[PATCH] PowerPC: PR libgcc/97543, build libgcc with -mno-gnu-attribute

[Michael Meissner]

PowerPC: PR libgcc/97543, fix 64-bit long double issues

There are two issues in PR libgcc/97543 which shows up if you build a GCC
compiler with long double defaulting to 64-bit instead of 128-bit with IBM
extended double:

    1)	The first issue was the t-linux file forced the entire libgcc library
	to be compiled with the -mlong-double-128 option.

    2)	The second issue is that the GNU attribute #4 is set to reflect using
	128-bit long doubles, and you get linker warnings when you use use the
	compiler, since libgcc_s.so indicates 128-bit IBM long doubles were
	used.  I ran into a similar issue with my patches to extend libgcc to
	work if long doubles were configured to use the 128-bit IEEE format
	instead of the 128-bit IBM format.

One feature of the current GNU attribute implementation is if you have a shared
library (such as libgcc_s.so), the GNU attributes for the shared library is an
inclusive OR of all of the modules within the library.  This means if any
module uses the -mlong-double-128 option and uses long double, the GNU
attributes for the library will indicate that it uses 128-bit IBM long
doubles.  If you have a static library, you will get the warning only if you
actually reference a module with the attribute set.

This patch does two things:

    1)	Instead of compiling the whole library with -mlong-double-128, it only
	compiles the modules that process the IBM extended double format with
	this switch.  It also specifies that these files must be compiled using
	the IBM format for long double.

    2)	I turned off GNU attributes for the whole library.  Originally, I just
	turned off GNU attributes for just the modules that process IBM
	extended format values.  But this doesn't work if the compiler defaults
	long double to 64-bits.  What happens is the logic in rs6000.c that
	sets the GNU attribute bits, will set the bits for 64-bit long double
	if a normal double (DFmode) is used.  So I just turned off the
	attributes for the whole library.

This patch replaces the patch I previously did for IEEE 128-bit to turn off
GNU attributes for just the ibm-ldouble.o module.
	https://gcc.gnu.org/pipermail/gcc-patches/2020-October/556863.html

I have tested this by building a compiler on a little endian power9 system
running Linux with long double defaulting to 64-bits using the configure
option: --without-long-double-128, and I verified that the warning no longer is
generated by the linker.

I then built a bootstrap compiler, by first building a non-bootstrap version.
With that non-bootstrap compiler, I built versions of the MPC and MPFR.  Using
those libraries, and the non-bootstrap compiler as the host compiler, I was
able to do a full bootstrap compiler.

There are differences in the regression test suite where the test implicitly
assumed long double was 128-bits or was a float128 test.

Can I install this patch into the master branch?  I would also like to install
it in the GCC 10 branch after an appropriate period.

libgcc/
2020-10-28  Michael Meissner  <meissner@linux.ibm.com>

	PR libgcc/97543
	* config/rs6000/t-linux (HOST_LIBGCC2_CFLAGS): Don't set
	-mlong-double-128 for all modules.  Instead set
	-mno-gnu-attributes.
	(IBM128_OBJS): New make variable for long double support.
	(IBM128_S_OBJS): New make variable for long double support.
	(IBM128_ALL_OBJS): New make variable for long double support.
	(IBM128_CFLAGS): New make variable for long double support.
---
 libgcc/config/rs6000/t-linux | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/libgcc/config/rs6000/t-linux b/libgcc/config/rs6000/t-linux
index ed821947b66..b2a079c6b54 100644
--- a/libgcc/config/rs6000/t-linux
+++ b/libgcc/config/rs6000/t-linux
@@ -1,6 +1,22 @@
 SHLIB_MAPFILES += $(srcdir)/config/rs6000/libgcc-glibc.ver
 
-HOST_LIBGCC2_CFLAGS += -mlong-double-128
+# On the modules that deal with IBM 128-bit values, we need to make sure that
+# TFmode uses the IBM extended double format.
+IBM128_OBJS	= ibm-ldouble$(objext) _powitf2$(objext) ppc64-fp$(objext) \
+		  _divtc3$(object) _multc3$(object) \
+		  _fixtfdi$(object) _fixunstfdi$(object) \
+		  _floatditf$(objext) _floatunsditf$(objext)
+
+IBM128_S_OBJS	= $(patsubst %$(objext),%_s$(objext),$(IBM128_OBJS))
+IBM128_ALL_OBJS	= $(IBM128_OBJS) $(IBM128_S_OBJS)
+
+IBM128_CFLAGS	= -mlong-double-128 -Wno-psabi -mabi=ibmlongdouble
+
+$(IBM128_ALL_OBJS) : INTERNAL_CFLAGS += $(IBM128_CFLAGS)
+
+# Turn off gnu attributes for the whole library.  This allows us to build
+# libgcc that supports the different long double formats.
+HOST_LIBGCC2_CFLAGS += -mno-gnu-attribute
 
 # This is a way of selecting -mcmodel=small for ppc64, which gives
 # smaller and faster libgcc code.  Directly specifying -mcmodel=small
-- 
2.22.0


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