gcc: if hostPlatform.isPower64, configure with "--with-long-double-64"

pkgs/development/compilers/gcc/10/backport-libgcc-fix-PR97653.patch

@@ -0,0 +1,75 @@
+This patch is included upstream in gcc 11.1.0 and later:
+
+  https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=cda41ce0e8414aec59e6b9fbe645d96e6e8193e2
+
+From cda41ce0e8414aec59e6b9fbe645d96e6e8193e2 Mon Sep 17 00:00:00 2001
+From: Jakub Jelinek <jakub@redhat.com>
+Date: Sat, 3 Apr 2021 10:05:32 +0200
+Subject: [PATCH] rs6000: Fix up libgcc ABI when built with
+ --with-long-double-format=ieee [PR97653]
+
+__floatunditf and __fixtfdi and a couple of other libgcc{.a,_s.so}
+entrypoints for backwards compatibility should mean IBM double double
+handling (i.e. IFmode), gcc emits such calls for that format and
+form IEEE long double emits *kf* instead.
+When gcc is configured without --with-long-double-format=ieee ,
+everything is fine, but when it is not, we need to compile those
+libgcc sources with -mno-gnu-attribute -mabi=ibmlongdouble.
+The following snippet in libgcc/config/rs6000/t-linux was attempting
+to ensure that, and for some routines it works fine (e.g. for _powitf2).
+But, due to 4 different types of bugs it doesn't work for most of those
+functions, which means that in --with-long-double-format=ieee
+configured gcc those *tf* entrypoints instead handle the long double
+arguments as if they were KFmode.
+
+The bugs are:
+1) the first few objs properly use $(objext) as suffix, but
+   several other contain a typo and use $(object) instead,
+   which is a variable that isn't set to anything, so we don't
+   add .o etc. extensions
+2) while unsigned fix are properly called _fixuns*, unsigned float
+   are called _floatun* (without s), but the var was using there
+   the extra s and so didn't match
+3) the variable didn't cover any of the TF <-> TI conversions,
+   only TF <-> DI conversions
+4) nothing in libgcc_s.so was handled, as those object files are
+   called *_s.o rather than *.o and IBM128_SHARED_OBJS used wrong
+   syntax of the GNU make substitution reference, which should be
+   $(var:a=b) standing for $(patsubst a,b,$(var)) but it used
+   $(var:a:b) instead
+
+2021-04-03  Jakub Jelinek  <jakub@redhat.com>
+
+	PR target/97653
+	* config/rs6000/t-linux (IBM128_STATIC_OBJS): Fix spelling, use
+	$(objext) instead of $(object).  Use _floatunditf instead of
+	_floatunsditf.  Add tf <-> ti conversion objects.
+	(IBM128_SHARED_OBJS): Use proper substitution reference syntax.
+---
+ libgcc/config/rs6000/t-linux | 10 ++++++----
+ 1 file changed, 6 insertions(+), 4 deletions(-)
+
+diff --git a/libgcc/config/rs6000/t-linux b/libgcc/config/rs6000/t-linux
+index 72e9c2770a6..500210ddaf2 100644
+--- a/libgcc/config/rs6000/t-linux
++++ b/libgcc/config/rs6000/t-linux
+@@ -11,10 +11,12 @@ HOST_LIBGCC2_CFLAGS += -mno-minimal-toc
+ # the IBM extended double format.  Also turn off gnu attributes on the static
+ # modules.
+ IBM128_STATIC_OBJS	= ibm-ldouble$(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$(objext))
++			  ppc64-fp$(objext) _divtc3$(objext) _multc3$(objext) \
++			  _fixtfdi$(objext) _fixunstfdi$(objext) \
++	                  _floatditf$(objext) _floatunditf$(objext) \
++			  _fixtfti$(objext) _fixunstfti$(objext) \
++	                  _floattitf$(objext) _floatuntitf$(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
+-- 
+2.27.0
+

pkgs/development/compilers/gcc/10/default.nix

@@ -67,6 +67,7 @@ let majorVersion = "10";
       ++ optional langAda ../gnat-cflags.patch
       ++ optional langFortran ../gfortran-driving.patch
       ++ optional (targetPlatform.libc == "musl" && targetPlatform.isPower) ../ppc-musl.patch
+      ++ optional (targetPlatform.isMusl         && targetPlatform.isPower64) ./backport-libgcc-fix-PR97653.patch
 
       # Obtain latest patch with ../update-mcfgthread-patches.sh
       ++ optional (!crossStageStatic && targetPlatform.isMinGW) ./Added-mcf-thread-model-support-from-mcfgthread.patch

pkgs/development/compilers/gcc/common/platform-flags.nix

@@ -10,7 +10,53 @@ in lib.concatLists [
   (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
   (lib.optional (p ? float) "--with-float=${p.float}")
   (lib.optional (p ? mode) "--with-mode=${p.mode}")
-  (lib.optional
-    (let tp = targetPlatform; in tp.isPower && tp.libc == "glibc" && tp.is64bit)
-    "--with-long-double-128")
+
+  # The powerpc64 ABI has an IBM-specific, non-IEEE "long double" type
+  # which causes many some test failures.  GCC and glibc have been
+  # working on migrating to 128-bit IEEE long doubles using a complex
+  # "fat binary" scheme that avoids having a new multiarch tuple.
+  # Progress with this scheme has been exceptionally slow: Fedora has
+  # been trying to "flip the switch" since Fedora 31 without success.
+  #
+  # As of version 2.34 glibc refuses to build using *only* the IEEE
+  # standard long-double type, and implementation of the "fat binary"
+  # scheme to support both IEEE and IBM long doubles simultaneously is
+  # troublesome and unreliable.  Therefore, we use the same approach
+  # with glibc that musl uses: simply have `long double` be the same
+  # size as `double`: 64 bits.  This is already the case on
+  # aarch64-darwin, 32-bit ARM, 32-bit MIPS, and Microsoft's C/C++
+  # compiler for both 64-bit ARM and 32-bit x86, so it is
+  # well-supported by the build scripts for essentially everything in
+  # nixpkgs.
+  #
+  # Musl specifically checks for non-IEEE `long double`s on powerpc
+  # and will refuse to build if they are found to be present:
+  #
+  #   https://git.musl-libc.org/cgit/musl/tree/configure#n732
+  #
+  # This has saved musl as great deal of trouble that glibc is
+  # currently struggling with.  By using the same approach for both
+  # musl and glibc we avoid a lot of headaches that come up when
+  # "cross-compiling" from *-linux-gnu to *-linux-musl due to the
+  # libgcc bundled into *-linux-gnu's being linked with the emitted
+  # binaries.
+  #
+  # More details:
+  #
+  #   https://gcc.gnu.org/wiki/Ieee128PowerPC
+  #   https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html
+  #   https://fedoraproject.org/wiki/Changes/PPC64LE_Float128_Transition
+  #   https://en.m.wikipedia.org/wiki/Long_double
+  #   https://gcc.gnu.org/install/configure.html
+  #
+  # Below are the three possible valid combinations of options, with
+  # two commented out:
+  #
+  (lib.optionals targetPlatform.isPower64 [ "--with-long-double-64" ])
+  #
+  # glibc gets very unhappy with these flags:
+  #(lib.optionals targetPlatform.isPower64 [ "--with-long-double-128"   "--with-long-double-format=ieee" ])
+  #
+  # this is the default; should be equivalent to not adding any flags at all:
+  #(lib.optionals targetPlatform.isPower64 [ "--with-long-double-128"   "--with-long-double-format=ibm" ])
 ]