Re: RFA (fold): PATCH for c++/49290 (folding *(T*)(ar+10))

[Richard Guenther <rguenther at suse dot de>]

On Tue, 14 Jun 2011, Mike Stump wrote:

> On Jun 13, 2011, at 3:57 AM, Richard Guenther wrote:
> > That's not exactly an example - I can't think of how you want or need
> > to use VIEW_CONVERT_EXPRs to implement said divmod instruction or why
> > you would need anything special for the _argument_ of said instruction.
> 
> Oh, I completely misunderstood your question.  In my case, as I previously stated, was with a vector type that was identical, save the name of the type:
> 
> 	mod = a%b
> 
> where mod didn't have the type of the expression (a%b), so someone created the VIEW_CONVERT_EXPR on the mod.  The person creating it _thought_ it would be a rvalue context, but ultimately, it was an lvalue context.  We discover the lvalue/rvalue state of the expression at target_fold_builtin time.  The actual code looks more like:
> 
>   __builtin_divmod (div, mod, a, b);
> 
> In fold_builtin, we do all the processing to handle the semantics.
> 
> > An
> > instruction or call with multiple outputs would simply be something
> > like
> > 
> > { div_1, mod_2 } = __builtin_divmod (arg_3);
> > 
> > with two SSA defs. A nice representation for the tree for { div_1,
> > mod_2 } remains to be found (if it should be a single tree at all, or
> > possibly multiple ones).
> 
> At target_fold_builtin time we regenerate it as:
> 
> s = builtin_divmod_final (a, b);
> div_1 = s.div
> mod_2 = s.mod
> 
> and generate a type { div, mod } on the fly.  We expect the optimizer to handle extra moves reasonably, and we want to keep the one instruction as one unit.
> 
> > We already play tricks for sincos for example via
> > 
> > tem_1 = __builtin_cexpi (arg_2);
> > sin_3 = REALPART_EXPR <tem_1>;
> > cos_4 = IMAGPART_EXPR <tem_1>;
> > 
> > which avoids the two defs by using a single def which is then decomposed.
> > 
> > So, can you elaborate a bit more on what you want to do with special
> > argument kinds?  Elaborate with an actual example, not words.
> 
> We support tagging any parameter to a builtin as define_outputs, define_inputs or define_in_outs in a part of the .md file that describes the builtins for the machine, the actual divmod builtin for example is:
> 
> (define_builtin "divmod<T_ALL_DI:sign_u>" "divmod<T_ALL_DI:sign_u>_<type>"
>   [
>     (define_outputs [(var_operand:T_ALL_DI 0)    ;;dividend                      
>                      (var_operand:T_ALL_DI 1)])  ;;mod                           
>     (define_inputs  [(var_operand:T_ALL_DI 2)
>                      (var_operand:T_ALL_DI 3)])
>     (define_rtl_pattern "<T_ALL_DI:sign_u>divmod<m_mode>4" [0 1 2 3])
>     (attributes [pure])
>   ]
> )
> 
> that's the actual code.  The testcase looks like:
> 
>   t_v4udi_0 = divmodu_t_v4udi (t_v4udi_1, t_v4udi_2, t_v4udi_3);

I see.  So it's the C side of the representation that needs the
special operands.

> The VIEW_CONVERT_EXPR looks like:
> 
> <view_convert_expr 0x7ffff5a872d0
>     type <vector_type 0x7ffff7f4b930 __attribute__((vector_size(32))) unsigned long
>         type <integer_type 0x7ffff7e8c690 long unsigned int public unsigned DI
>             size <integer_cst 0x7ffff7e76730 constant 64>
>             unit size <integer_cst 0x7ffff7e76758 constant 8>
>             align 64 symtab 0 alias set -1 canonical type 0x7ffff7e8c690 precision 64 min <integer_cst 0x7ffff7e76780 0> max\
>  <integer_cst 0x7ffff7e76708 18446744073709551615>
>             pointer_to_this <pointer_type 0x7ffff7f50738> reference_to_this <reference_type 0x7ffff7f513f0>>
>         unsigned V4DI
>         size <integer_cst 0x7ffff7f43de8 constant 256>
>         unit size <integer_cst 0x7ffff7e76348 constant 32>
>         align 256 symtab 0 alias set -1 canonical type 0x7ffff7f4b930 nunits 4 reference_to_this <reference_type 0x7ffff7f51\
> 540>>
> 
>     arg 0 <var_decl 0x7ffff59d9640 t_v4udi_1
>         type <vector_type 0x7ffff5ac3888 type <integer_type 0x7ffff7e8c690 long unsigned int>
>             unsigned V4DI size <integer_cst 0x7ffff7f43de8 256> unit size <integer_cst 0x7ffff7e76348 32>
>             align 256 symtab 0 alias set -1 canonical type 0x7ffff5ac3888 nunits 4>
>         used public static unsigned V4DI defer-output file t22.c line 262 col 48 size <integer_cst 0x7ffff7f43de8 256> unit \
> size <integer_cst 0x7ffff7e76348 32>
>         align 256>>

This VIEW_CONVERT_EXPR looks useless - in fact useless_type_conversion_p
will tell you that, so you can omit it.

Richard.