Print bfloat16 DWARF types correctly

Even if the DWARF information contains a bfloat16 base type (__bf16), a
variable of such type will still be printed using the IEEE half float format,
which is wrong.

This patch teaches GDB how to pick the bfloat16 format for __bf16 types in
DWARF (based on the base type name) and uses IEEE half float for all the other
16-bit float formats.

Tested on aarch64-linux/x86_64-linux.

OK?

gdb/ChangeLog:

2021-04-16  Luis Machado  <luis.machado@linaro.org>

	* arch-utils.c (default_floatformat_for_type): Handle bfloat16.

gdb/testsuite:

2021-04-16  Luis Machado  <luis.machado@linaro.org>

	* gdb.dwarf2/dw2-bfloat16.exp: New file.

4 files changed, 97 insertions, 1 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 2a8ca58..c8630a4 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2021-04-16  Luis Machado  <luis.machado@linaro.org>
+
+	* arch-utils.c (default_floatformat_for_type): Handle bfloat16.
+
 2021-04-15  John Baldwin  <jhb@FreeBSD.org>
 
 	* fbsd-nat.c (fbsd_lwp_debug_printf, fbsd_nat_debug_printf): New,
diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 0017e70..e1d5afd 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -279,7 +279,13 @@ default_floatformat_for_type (struct gdbarch *gdbarch,
 {
   const struct floatformat **format = NULL;
 
-  if (len == gdbarch_half_bit (gdbarch))
+  /* Check if this is a bfloat16 type.  It has the same size as the
+     IEEE half float type, so we use the base type name to tell them
+     apart.  */
+  if (name != nullptr && strcmp (name, "__bf16") == 0
+      && len == gdbarch_bfloat16_bit (gdbarch))
+    format = gdbarch_bfloat16_format (gdbarch);
+  else if (len == gdbarch_half_bit (gdbarch))
     format = gdbarch_half_format (gdbarch);
   else if (len == gdbarch_float_bit (gdbarch))
     format = gdbarch_float_format (gdbarch);
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index fdb02b7..9e490aa 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2021-04-16  Luis Machado  <luis.machado@linaro.org>
+
+	* gdb.dwarf2/dw2-bfloat16.exp: New file.
+
 2021-04-15  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.python/py-parameter.exp: Give a test a proper name to avoid
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-bfloat16.exp b/gdb/testsuite/gdb.dwarf2/dw2-bfloat16.exp
new file mode 100644
index 0000000..b7c394a
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-bfloat16.exp
@@ -0,0 +1,82 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This test checks if GDB can identify bfloat16 and IEEE half float types and
+# print them correctly.
+#
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile main.c -dw.S
+
+# Make some DWARF for the test.
+# 0x4049 is the equivalent of 3.141 for bfloat16.
+# 0x4248 is the equivalent of 3.1406 for IEEE half float.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    cu {} {
+	DW_TAG_compile_unit {
+                {DW_AT_language @DW_LANG_C}
+                {DW_AT_name     $srcfile}
+                {DW_AT_comp_dir /tmp}
+        } {
+	    declare_labels bf16_type fp16_type \
+		bf16_var fp16_var
+
+            bf16_type: DW_TAG_base_type {
+                {DW_AT_name __bf16}
+		{encoding @DW_ATE_float}
+                {DW_AT_byte_size 2 DW_FORM_sdata}
+            }
+
+            fp16_type: DW_TAG_base_type {
+                {DW_AT_name __fp16}
+		{encoding @DW_ATE_float}
+                {DW_AT_byte_size 2 DW_FORM_sdata}
+            }
+
+	    bf16_var: DW_TAG_variable {
+		{DW_AT_name "bf16_1"}
+		{DW_AT_type :${bf16_type}}
+		{DW_AT_const_value 0x4049 DW_FORM_sdata}
+	    }
+
+	    fp16_var: DW_TAG_variable {
+		{DW_AT_name "fp16_1"}
+		{DW_AT_type :${fp16_type}}
+		{DW_AT_const_value 0x4248 DW_FORM_sdata}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+# Make sure we can print both types correctly.
+gdb_test "print bf16_1" " = 3.141"
+gdb_test "print fp16_1" " = 3.1406"