[PATCH], Enable IBM/IEEE long double format to overriden more easily

[Michael Meissner <meissner@linux.vnet.ibm.com>]

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As Segher and I were discussing off-line, I have some problems with the current
-mabi={ieee,ibm}longdouble switches as we start to planning to modify GCC 9 and
GLIBC 2.27/2.28 to support __float128 as the default long double format for
power server systems.

My gripes are:

    1)	Using Warn() in rs6000.opt means that you get two warning messages when
	you use the switches (one from the driver, and one from cc1).

    2)	I feel you should not get a warning if you select the option that
	reflects the current default behavior (i.e. -mabi=ibmlongdouble
	currently for power server systems).

    3)	There is no way to silenece the warning (i.e. -w doesn't work on
	warnings in the .opt file).  Both GLIBC and LIBGCC will need the
	ability to build support modules with an explicit long double format.

    4)	In the future we will need a little more flexibility in how the default
	is set.

    5)	There is a mis-match between the documentation and rs6000.opt, as these
	switches are documented, but use Undocumented in the rs6000.opt.

These patches fix these issues.  If you use -Wno-psabi, it will silence the
warning.  I have built these patches on a little endian power8 system, and
there were no regressions.  Can I check these patches into the trunk?

2017-10-21  Michael Meissner  <meissner@linux.vnet.ibm.com>

	* config/rs6000/aix.h (TARGET_IEEEQUAD_DEFAULT): Set long double
	default to IBM.
	* config/rs6000/darwin.h (TARGET_IEEEQUAD_DEFAULT): Likewise.
	* config/rs6000/rs6000.opt (-mabi=ieeelongdouble): Move the
	warning to rs6000.c.  Remove the Undocumented flag, since it has
	been documented.
	(-mabi=ibmlongdouble): Likewise.
	* config/rs6000/rs6000.c (TARGET_IEEEQUAD_DEFAULT): If it is not
	already set, set the default format for long double.
	(rs6000_debug_reg_global): Print whether long double is IBM or
	IEEE.
	(rs6000_option_override_internal): Rework setting long double
	format.  Only warn if the user is changing the long double default
	and they did not use -Wno-psabi.
	* doc/invoke.texi (PowerPC options): Update the documentation for
	-mabi=ieeelongdouble and -mabi=ibmlongdouble.

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797

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Index: gcc/config/rs6000/aix.h
===================================================================
--- gcc/config/rs6000/aix.h	(revision 253961)
+++ gcc/config/rs6000/aix.h	(working copy)
@@ -78,6 +78,9 @@
 #undef  TARGET_IEEEQUAD
 #define TARGET_IEEEQUAD 0
 
+#undef  TARGET_IEEEQUAD_DEFAULT
+#define TARGET_IEEEQUAD_DEFAULT 0
+
 /* The AIX linker will discard static constructors in object files before
    collect has a chance to see them, so scan the object files directly.  */
 #define COLLECT_EXPORT_LIST
Index: gcc/config/rs6000/rs6000.opt
===================================================================
--- gcc/config/rs6000/rs6000.opt	(revision 253961)
+++ gcc/config/rs6000/rs6000.opt	(working copy)
@@ -381,10 +381,10 @@ mabi=d32
 Target RejectNegative Undocumented Warn(using old darwin ABI) Var(rs6000_darwin64_abi, 0)
 
 mabi=ieeelongdouble
-Target RejectNegative Undocumented Warn(using IEEE extended precision long double) Var(rs6000_ieeequad) Save
+Target RejectNegative Var(rs6000_ieeequad) Save
 
 mabi=ibmlongdouble
-Target RejectNegative Undocumented Warn(using IBM extended precision long double) Var(rs6000_ieeequad, 0)
+Target RejectNegative Var(rs6000_ieeequad, 0)
 
 mcpu=
 Target RejectNegative Joined Var(rs6000_cpu_index) Init(-1) Enum(rs6000_cpu_opt_value) Save
Index: gcc/config/rs6000/darwin.h
===================================================================
--- gcc/config/rs6000/darwin.h	(revision 253961)
+++ gcc/config/rs6000/darwin.h	(working copy)
@@ -274,6 +274,9 @@ extern int darwin_emit_branch_islands;
 #undef  TARGET_IEEEQUAD
 #define TARGET_IEEEQUAD 0
 
+#undef  TARGET_IEEEQUAD_DEFAULT
+#define TARGET_IEEEQUAD_DEFAULT 0
+
 /* Since Darwin doesn't do TOCs, stub this out.  */
 
