Issue a warning for conversion between short and __bf16 under TARGET_AVX512BF16.

__bfloat16 is redefined from typedef short to real __bf16 since GCC
V13. The patch issues an warning for potential silent implicit
conversion between __bf16 and short where users may only expect a
data movement.

To avoid too many false positive, warning is only under
TARGET_AVX512BF16.

Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}
Ok for trunk?

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_invalid_conversion): New function.
	(TARGET_INVALID_CONVERSION): Define as
	ix86_invalid_conversion.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/bf16_short_warn.c: New test.
---
 gcc/config/i386/i386.cc                       | 32 +++++++++++++++++++
 .../gcc.target/i386/bf16_short_warn.c         | 17 ++++++++++
 2 files changed, 49 insertions(+)
 create mode 100644 gcc/testsuite/gcc.target/i386/bf16_short_warn.c

On Mon, Jun 26, 2023 at 4:54 PM liuhongt <hongtao.liu@intel.com> wrote:
>
> __bfloat16 is redefined from typedef short to real __bf16 since GCC
> V13. The patch issues an warning for potential silent implicit
> conversion between __bf16 and short where users may only expect a
> data movement.
>
> To avoid too many false positive, warning is only under
> TARGET_AVX512BF16.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}
> Ok for trunk?
Ready to install.
>
> gcc/ChangeLog:
>
>         * config/i386/i386.cc (ix86_invalid_conversion): New function.
>         (TARGET_INVALID_CONVERSION): Define as
>         ix86_invalid_conversion.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/i386/bf16_short_warn.c: New test.
> ---
>  gcc/config/i386/i386.cc                       | 32 +++++++++++++++++++
>  .../gcc.target/i386/bf16_short_warn.c         | 17 ++++++++++
>  2 files changed, 49 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.target/i386/bf16_short_warn.c
>
> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> index 0761965344b..dc02eac6203 100644
> --- a/gcc/config/i386/i386.cc
> +++ b/gcc/config/i386/i386.cc
> @@ -22718,6 +22718,35 @@ x86_emit_floatuns (rtx operands[2])
>
>    emit_label (donelab);
>  }
> +
> +/* Return the diagnostic message string if conversion from FROMTYPE to
> +   TOTYPE is not allowed, NULL otherwise.
> +   Currently it's used to warn for silent implicit conversion between __bf16
> +   and short, since __bfloat16 is refined as real __bf16 instead of short
> +   since GCC13.  */
> +
> +static const char *
> +ix86_invalid_conversion (const_tree fromtype, const_tree totype)
> +{
> +  if (element_mode (fromtype) != element_mode (totype)
> +      && (TARGET_AVX512BF16 || TARGET_AVXNECONVERT))
> +    {
> +      /* Warn for silent implicit conversion where user may expect
> +        a bitcast.  */
> +      if ((TYPE_MODE (fromtype) == BFmode
> +          && TYPE_MODE (totype) == HImode)
> +         || (TYPE_MODE (totype) == BFmode
> +             && TYPE_MODE (fromtype) == HImode))
> +       warning (0, "%<__bfloat16%> is redefined from typedef %<short%> "
> +               "to real %<__bf16%> since GCC V13, be careful of "
> +                "implicit conversion between %<__bf16%> and %<short%>; "
> +                "a explicit bitcast may be needed here");
> +    }
> +
> +  /* Conversion allowed.  */
> +  return NULL;
> +}
> +
>
>  /* Target hook for scalar_mode_supported_p.  */
>  static bool
> @@ -25009,6 +25038,9 @@ ix86_run_selftests (void)
>  #  define TARGET_MERGE_DECL_ATTRIBUTES merge_dllimport_decl_attributes
>  #endif
>
> +#undef TARGET_INVALID_CONVERSION
> +#define TARGET_INVALID_CONVERSION ix86_invalid_conversion
> +
>  #undef TARGET_COMP_TYPE_ATTRIBUTES
>  #define TARGET_COMP_TYPE_ATTRIBUTES ix86_comp_type_attributes
>
> diff --git a/gcc/testsuite/gcc.target/i386/bf16_short_warn.c b/gcc/testsuite/gcc.target/i386/bf16_short_warn.c
> new file mode 100644
> index 00000000000..3e47a815200
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/bf16_short_warn.c
> @@ -0,0 +1,17 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +
> +#include<immintrin.h>
> +typedef struct {
> +short payload;
> +} BFloat16;
> +
> +__attribute__((target("avx512vl,avx512bf16")))
> +BFloat16 tobf16_avx512(float f)
> +{
> +    BFloat16 r;
> +    __m128bh m = _mm_cvtneps_pbh(_mm_set_ss(f));
> +    r.payload = m[0]; /* { dg-warning " be careful of implicit conversion between '__bf16' and 'short'" } */
> +    return r;
> +}
> +
> --
> 2.39.1.388.g2fc9e9ca3c
>