Add a RTS option -xp to load PIC object anywhere in address space

Add a RTS option -xp to load PIC object anywhere in address space. We do
this by relaxing the requirement of <0x80000000 result of
`mmapForLinker` and implying USE_CONTIGUOUS_MMAP.

We also need to change calls to `ocInit` and `ocGetNames` to avoid
dangling pointers when the address of `oc->image` is changed by
`ocAllocateSymbolExtra`.

Test Plan:
```
$ uname -a
Linux localhost 4.18.8-arch1-1-ARCH #1 SMP PREEMPT Sat Sep 15 20:34:48
UTC 2018 x86_64 GNU/Linux
$ cat mk/build.mk
DYNAMIC_GHC_PROGRAMS = NO
DYNAMIC_BY_DEFAULT = NO

GhcRTSWays += thr_debug
EXTRA_HC_OPTS += -debug
WAY_p_HC_OPTS += -fPIC -fexternal-dynamic-refs
$ inplace/bin/ghc-stage2 --interactive -prof +RTS -xp
GHCi, version 8.7.20180928: http://www.haskell.org/ghc/  :? for help
ghc-stage2: R_X86_64_32 relocation out of range:
ghczmprim_GHCziTypes_ZMZN_closure = 7f690bffab59
Recompile
/data/users/watashi/ghc/libraries/ghc-prim/dist-install/build/HSghc-prim
-0.5.3.o with -fPIC -fexternal-dynamic-refs.
ghc-stage2: unable to load package `ghc-prim-0.5.3'
$ strace -f -e open,mmap inplace/bin/ghc-stage2 --interactive -prof
-fexternal-interpreter  -opti+RTS -opti-xp
...
[pid 1355283]
open("/data/users/watashi/ghc/libraries/base/dist-install/build/libHSbas
e-4.12.0.0_p.a", O_RDONLY) = 14
[pid 1355283] mmap(NULL, 8192, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6a84842000
[pid 1355283]
open("/data/users/watashi/ghc/libraries/base/dist-install/build/libHSbas
e-4.12.0.0_p.a", O_RDONLY) = 14
[pid 1355283] mmap(NULL, 8192, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6a84676000
...
Prelude> System.Posix.Process.getProcessID
...
[pid 1355283]
open("/data/users/watashi/ghc/libraries/unix/dist-install/build/libHSuni
x-2.7.2.2_p.a", O_RDONLY) = 14
[pid 1355283] mmap(NULL, 45056, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6a67d60000
[pid 1355283]
open("/data/users/watashi/ghc/libraries/unix/dist-install/build/libHSuni
x-2.7.2.2_p.a", O_RDONLY) = 14
[pid 1355283] mmap(NULL, 57344, PROT_READ|PROT_WRITE|PROT_EXEC,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6a67d52000
...
```

```
$ uname -a
Darwin watashis-iMac.local 18.0.0 Darwin Kernel Version 18.0.0: Wed Aug
22 20:13:40 PDT 2018; root:xnu-4903.201.2~1/RELEASE_X86_64 x86_64
$ mv
/Users/watashi/gao/ghc/libraries/integer-gmp/dist-install/build/HSintege
r-gmp-1.0.2.0.o{,._DISABLE_GHC_ISSUE_15105}
$ inplace/bin/ghc-stage2 --interactive  +RTS -xp
GHCi, version 8.7.20181003: http://www.haskell.org/ghc/  :? for help
Prelude> System.Posix.Process.getProcessID
42791
Prelude> Data.Set.fromList [1 .. 10]
fromList [1,2,3,4,5,6,7,8,9,10]
Prelude>
Leaving GHCi.
$ inplace/bin/ghc-stage2 --interactive -prof -fexternal-interpreter
GHCi, version 8.7.20181003: http://www.haskell.org/ghc/  :? for help
Prelude> System.Posix.Process.getProcessID
42806
Prelude> Data.Set.fromList [1 .. 10]
fromList [1,2,3,4,5,6,7,8,9,10]
Prelude>
Leaving GHCi.
```

Also test with something that used to hit the 2Gb limit and it loads
and runs without problem.

Reviewers: simonmar, bgamari, angerman, Phyx, hvr, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5195

2 files changed, 30 insertions, 2 deletions

diff --git a/docs/users_guide/8.8.1-notes.rst b/docs/users_guide/8.8.1-notes.rst
index 37bad13b5b..75645e4690 100644
--- a/docs/users_guide/8.8.1-notes.rst
+++ b/docs/users_guide/8.8.1-notes.rst
@@ -80,6 +80,10 @@ Runtime system
   alignment, lower the amount of wasted memory and lower the amount of in use memory.
   See :ghc-ticket:`13617`. Note that committed memory may be slightly higher.
 
+- A new flag ``-xp`` is added on x86_64. When it is passed, the runtime linker
+  can load object files compiled with ``-fPIC -fexternal-dynamic-refs``
+  anywhere in the address space. This used to be restricted to the low 2Gb.
+
 
 Template Haskell
 ~~~~~~~~~~~~~~~~
diff --git a/docs/users_guide/runtime_control.rst b/docs/users_guide/runtime_control.rst
index 0c38ac5919..674b8f73ac 100644
--- a/docs/users_guide/runtime_control.rst
+++ b/docs/users_guide/runtime_control.rst
@@ -241,6 +241,28 @@ Miscellaneous RTS options
     crashes if exception handling are enabled. In order to get more information
     in compiled executables, C code or DLLs symbols need to be available.
 
+
+.. rts-flag:: -xp
+
+    On 64-bit machines, the runtime linker usually needs to map object code
+    into the low 2Gb of the address space, due to the x86_64 small memory model
+    where most symbol references are 32 bits. The problem is that this 2Gb of
+    address space can fill up, especially if you're loading a very large number
+    of object files into GHCi.
+
+    This flag offers a workaround, albeit a slightly convoluted one. To be able
+    to load an object file outside of the low 2Gb, the object code needs to be
+    compiled with ``-fPIC -fexternal-dynamic-refs``. When the ``+RTS -xp`` flag
+    is passed, the linker will assume that all object files were compiled with
+    ``-fPIC -fexternal-dynamic-refs`` and load them anywhere in the address
+    space. It's up to you to arrange that the object files you load (including
+    all packages) were compiled in the right way. If this is not the case for
+    an object, the linker will probably fail with an error message when the
+    problem is detected.
+
+    On some platforms where PIC is always the case, e.g. x86_64 MacOS X, this
+    flag is enabled by default.
+
 .. rts-flag:: -xm ⟨address⟩
 
     .. index::
@@ -250,8 +272,10 @@ Miscellaneous RTS options
 
         This option is for working around memory allocation
         problems only. Do not use unless GHCi fails with a message like
-        “\ ``failed to mmap() memory below 2Gb``\ ”. If you need to use this
-        option to get GHCi working on your machine, please file a bug.
+        “\ ``failed to mmap() memory below 2Gb``\ ”. Consider recompiling
+        the objects with ``-fPIC -fexternal-dynamic-refs`` and using the
+        ``-xp`` flag instead. If you need to use this option to get GHCi
+        working on your machine, please file a bug.
 
     On 64-bit machines, the RTS needs to allocate memory in the low 2Gb
     of the address space. Support for this across different operating