 #define ASM_OUTPUT_SPECIAL_POOL_ENTRY_P(X, MODE)  ((void)X, (void)MODE, 0)
Index: gcc/config/rs6000/rs6000.c
===================================================================
--- gcc/config/rs6000/rs6000.c	(revision 253961)
+++ gcc/config/rs6000/rs6000.c	(working copy)
@@ -86,6 +86,20 @@
 #define TARGET_NO_PROTOTYPE 0
 #endif
 
+  /* Set -mabi=ieeelongdouble on some old targets.  In the future, power server
+     systems will also set long double to be IEEE 128-bit.  AIX and Darwin
+     explicitly redefine TARGET_IEEEQUAD and TARGET_IEEEQUAD_DEFAULT to 0, so
+     those systems will not pick up this default.  This needs to be after all
+     of the include files, so that POWERPC_LINUX and POWERPC_FREEBSD are
+     properly defined.  */
+#ifndef TARGET_IEEEQUAD_DEFAULT
+#if !defined (POWERPC_LINUX) && !defined (POWERPC_FREEBSD)
+#define TARGET_IEEEQUAD_DEFAULT 1
+#else
+#define TARGET_IEEEQUAD_DEFAULT 0
+#endif
+#endif
+
 #define min(A,B)	((A) < (B) ? (A) : (B))
 #define max(A,B)	((A) > (B) ? (A) : (B))
 
@@ -2875,6 +2889,13 @@ rs6000_debug_reg_global (void)
   fprintf (stderr, DEBUG_FMT_D, "tls_size", rs6000_tls_size);
   fprintf (stderr, DEBUG_FMT_D, "long_double_size",
 	   rs6000_long_double_type_size);
+  if (rs6000_long_double_type_size == 128)
+    {
+      fprintf (stderr, DEBUG_FMT_S, "long double type",
+	       TARGET_IEEEQUAD ? "IEEE" : "IBM");
+      fprintf (stderr, DEBUG_FMT_S, "default long double type",
+	       TARGET_IEEEQUAD_DEFAULT ? "IEEE" : "IBM");
+    }
   fprintf (stderr, DEBUG_FMT_D, "sched_restricted_insns_priority",
 	   (int)rs6000_sched_restricted_insns_priority);
   fprintf (stderr, DEBUG_FMT_D, "Number of standard builtins",
@@ -4557,13 +4578,26 @@ rs6000_option_override_internal (bool gl
 	rs6000_long_double_type_size = RS6000_DEFAULT_LONG_DOUBLE_SIZE;
     }
 
-  /* Set -mabi=ieeelongdouble on some old targets.  Note, AIX and Darwin
-     explicitly redefine TARGET_IEEEQUAD to 0, so those systems will not
-     pick up this default.  */
-#if !defined (POWERPC_LINUX) && !defined (POWERPC_FREEBSD)
+  /* Set -mabi=ieeelongdouble on some old targets.  In the future, power server
+     systems will also set long double to be IEEE 128-bit.  AIX and Darwin
+     explicitly redefine TARGET_IEEEQUAD and TARGET_IEEEQUAD_DEFAULT to 0, so
+     those systems will not pick up this default.  Warn if the user changes the
+     default unless -Wno-psabi.  */
   if (!global_options_set.x_rs6000_ieeequad)
-    rs6000_ieeequad = 1;
-#endif
+    rs6000_ieeequad = TARGET_IEEEQUAD_DEFAULT;
+
+  else if (rs6000_ieeequad != TARGET_IEEEQUAD_DEFAULT && TARGET_LONG_DOUBLE_128)
+    {
+      static bool warned_change_long_double;
+      if (!warned_change_long_double && warn_psabi)
+	{
+	  warned_change_long_double = true;
+	  if (TARGET_IEEEQUAD)
+	    warning (0, "Using IEEE extended precision long double");
+	  else
+	    warning (0, "Using IBM extended precision long double");
+	}
+    }
 
   /* Enable the default support for IEEE 128-bit floating point on Linux VSX
      sytems.  In GCC 7, we would enable the the IEEE 128-bit floating point
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 253961)
+++ gcc/doc/invoke.texi	(working copy)
@@ -22607,12 +22607,14 @@ Disable Book-E SPE ABI extensions for th
 @item -mabi=ibmlongdouble
 @opindex mabi=ibmlongdouble
 Change the current ABI to use IBM extended-precision long double.
-This is a PowerPC 32-bit SYSV ABI option.
+This is not likely to work if your system defaults to using IEEE
+extended-precision long double.
 
 @item -mabi=ieeelongdouble
 @opindex mabi=ieeelongdouble
 Change the current ABI to use IEEE extended-precision long double.
-This is a PowerPC 32-bit Linux ABI option.
+This is not likely to work if your system defaults to using IBM
+extended-precision long double.
 
 @item -mabi=elfv1
 @opindex mabi=elfv1