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

[liuhongt <hongtao.liu@intel.com>]

__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

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;
+}
+
 \f
 /* 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