Re: Handle CONSTRUCTOR in operand_equal_p

[Richard Biener <rguenther@suse.de>]

On Wed, 21 Oct 2015, Jan Hubicka wrote:

> Hi,
> this is updated patch I am going to commit.  As discussed, we also need to match
> non-empty CONSTRUCTOR of vectors, but those should never be having CONSTANT flags
> set, so they need care in the other path trhough operand_equal_p, so I will post
> separate patch for that.
> 
> I updated the comment per Richard's comment and added a testcase.  Vuriously
> enoug we fold "val? (struct a){} : (struct a){}" only in PRE (tailmerging).
> W/o the patch even .optimized dump has the test and we fold it away only after
> lowering to RTL:
> ret (int val)
> {
>   struct a D.1764;
> 
>   <bb 2>:
>   if (val_2(D) != 0)
>     goto <bb 3>;
>   else
>     goto <bb 4>;
> 
>   <bb 3>:
>   D.1764 = {};
>   goto <bb 5>;
> 
>   <bb 4>:
>   D.1764 = {};
> 
>   <bb 5>:
>   return D.1764;
> 
> }
> 
> ret:
> .LFB0:
>         .cfi_startproc
>         xorl    %eax, %eax
>         ret
>         .cfi_endproc
> 
> With the patch we get:
> 
> ret (int val)
> {
>   struct a D.1764;
> 
>   <bb 2>:
>   D.1764 = {};
>   return D.1764;
> 
> }

But only via GENERIC folding I suppose.  Yes, we don't value-number
aggregates and generally PRE (and DOM via excessive jump-threading)
is the only pass that remotely handles this kind of situation.

code hoisting/sinking would maybe catch this but as this involves
memory I'm not sure the implementation ontop of PRE that is stuck
in some PR would catch it.

Richard.

> Honza
> 
> 	* fold-const.c (operand_equal_p): Add code matching empty
> 	constructors.
> 	* gcc.dg/tree-ssa/operand-equal-1.c: Verify that empty constructors
> 	are matched.
> 
> Index: fold-const.c
> ===================================================================
> --- fold-const.c	(revision 229133)
> +++ fold-const.c	(working copy)
> @@ -2892,6 +2892,11 @@ operand_equal_p (const_tree arg0, const_
>  	return operand_equal_p (TREE_OPERAND (arg0, 0), TREE_OPERAND (arg1, 0),
>  				flags | OEP_ADDRESS_OF
>  				| OEP_CONSTANT_ADDRESS_OF);
> +      case CONSTRUCTOR:
> +	/* In GIMPLE empty constructors are allowed in initializers of
> +	   aggregates.  */
> +	return (!vec_safe_length (CONSTRUCTOR_ELTS (arg0))
> +		&& !vec_safe_length (CONSTRUCTOR_ELTS (arg1)));
>        default:
>  	break;
>        }
> Index: testsuite/gcc.dg/tree-ssa/operand-equal-1.c
> ===================================================================
> --- testsuite/gcc.dg/tree-ssa/operand-equal-1.c	(revision 0)
> +++ testsuite/gcc.dg/tree-ssa/operand-equal-1.c	(revision 0)
> @@ -0,0 +1,8 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-pre" } */
> +struct a {int a,b;};
> +struct a ret(int val)
> +{
> +   return val? (struct a){} : (struct a){};
> +}
> +/* { dg-final { scan-tree-dump-not "if " "pre"} } */
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)