bug#69770: [PATCH] build: strengthen 16 bit float support checks

[Grisha Levit]

Recent clang provides __bf16 on aarch64 but it is broken.

If built with -O0, the conversion is wrong:

    $ printf '\x3F\x80' | od --end=big -An -tfB | tr -d ' '
    1.875

If built with -O1 or higher, compilation fails:

    fatal error: error in backend: Cannot select: 0xb400007a58d29780: f32 = 
fp_extend 0xb400007a58d31720
      0xb400007a58d31720: bf16,ch = CopyFromReg 0xb400007b78c53720, 
Register:bf16 %13
        0xb400007a58d29470: bf16 = Register %13
    In function: print_bfloat

The latter issue does not cause the existing configure test to fail
because the promotion is optimized out.

* configure.ac: Ensure 16 bit float promotion code does not get
optimized out, and produces an expected result.
---
 configure.ac | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index 248e30ca2..21bee28d1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -569,13 +569,14 @@ ac_c_werror_flag=$cu_save_c_werror_flag
 
 # Test compiler support for half precision floating point types (for od)
 AC_MSG_CHECKING([IEEE 16 bit floating point])
- AC_COMPILE_IFELSE(
+ AC_RUN_IFELSE(
    [AC_LANG_SOURCE([[
      int
      main (void)
      {
-        _Float16 hf;
+        volatile _Float16 hf = 1;
         float f = hf;  /* Ensure compiler can promote to float.  */
+        return !(f == 1.0f);
      }
   ]])
   ],[
@@ -589,13 +590,14 @@ if test $ieee_16_bit_supported = yes; then
 fi
 
 AC_MSG_CHECKING([Brain 16 bit floating point])
- AC_COMPILE_IFELSE(
+ AC_RUN_IFELSE(
    [AC_LANG_SOURCE([[
      int
      main (void)
      {
-        __bf16 hf;
+        volatile __bf16 hf = 1;
         float f = hf;  /* Ensure compiler can promote to float.  */
+        return !(f == 1.0f);
      }
   ]])
   ],[
-- 
2.44.0