Re: [PATCH v2 1/2] aarch64: Don't return invalid GIMPLE assign statements

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

On Tue, Jul 12, 2022 at 4:38 PM Andrew Carlotti <andrew.carlotti@arm.com> wrote:
>
> aarch64_general_gimple_fold_builtin doesn't check whether the LHS of a
> function call is null before converting it to an assign statement. To avoid
> returning an invalid GIMPLE statement in this case, we instead assign the
> expression result to a new (unused) variable.
>
> This change only affects code that:
> 1) Calls an intrinsic function that has no side effects;
> 2) Does not use or store the value returned by the intrinsic;
> 3) Uses parameters that prevent the front-end eliminating the call prior to
> gimplification.
>
> The ICE is unlikely to have occurred in the wild, as it relies on the presence
> of a redundant intrinsic call.

Other targets usually simply refrain from folding intrinsic calls with no LHS.
Another option is to just drop it on the floor if it does not have any
side-effects which for the gimple_fold_builtin hook means folding it to
a GIMPLE_NOP (gimple_build_nop ()).

> gcc/ChangeLog:
>
>  * config/aarch64/aarch64-builtins.cc
>  (aarch64_general_gimple_fold_builtin): Add fixup for invalid GIMPLE.
>
> gcc/testsuite/ChangeLog:
>
>  * gcc.target/aarch64/advsimd-intrinsics/ignored_return_1.c: New test.
>
> ---
>
> diff --git a/gcc/config/aarch64/aarch64-builtins.cc b/gcc/config/aarch64/aarch64-builtins.cc
> index e0a741ac663188713e21f457affa57217d074783..5753988a9964967c27a03aca5fddb9025fd8ed6e 100644
> --- a/gcc/config/aarch64/aarch64-builtins.cc
> +++ b/gcc/config/aarch64/aarch64-builtins.cc
> @@ -3022,6 +3022,16 @@ aarch64_general_gimple_fold_builtin (unsigned int fcode, gcall *stmt,
>      default:
>        break;
>      }
> +
> +  /* GIMPLE assign statements (unlike calls) require a non-null lhs. If we
> +     created an assign statement with a null lhs, then fix this by assigning
> +     to a new (and subsequently unused) variable. */
> +  if (new_stmt && is_gimple_assign (new_stmt) && !gimple_assign_lhs (new_stmt))
> +    {
> +      tree new_lhs = make_ssa_name (gimple_call_return_type (stmt));
> +      gimple_assign_set_lhs (new_stmt, new_lhs);
> +    }
> +
>    return new_stmt;
>  }
>
> diff --git a/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/ignored_return_1.c b/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/ignored_return_1.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..345307456b175307f5cb22de5e59cfc6254f2737
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/advsimd-intrinsics/ignored_return_1.c
> @@ -0,0 +1,9 @@
> +/* { dg-do compile { target { aarch64*-*-* } } } */
> +
> +#include <arm_neon.h>
> +
> +int8_t *bar();
> +
> +void foo() {
> +  __builtin_aarch64_ld1v16qi(bar());
> +}