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 <[email protected]>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
