build: strengthen 16 bit float support checks

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 0xb40000...
    0xb40000...: bf16,ch = CopyFromReg 0xb40000..., Register:bf16 %13
    0xb40000...: 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.

diff --git a/configure.ac b/configure.ac
index 248e30ca2b99e9da2bb78fcf130829550c006071..21bee28d1085f7f3471e4039a0ff4da27481fd00 100644 (file)
--- 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);
      }
   ]])
   ],[