[PowerPC] Detect different long double floating-point formats

Current versions of GCC support switching the format used for "long double"
to either IBM double double or IEEE-128.  The resulting binary is marked
via different setting of the Tag_GNU_Power_ABI_FP GNU attribute.

This patch checks this attribute to detect the format of the default
"long double" type and sets GDB's notion of the format accordingly.

The patch also adds support for the "__ibm128" type, which always uses
IBM double double format independent of the format used for "long double".

A new test case verifies that all three types, "long double", "__float128",
and "__ibm128" are correctly detected in all three compiler settings,
the default setting, -mabi=ieeelongdouble, and -mabi=ibmlongdouble.

gdb/ChangeLog:
2017-11-21  Ulrich Weigand  <uweigand@de.ibm.com>

	* ppc-tdep.h (enum powerpc_long_double_abi): New data type.
	(struct gdbarch_tdep): New member long_double_abi.
	* rs6000-tdep.c (rs6000_gdbarch_init): Initialize long_double_abi
	member of tdep struct based on Tag_GNU_Power_ABI_FP attribute.
	* ppc-linux-tdep.c (ppc_linux_init_abi): Install long double data
	format depending on long_double_abi tdep member.
	(ppc_floatformat_for_type): Handle __ibm128 type.

gdb/testsuite/ChangeLog:
2017-11-21  Ulrich Weigand  <uweigand@de.ibm.com>

	* gdb.arch/ppc-longdouble.exp: New file.
	* gdb.arch/ppc-longdouble.c: Likewise.

1 files changed, 19 insertions, 11 deletions

diff --git a/gdb/ppc-linux-tdep.c b/gdb/ppc-linux-tdep.c
index ee80a71..0e43a64 100644
--- a/gdb/ppc-linux-tdep.c
+++ b/gdb/ppc-linux-tdep.c
@@ -1623,12 +1623,17 @@ ppc_floatformat_for_type (struct gdbarch *gdbarch,
                           const char *name, int len)
 {
   if (len == 128 && name)
-    if (strcmp (name, "__float128") == 0
-        || strcmp (name, "_Float128") == 0
-        || strcmp (name, "_Float64x") == 0
-        || strcmp (name, "complex _Float128") == 0
-        || strcmp (name, "complex _Float64x") == 0)
-      return floatformats_ia64_quad;
+    {
+      if (strcmp (name, "__float128") == 0
+	  || strcmp (name, "_Float128") == 0
+	  || strcmp (name, "_Float64x") == 0
+	  || strcmp (name, "complex _Float128") == 0
+	  || strcmp (name, "complex _Float64x") == 0)
+	return floatformats_ia64_quad;
+
+      if (strcmp (name, "__ibm128") == 0)
+	return floatformats_ibm_long_double;
+    }
 
   return default_floatformat_for_type (gdbarch, name, len);
 }
@@ -1648,12 +1653,15 @@ ppc_linux_init_abi (struct gdbarch_info info,
   linux_init_abi (info, gdbarch);
 
   /* PPC GNU/Linux uses either 64-bit or 128-bit long doubles; where
-     128-bit, they are IBM long double, not IEEE quad long double as
-     in the System V ABI PowerPC Processor Supplement.  We can safely
-     let them default to 128-bit, since the debug info will give the
-     size of type actually used in each case.  */
+     128-bit, they can be either IBM long double or IEEE quad long double.
+     The 64-bit long double case will be detected automatically using
+     the size specified in debug info.  We use a .gnu.attribute flag
+     to distinguish between the IBM long double and IEEE quad cases.  */
   set_gdbarch_long_double_bit (gdbarch, 16 * TARGET_CHAR_BIT);
-  set_gdbarch_long_double_format (gdbarch, floatformats_ibm_long_double);
+  if (tdep->long_double_abi == POWERPC_LONG_DOUBLE_IEEE128)
+    set_gdbarch_long_double_format (gdbarch, floatformats_ia64_quad);
+  else
+    set_gdbarch_long_double_format (gdbarch, floatformats_ibm_long_double);
 
   /* Support for floating-point data type variants.  */
   set_gdbarch_floatformat_for_type (gdbarch, ppc_floatformat_for_type);