[PATCH] PowerPC: PR libgcc/97543, fix 64-bit long double issues
Thu Nov 19 23:43:02 GMT 2020
PowerPC: PR libgcc/97543, fix 64-bit long double issues
I meant to post this to the gcc-patches mailing list last Thursday, but I see I
posted this to an internal IBM mailing list.
This patch replaces the previous iterations of this patch:
October 22nd, 2020:
October 28th, 2020:
If you use a compiler with long double defaulting to 64-bit instead of 128-bit
with IBM extended double, you get linker warnings about mis-matches in the gnu
attributes for long double (PR libgcc/97543). Even if the compiler is
configured to have long double be 64 bit as the default with the configuration
option '--without-long-double-128' you get the warnings.
You also get the same issues if you use a compiler with long double defaulting
to IEEE 128-bit instead of IBM extended double (PR libgcc/97643).
The issue is the way libgcc.a/libgcc.so is built. Right now when building
libgcc under Linux, the long double size is set to 128-bits when building
libgcc. However, the gnu attributes are set, leading to the warnings.
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) All of the modules that support IBM 128-bit long doubles explicitly set
the ABI to IBM extended double.
2) I turned off GNU attributes for building the shared library or for
building the IBM 128-bit long double support.
I have discussed this patch with Alan Modra, and made several changes based on
I have tested this on a little endian power9 system running Linux by building
three separate compilers, using the Advance Toolchain AT14.0 which uses GLIBC
1) A compiler where the long double default is IBM 128-bit double;
2) A compiler where the long double default is IEEE 128-bit double; (and)
3) A compiler where the long double default is 64-bit.
Note, for the IEEE build (#2), the other patches that I will be submitting are
needed to enable the full build.
For each of the 3 compilers, I then tested some code with long double's and
verified that each of the long double options worked without generating
In addition, I have tested this patch on a big endian power8 system running
Linux, and there were no regressions.
Can I install this patch into the master branch? Since this is a bug for
64-bit long doubles, I would like to back port it to GCC 10, and GCC 9 after a
2020-11-17 Michael Meissner <email@example.com>
* config/rs6000/t-linux (IBM128_STATIC_OBJS): New make variable.
(IBM128_SHARED_OBJS): New make variable.
(IBM128_OBJS): New make variable. Set all objects to use the
explicit IBM format, and disable gnu attributes.
(IBM128_CFLAGS): New make variable.
(gcc_s_compile): Add -mno-gnu-attribute to all shared library
libgcc/config/rs6000/t-linux | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/libgcc/config/rs6000/t-linux b/libgcc/config/rs6000/t-linux
index ed821947b66..72e9c2770a6 100644
@@ -6,3 +6,25 @@ HOST_LIBGCC2_CFLAGS += -mlong-double-128
# smaller and faster libgcc code. Directly specifying -mcmodel=small
# would need to take into account targets for which -mcmodel is invalid.
HOST_LIBGCC2_CFLAGS += -mno-minimal-toc
+# On the modules that deal with IBM 128-bit values, make sure that TFmode uses
+# the IBM extended double format. Also turn off gnu attributes on the static
+IBM128_STATIC_OBJS = ibm-ldouble$(objext) _powitf2$(objext) \
+ ppc64-fp$(objext) _divtc3$(object) _multc3$(object) \
+ _fixtfdi$(object) _fixunstfdi$(object) \
+ _floatditf$(objext) _floatunsditf$(objext)
+IBM128_SHARED_OBJS = $(IBM128_STATIC_OBJS:$(objext):_s$(objext))
+IBM128_OBJS = $(IBM128_STATIC_OBJS) $(IBM128_SHARED_OBJS)
+IBM128_CFLAGS = -Wno-psabi -mabi=ibmlongdouble -mno-gnu-attribute
+$(IBM128_OBJS) : INTERNAL_CFLAGS += $(IBM128_CFLAGS)
+# Turn off gnu attributes for long double size on all of the shared library
+# modules, but leave it on for the static modules, except for the functions
+# that explicitly process IBM 128-bit floating point. Shared libraries only
+# have one gnu attribute for the whole library, and it can lead to warnings if
+# somebody changes the long double format. We leave it on for the static
+# modules to catch mis-compilation errors.
+gcc_s_compile += -mno-gnu-attribute
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: firstname.lastname@example.org, phone: +1 (978) 899-4797
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