Re: [PATCH] combine: Don't optimize SIGN_EXTEND of MEM on WORD_REGISTER_OPERATIONS targets [PR113010]

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Jan 16, 2024 at 11:20 PM Greg McGary <gkm@rivosinc.com> wrote:
>
> The sign bit of a sign-extending load cannot be known until runtime,
> so don't attempt to simplify it in the combiner.

It feels like this papers over an issue downstream?

> 2024-01-11  Greg McGary  <gkm@rivosinc.com>
>
>         PR rtl-optimization/113010
>         * combine.cc (expand_compound_operation): Don't simplify
>         SIGN_EXTEND of a MEM on WORD_REGISTER_OPERATIONS targets
>
>         * gcc.c-torture/execute/pr113010.c: New test.
> ---
>  gcc/combine.cc                                 | 5 +++++
>  gcc/testsuite/gcc.c-torture/execute/pr113010.c | 9 +++++++++
>  2 files changed, 14 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.c-torture/execute/pr113010.c
>
> diff --git a/gcc/combine.cc b/gcc/combine.cc
> index 812553c091e..ba587184dfc 100644
> --- a/gcc/combine.cc
> +++ b/gcc/combine.cc
> @@ -7208,6 +7208,11 @@ expand_compound_operation (rtx x)
>        if (len == 0)
>         return x;
>
> +      /* Sign-extending loads can never be simplified at compile time.  */
> +      if (WORD_REGISTER_OPERATIONS && MEM_P (XEXP (x, 0))
> +         && load_extend_op (inner_mode) == SIGN_EXTEND)
> +       return x;
> +
>        break;
>
>      case ZERO_EXTRACT:
> diff --git a/gcc/testsuite/gcc.c-torture/execute/pr113010.c b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
> new file mode 100644
> index 00000000000..a95c613c1df
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/execute/pr113010.c
> @@ -0,0 +1,9 @@
> +int minus_1 = -1;
> +
> +int
> +main ()
> +{
> +  if ((0, 0xfffffffful) >= minus_1)
> +    __builtin_abort ();
> +  return 0;
> +}
> --
> 2.34.1
>