Re: [PATCH] fold-const: Optimize &"foo"[0] == "foo" [PR89074]

[Richard Biener via Gcc-patches]

> Am 19.01.2022 um 00:50 schrieb Jakub Jelinek via Gcc-patches 
> <gcc-patches@gcc.gnu.org>:
> 
> Hi!
> 
> This is a non-C++ related part from the PR89074 address_compare changes.
> For "foo" == "foo" we already optimize this from the (cmp @0 @0)
> simplification, because we use operand_equal_p in that case
> and operand_equal_p also compares the STRING_CSTs bytes rather than
> just addresses.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Ok.

Thanks,
Richard 


> 2022-01-18  Jakub Jelinek  <ja...@redhat.com>
> 
>    PR c++/89074
>    * fold-const.cc (address_compare): Consider different STRING_CSTs
>    with the same lengths that memcmp the same as equal, not different.
> 
>    * gcc.dg/tree-ssa/pr89074.c: New test.
> 
> --- gcc/fold-const.cc.jj    2022-01-18 13:25:08.057491249 +0100
> +++ gcc/fold-const.cc    2022-01-18 13:53:26.106261913 +0100
> @@ -16587,6 +16587,15 @@ address_compare (tree_code code, tree ty
>           || TREE_CODE (base1) == SSA_NAME
>           || TREE_CODE (base1) == STRING_CST))
>     equal = (base0 == base1);
> +  /* Assume different STRING_CSTs with the same content will be
> +     merged.  */
> +  if (equal == 0
> +      && TREE_CODE (base0) == STRING_CST
> +      && TREE_CODE (base1) == STRING_CST
> +      && TREE_STRING_LENGTH (base0) == TREE_STRING_LENGTH (base1)
> +      && memcmp (TREE_STRING_POINTER (base0), TREE_STRING_POINTER (base1),
> +         TREE_STRING_LENGTH (base0)) == 0)
> +    equal = 1;
>   if (equal == 1)
>     {
>       if (code == EQ_EXPR
> --- gcc/testsuite/gcc.dg/tree-ssa/pr89074.c.jj    2022-01-18 
> 18:42:05.125004996 +0100
> +++ gcc/testsuite/gcc.dg/tree-ssa/pr89074.c    2022-01-18 18:43:39.249694890 
> +0100
> @@ -0,0 +1,12 @@
> +/* PR c++/89074 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +/* { dg-final { scan-tree-dump "return 1;" "optimized" } } */
> +
> +int
> +foo (void)
> +{
> +  const char *a = &"foo"[0];
> +  const char *b = "foo";
> +  return a == b;
> +}
> 
>    Jakub
>