Re: [PATCH] Mark asm goto with outputs as volatile

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

On Tue, Jun 27, 2023 at 5:26 AM Andrew Pinski via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> The manual references asm goto as being implicitly volatile already
> and that was done when asm goto could not have outputs. When outputs
> were added to `asm goto`, only asm goto without outputs were still being
> marked as volatile. Now some parts of GCC decide, removing the `asm goto`
> is ok if the output is not used, though not updating the CFG (this happens
> on both the RTL level and the gimple level). Since the biggest user of `asm goto`
> is the Linux kernel and they expect them to be volatile (they use them to
> copy to/from userspace), we should just mark the inline-asm as volatile.
>
> OK? Bootstrapped and tested on x86_64-linux-gnu.

OK.

>         PR middle-end/110420
>         PR middle-end/103979
>         PR middle-end/98619
>
> gcc/ChangeLog:
>
>         * gimplify.cc (gimplify_asm_expr): Mark asm with labels as volatile.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.c-torture/compile/asmgoto-6.c: New test.
> ---
>  gcc/gimplify.cc                               |  7 ++++-
>  .../gcc.c-torture/compile/asmgoto-6.c         | 26 +++++++++++++++++++
>  2 files changed, 32 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.c-torture/compile/asmgoto-6.c
>
> diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
> index 0e24b915b8f..dc6a00e8bd9 100644
> --- a/gcc/gimplify.cc
> +++ b/gcc/gimplify.cc
> @@ -6935,7 +6935,12 @@ gimplify_asm_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
>        stmt = gimple_build_asm_vec (TREE_STRING_POINTER (ASM_STRING (expr)),
>                                    inputs, outputs, clobbers, labels);
>
> -      gimple_asm_set_volatile (stmt, ASM_VOLATILE_P (expr) || noutputs == 0);
> +      /* asm is volatile if it was marked by the user as volatile or
> +        there is no outputs or this is an asm goto.  */
> +      gimple_asm_set_volatile (stmt,
> +                              ASM_VOLATILE_P (expr)
> +                              || noutputs == 0
> +                              || labels);
>        gimple_asm_set_input (stmt, ASM_INPUT_P (expr));
>        gimple_asm_set_inline (stmt, ASM_INLINE_P (expr));
>
> diff --git a/gcc/testsuite/gcc.c-torture/compile/asmgoto-6.c b/gcc/testsuite/gcc.c-torture/compile/asmgoto-6.c
> new file mode 100644
> index 00000000000..0652bd4e4e1
> --- /dev/null
> +++ b/gcc/testsuite/gcc.c-torture/compile/asmgoto-6.c
> @@ -0,0 +1,26 @@
> +
> +/* { dg-do compile } */
> +/* PR middle-end/110420 */
> +/* PR middle-end/103979 */
> +/* PR middle-end/98619 */
> +/* Test that the middle-end does not remove the asm goto
> +   with an output. */
> +
> +static int t;
> +void g(void);
> +
> +void f(void)
> +{
> +  int  __gu_val;
> +  asm goto("#my asm "
> +     : "=&r"(__gu_val)
> +     :
> +     :
> +     : Efault);
> +  t = __gu_val;
> +  g();
> +Efault:
> +}
> +
> +/* Make sure "my asm " is still in the assembly. */
> +/* { dg-final { scan-assembler "my asm " } } */
> --
> 2.31.1
>