expansion: Use __trunchfbf2 calls rather than __extendhfbf2 [PR114907]

The HF and BF modes have the same size/precision and neither is
a subset nor superset of the other.
So, using either __extendhfbf2 or __trunchfbf2 is weird.
The expansion apparently emits __extendhfbf2, but on the libgcc side
we apparently have __trunchfbf2 implemented.

I think it is easier to switch to using what is available rather than
adding new entrypoints to libgcc, even alias, because this is backportable.

2024-05-07  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/114907
	* expr.cc (convert_mode_scalar): Use trunc_optab rather than
	sext_optab for HF->BF conversions.
	* optabs-libfuncs.cc (gen_trunc_conv_libfunc): Likewise.

	* gcc.dg/pr114907.c: New test.

3 files changed, 40 insertions, 3 deletions

diff --git a/gcc/expr.cc b/gcc/expr.cc
index d4414e2..9f66d47 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -355,8 +355,16 @@ convert_mode_scalar (rtx to, rtx from, int unsignedp)
 		      && REAL_MODE_FORMAT (from_mode) == &ieee_half_format));
 
       if (GET_MODE_PRECISION (from_mode) == GET_MODE_PRECISION (to_mode))
-	/* Conversion between decimal float and binary float, same size.  */
-	tab = DECIMAL_FLOAT_MODE_P (from_mode) ? trunc_optab : sext_optab;
+	{
+	  if (REAL_MODE_FORMAT (to_mode) == &arm_bfloat_half_format
+	      && REAL_MODE_FORMAT (from_mode) == &ieee_half_format)
+	    /* libgcc implements just __trunchfbf2, not __extendhfbf2.  */
+	    tab = trunc_optab;
+	  else
+	    /* Conversion between decimal float and binary float, same
+	       size.  */
+	    tab = DECIMAL_FLOAT_MODE_P (from_mode) ? trunc_optab : sext_optab;
+	}
       else if (GET_MODE_PRECISION (from_mode) < GET_MODE_PRECISION (to_mode))
 	tab = sext_optab;
       else
diff --git a/gcc/optabs-libfuncs.cc b/gcc/optabs-libfuncs.cc
index 02e9bf6..2672991 100644
--- a/gcc/optabs-libfuncs.cc
+++ b/gcc/optabs-libfuncs.cc
@@ -589,7 +589,9 @@ gen_trunc_conv_libfunc (convert_optab tab,
   if (GET_MODE_CLASS (float_tmode) != GET_MODE_CLASS (float_fmode))
     gen_interclass_conv_libfunc (tab, opname, float_tmode, float_fmode);
 
-  if (GET_MODE_PRECISION (float_fmode) <= GET_MODE_PRECISION (float_tmode))
+  if (GET_MODE_PRECISION (float_fmode) <= GET_MODE_PRECISION (float_tmode)
+      && (REAL_MODE_FORMAT (float_tmode) != &arm_bfloat_half_format
+	  || REAL_MODE_FORMAT (float_fmode) != &ieee_half_format))
     return;
 
   if (GET_MODE_CLASS (float_tmode) == GET_MODE_CLASS (float_fmode))
diff --git a/gcc/testsuite/gcc.dg/pr114907.c b/gcc/testsuite/gcc.dg/pr114907.c
new file mode 100644
index 0000000..628746e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr114907.c
@@ -0,0 +1,27 @@
+/* PR middle-end/114907 */
+/* { dg-do run } */
+/* { dg-options "" } */
+/* { dg-add-options float16 } */
+/* { dg-require-effective-target float16_runtime } */
+/* { dg-add-options bfloat16 } */
+/* { dg-require-effective-target bfloat16_runtime } */
+
+__attribute__((noipa)) _Float16
+foo (__bf16 x)
+{
+  return (_Float16) x;
+}
+
+__attribute__((noipa)) __bf16
+bar (_Float16 x)
+{
+  return (__bf16) x;
+}
+
+int
+main ()
+{
+  if (foo (11.125bf16) != 11.125f16
+      || bar (11.125f16) != 11.125bf16)
+    __builtin_abort ();
+}