Re: [PATCH] relocation truncated to fit: citus build failure on s390x

Re: To Andres Freund 2016-04-28 <20160428080824(dot)GA22412(at)msg(dot)df7cb(dot)de>
> > I'm not clear why citus triggers this, when other extensions don't?
>
> Maybe it's simply because citus.so is bigger than all the other
> extension .so files:
>
> -fpic
> Generate position-independent code (PIC) suitable for use
> in a shared library, if supported for the target machine.
> Such code accesses all constant addresses through a global
> offset table (GOT). The dynamic loader resolves the GOT
> entries when the program starts (the dynamic loader is not
> part of GCC; it is part of the operating system). If the
> GOT size for the linked executable exceeds a machine-
> specific maximum size, you get an error message from the
> linker indicating that -fpic does not work; in that case,
> recompile with -fPIC instead. (These maximums are 8k on
> the SPARC and 32k on the m68k and RS/6000. The 386 has no
> such limit.)
>
> Position-independent code requires special support, and
> therefore works only on certain machines. For the 386, GCC
> supports PIC for System V but not for the Sun 386i. Code
> generated for the IBM RS/6000 is always
> position-independent.
>
> When this flag is set, the macros "__pic__" and "__PIC__"
> are defined to 1.
>
> -fPIC
> If supported for the target machine, emit
> position-independent code, suitable for dynamic linking and
> avoiding any limit on the size of the global offset table.
> This option makes a difference on the m68k, PowerPC and
> SPARC.
>
> Position-independent code requires special support, and
> therefore works only on certain machines.
>
> When this flag is set, the macros "__pic__" and "__PIC__"
> are defined to 2.
>
> This doesn't mention s390(x), but citus.so 382952 bytes (on amd64) is
> well beyond the 8k/32k limits mentioned above.
>
> PostgreSQL itself links correctly on s390x:
> ... -I/usr/include/mit-krb5 -fPIC -pie -I../../../../src/include
>
> I'm not an expert in compiler flags, but it seems to me CFLAGS_SL is
> wrong on s390(x) in Makefile.port, it should use -fPIC like sparc.

After talking to a s390x Debian porter, -fPIC is the correct flag to
use. The quoted patch made the previously failing builds for citus and
pglogical (which have larger-than-average .so files) on s390x succeed
(and the sparc64 case still works):

--- a/src/makefiles/Makefile.linux
+++ b/src/makefiles/Makefile.linux
@@ -5,7 +5,8 @@ export_dynamic = -Wl,-E
rpath = -Wl,-rpath,'$(rpathdir)',--enable-new-dtags
DLSUFFIX = .so

-ifeq "$(findstring sparc,$(host_cpu))" "sparc"
+# Enable -fPIC to avoid "relocation truncated to fit" linker errors
+ifneq "$(filter sparc% s390%,$(host_cpu))" ""
CFLAGS_SL = -fPIC
else
CFLAGS_SL = -fpic

The patch was made against 9.6; I'd opt to include it in master and
the back branches.

https://buildd.debian.org/status/logs.php?pkg=citus&arch=s390x
https://buildd.debian.org/status/logs.php?pkg=pglogical&arch=s390x

Christoph