@ rs6000_gimple_fold_builtin (gimple_stmt_iterator *gsi)
> >         gsi_replace (gsi, g, true);
> >         return true;
> >        }
<snip>
> > +    /* Flavors of vector shift right.  */
> > +    case ALTIVEC_BUILTIN_VSRB:
> > +    case ALTIVEC_BUILTIN_VSRH:
> > +    case ALTIVEC_BUILTIN_VSRW:
> > +    case P8V_BUILTIN_VSRD:
> > +      {
> > +       arg0 = gimple_call_arg (stmt, 0);
> > +       arg1 = gimple_call_arg (stmt, 1);
> > +       lhs = gimple_call_lhs (stmt);
> > +       gimple *g;
> > +       /* convert arg0 to unsigned.  */
> > +       arg0 = convert (unsigned_type_for (TREE_TYPE (arg0)), arg0);
> 
> Please do not use 'convert', instead do ...

Hi Richard, 

V3 of this patch , using the gimple_build() convenience helper function
has been posted, and is the direction I'm going for with this patch.  I
wanted to make sure I fully understood the other options though, so I
have a question/clarification on the other suggestions:

> > +       tree arg0_uns = create_tmp_reg_or_ssa_name
> > +          (unsigned_type_for (TREE_TYPE (arg0)));
> > +       g = gimple_build_assign (arg0_uns, arg0);
> 
>    g = gimple_build_assign (arg0_uns, VIEW_CONVERT_EXPR, usigned_type, arg0);

I tried a few trivial variations of this:
	g = gimple_build_assign (arg0_uns, VIEW_CONVERT_EXPR,
		 unsigned_type_for (TREE_TYPE(arg0_uns)), arg0);

which lookd good, but it asserts in gimple_build_assign_1(), on the
check
"if (op2)
 {
 gcc_assert (num_ops > 2);
...

Trolling around the other code for references, i found and tried this,
which uses the build1() helper, and appears to work.  Is this the gist
of what you suggested, or would there be another alternative?

   g = gimple_build_assign (arg0_uns, 
      build1(VIEW_CONVERT_EXPR,
         unsigned_type_for (TREE_TYPE(arg0_uns)), arg0));

Thanks for the feedback, etc.  :-)

-Will


> You also want to avoid spitting out useless copies here if the
> arg/result is already unsigned,
> like via
> 
>     tree arg0_uns = arg0;
>     if (! TYPE_UNSIGNED (TREE_TYPE (arg0_uns)))
>      {
> ...
>      }
> 
> > +       gimple_set_location (g, gimple_location (stmt));
> > +       gsi_insert_before (gsi, g, GSI_SAME_STMT);
> > +       /* convert lhs to unsigned and do the shift.  */
> 
> Just use lhs if it has the same sign as arg0_uns.
> 
> > +       tree lhs_uns = create_tmp_reg_or_ssa_name
> > +          (unsigned_type_for (TREE_TYPE (lhs)));
> 
> You can re-use the type of arg0_uns here.
> 
> > +       g = gimple_build_assign (lhs_uns, RSHIFT_EXPR, arg0_uns, arg1);
> > +       gimple_set_location (g, gimple_location (stmt));
> > +       gsi_insert_before (gsi, g, GSI_SAME_STMT);
> > +       /* convert lhs back to a signed type for the return.  */
> > +       lhs_uns = convert (signed_type_for (TREE_TYPE (lhs)),lhs_uns);
> > +       g = gimple_build_assign (lhs, lhs_uns);
> 
> See above for how to perform the conversion.
> 
> Note that you could use the gimple_build convenience to shorten the code
> sequence above to
> 
>     gimple_seq stmts = NULL;
>     tree arg0_unsigned = gimple_build (&stmts, VIEW_CONVERT_EXPR,
> 
> unsigned_type_for (...), arg0);
>     tree res = gimple_build (&stmts, RSHIFT_EXPR, TREE_TYPE (arg0_uns),
>                                        arg0_uns, arg1);
>     res = gimple_build (&stmts, VIEW_CONVERT_EXPR, TREE_TYPE (lhs), res);
>     gsi_insert_seq_before (gsi, stmts, GSI_SAME_STMT);
>     update_call_from_tree (gsi, res);
> 
> The above gimple_build sequence will fold all the stmts thus remove
> useless conversions and apply constant folding, etc.
> 
> Richard.
> 
> > +       gimple_set_location (g, gimple_location (stmt));
> > +       gsi_replace (gsi, g, true);
> > +       return true;
> > +      }
> >      default:
> >        break;
> >      }